Part 2] OXOLOGY of NORTH-WEST PEMBROKESHIRE. 273

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part 2] OXOLOGYOF NORTH-WEST PEMBROKESHIRE. 273

12. T/se GEOLOGY of the DISTRICT betwee~ ABE~tEIDDY and ArERCASTLE (I~MrRO~ESmRE). By ARTHUR I-I~B~.RT Cox, l~.Se., Ph.D., F.G.S. (Read January 20th, 1915.) [PLATES XXII-XXT~I.]

CONTENTS. Pag~- I. Introduction ...... 9,73 (a) Historical Review. (b) Bibliography. II. Stratigraphy ...... 279 (A) The IAng~da Flags. (B) The Arenig Series. (C) The Llanvirn Series. (D) The Dicranooraptit8 Beds. IIL Petrology ...... 307 (A) The Keratophyres. (B) The Rhyolites. (C) The Pyroclastic Rocks. (D) The Diabases. (E) Other Igneous Rocks. IV. The Tectonics...... 332 V. The Physical Features ...... 336 VI. Summaryand Conclusions ...... 337

I. I~RODUCTION. THE area with which this paper is mainly concerned is a strip of country about 5 miles long, bordering on the nor~h-wesfern coast of Pembrokeshire. The eastern-end of this strip is about 6 miles south-west of Fishguard, while its wesbrn end lies some 4 miles north-east of St. David's. The country is represented on the 6-inch Ordnance Survey Maps, Pembrokeshire Sheets 8 S.W. & S.E., 15 N.W. & N.E., and comes within' Sheet 40 (Old Series) of the !-inch Geolo~cal Survey Map. The district is one of low relief, lying as it does for the greater part between the 100- and the 800-foot conbur-lines. The ground rarely falls below the former level, and accordingly the very irregular coast-line is largely formed by precipitous eli~s. The valleys are broad, and choked with Drift or alluvium ; and, as the watershed (the average elevation of which is between 300 and 400 feet) lies only about 2 miles south of the coast, the few northward-flowing streams have very short courses and li~le or no erosive power. The streams are hence almost lost in boggy, ground. The surface configuration has no very direct bearing on the geological structure, while inland exposures are poor and none too plentiful. The district is several miles disf~nt from the nearest railway. The population is scanty, and the only industry, apart from farming, is that connecbd with the diabase- or ' granib '-quarries at Porth Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016 QUART. JOURN. GEOL. SOC. VOL. LXXI, PL. XXII.

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274 GXOLOG~OF ~O~TmWEST PEMBROUS~ra~. [voL ~1,~,

(~tain, where there is a small harbour, the only one now used ~)n this coast. True, there is another harbour along this coast at Abereastle, but it has fallen into disuse since the development of Fishguard. Abercastle village is, in consequence, now almost deserted. My introduction to this area was as a member of the Geo- logists' Association party conducted over the St. David's district by Mr. J. F. N. Green and Prof. O. T. Jones at Easter 1911. Observar then made showed that the area required investigation or re-investigation. Indeed, the stratigraphy of the greater l~rt of this stri'p of country is now described for the first time.

(a) Historical Review. Although the neighbouring district of St. David's has been the ~ubject of very numerous investigations, the area here considered has, strange to say, been almost entirely neglected, the majority of the authors cited below having merely touched on its western r Mention is made of the trap (greenstone) rocks of the northern "coast by Kidd 1 and H. de la Beche, ~ while ]Kurchison 3 in 1839 mentions the presence of fossils in the slates at Abereiddy Bay, .and, in addition, gives some notes on the trap rocks of the Fish- ~rd and St. David's Head districts. In 1842, MacLauchlan 4 records Graptolithus murchisoni and Gr. foliaceus, with other fossils, from Abereidd~,,~ and refers the fossiliferous beds to the Llandeilo Flags, despite the differences in lithological character. In the first edition of the Geological Survey Map (sheet 40), l~ublished in 1845, the whole area was mapped as b., ' Llandeilo.' 5 Most of the igneous rocks were indicated, although not. differentiated one from the other. In the revised edition of this map (1857), the igneous rocks were separated into greenstones and felsl~thic traps, the Pwll Strodyr Fault was marked, and the boundary of the Linyula Flags, near Porth Gain, was inserted. Little further progress was made, until Hicks, following on his famous researches among the Cambrian rocks of St. David's, turned his attention to the younger beds of the district. In 1866, he and ~al~er~ outlined the succession of formations around St. David's,

1 , Note on the Mineralogy of the Neighbourhood of St. David's' Trans. Geol. Soc. set. 1, vol. ii (1814) p. 79. 2 , On the Geology of Southern Pembrokeshire' Trans. Geol. Soc. set. 2, 9vol. ii, pt. 1 (1826) p. 1. s , The Siluriau System' 1839, p. 401. 4 , Notes to accompany some Fossils collected by the author & Mr. H. Still during their employment on the Ordnauce Survey in Pembrokeshire' Trans. Geol. Soc. set. 2, vol. vi, pt. 2 (1842) p. 557. 5 Th~ term Llandeilo at that date was used to embrace the series now separated as Arenig and Llandeilo. 6 , Second Report on the "Menevian Group "& the other Formations of :St. I)avid~s ' Rep. Brit. Assoc. (Nottingham) 1866, p. 182. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

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276 yR. A. H. COX ON THE GEOLOGY OF THE DISTRICT [voL ]x~ classifying the st~ta above the I~'ngula Flags into Tremadoe~ Arenig, and Llandeilo, and mentioning some of the characteristic fossils. The ' Tremadoc' rocks of Ramsey Island were thought to. graduate into the Linyula Flags below, and into the Arenig Slates above. !t was recognized, however, that, whether considered from a palmontological or from a lithological standpoint, these ' Ramsey Island Tremadocs' (or _~esuretus Beds, as they afterwards came to be called) dif[ered from the Tremadoc Beds of North Wales; and further, that the upper beds of the Linyula Flags of North Wales were either not present in the St. David's district, or were there represented by di~erent rock-types. The Arenig Series of Whitesand Bay was compared with tha~ of Shropshire and North Wales, and with the Skiddaw Slates of the Lake District; while the fauna of the Llandeilo Beds at Aber- eiddy was correlated with that found in the Llandeilo of Builth and Llandeilo. Some account of the types of faulting was also given, and to this there is but little more to be added now. The various stratigraphical divisions were subsequently described in greater detail by Hicks in a succession of papers, of which the most important appeared in 1872 and 1875. The first of these papers 1 dealt with the' Tremadoc' Series of Ramsey Isls and the mainland. It was accompanied by a map of the St. David's district, including the country as far north as Llanrian. The l~pe r of 1875 ~ contained a more detailed map of the same area and in it the Ordovician s~rata, as developed in the coastal district between St. David's and Abereiddy, were divided into Lower, Middle, and Upper Arenig, and Lower, Middle, and Upper Llandeilo, while the presence of contemporaneous igneous rocks at the top of the Arenig and in the Upper Llandeilo was recorded. The numerous graptolites which had meanwhile been collected were" described by Mr. Hopkinson & Prof. Lapworth,S and the various stratigraphieal divisions correlated with those of Shropshire and elsewhere. At a s6_ll later date, Hicks, 4 on the ground that the faunas of the Upper Arenig and Lower Llandeilo divisions were closely connected, suggested that the two divisions should be united to form a new series, to be known as the Llanvirn Series, a classifica. tion which has since been adopted by the officers of the Geological Survey. 5 In 1904, Miss Elles, in her paper on ' Some Graptolite Zones in

I , On the Tremadoc Rocks in the Neighbourhood of St. David's, & their Fossil Contents' Q. J. G. S. vol. ~x~x (1873) p. 39. 2 , On the Succession of the Ancient Rocks in the Vicinity of St. David's' /b/d. vol. xxxi (1875) p. 167. a 'Descriptions of the Graptolites of the Arenig & Llandeflo Rocks of St. David's' /b/d. p. 631. 4 , The Classification of the Eozoic & Lower Pal~ozoic' Pop. Sci. Rev. n. s. vol, v (1881) p. 302. 5 T. C. Cantri]l ~& H. H. Thomas, ' Geology of the Country around Car- marthen' Mere. Geol. Surv. 1909, p. 24. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

par~ 2] BETWEEI~ABEREIDI)Y AND ABERCASTLE. 27~ the Arenig Rocks of Wales,' 1 dealt with the graptolites of Llanvirn, Ramsey Island, and Whitesand Bay near St. David's, and pointed out that the graptolitie slates yielding DidymograTtus extensus and, at a higher level, /). biJidus, probably did not represent the whole of the Arenig Series as developed in other districts. This was confirmed by Mr. J. Pringle ~ in 1911, when he showed that the ' Tremadoc ' or _~esuretus Beds of Ramscy Island contained an Arenig fauna. All these authors were concerned with areas lying west of Llanrlan, while the whole district east of that village remained undescribed. In fact, the nearest district in this direct;ion of which we have any account is the area ex~ending from Shxunble Head through Fishguard, described by Dr. Cowper Reed a in 1895. In that paper some account of the sh-atigmphical sequence round Fishguard and a detailed account of the igneous rocks were given. No fossils, however, were found anywhere west of Fishguard, and the correctness of Hicks's sequence at Abereiddy was assumed. Accordingly, the s~atified rocks of Pen Caer were assigned pro- visionally to the ' Bala and Upper Llandeilo' of Hicks. This work was not extended into the area at present under consideration. Dr. J. V. Elsden, in 1905, in his paper on ' The Igneous Rocks occurring between St. David's Head & Strumble Head,' 4 accepted Dr. Reed's suggestfion as to the age of the Pen Caer rocks, and remarked that 9Bala" Beds probably occupy Eae grea~er par~ of ~he area between Trevine and S~rmnble Head .... ' His paper was, however, mainly concerned with the igneous rocks; the Llam~an lavas were briefly described, and correlated with the supposed'Upper Llandeilo and Bala'lavas of Goodwick. Many of the basic intrusive rocks were described in some detail, and, judging from their distribution and from the apparent absence of any signs of breccia~on or of shearing, the author considered it likely that they were intruded after the main faulting of the district had "taken place. The peculiar rocks in the neighbourhood of Abereastle were described under the term 'lime-bostonites' and were compared with the m~enaites of Prof. W. 0. BrCigger. They were further compared with rocks in the Waterford district, and especially with the intrusive tuff-like rocks described from that part of Ireland by Kilroe & ]M:cHenry. 5 Dr. Elsden considered these' lime-bostonites' to be intrusive rocks, and from the brecciated character often shown by them, as also from their distribution in the field, he was inclined ~o assign their intrusion to a period earlier than the main faulting. 1 Geol. ~[ag. dec. 5, vol. i (1904) p. 208. ' Note, on the "Lower Tremadoc" Rocks of St. David's' Geol. ]Kag. dec. 5, vol. viii (1911) p. 556. '~ Q. J. G. S. vol. li (1895) p. 149. a Q. j. G. S. vol. lxi (1905) p. 579. 5 , Intrusive Tuff-like Igneous Rocks & Breccias in Ireland' Q. J. G. S. vol. lvii (1901) p. 484. Q. J. G. S. :No. 282. v Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

~8 DR. A~ If. CO~ ON THE GEOLOOI OF THE DISTRICT Lvol. IIli,

In a sul~equent paper Dr. Elsden I described in greater detail the various rock-i~pes biotite-norites, quartz-enstatite-diorites, etc. --wtxich go to form the large intrusive masses of St. David's Head and Cam Lidi near Whitesand Bay. From the absence of any shearing in these rocks he concluded that they have not been affected by any great earth-movement, and he expressed the opinion that they were probably intruded in connexion with the last great movement affecting this area, and that therefore they are at ,any rate of post-Bala, possibly of post-Carboniferous, or even of still later, age. In 1911, Prof. 0. T. Jones, while conducting the members of the Geologists' Association over the Abereiddy section, discovered ~Didymograptus bij~dus, etc. in the slates of the large quarry on the eastern side of the bay. These slates, with the overlying volcanic rocks, comprised the'Upper Llandeilo' of Hicks. This discovery was recorded by Mr. J. F. N. Green in his 'gepert on an Excursion to St. David's.' u In consequence of this discovery., which indicated repetition of- Llanvirn Beds on a large scale, a repetition likely to account for the difficulty of correlating part of the Abereiddy section with the Ordovieian stra~ of Carmarthenshire, etc., it was decided to re- investigate the sequence, and to examine the practically unknown district which lies to the east. Prof. Jones, who was to have taken part in this work, was unfortunately prevented from doing so for some considerable time. During the interval, the mapping of the ground between Abereiddy and Pwll Strodyr. was taken in hand. The general dip over this area appears to be in a northward direction at high angles ; but, as the mapping progressed, it became evident that the apparent sequence was extremely complicated by folding and strike-faulting, so that north-east of Abereiddy (that is, around Porth Gain, Trevine, and Abercastle) the grouud is occupied not by higher beds (Llandeilo and Bala) but by various rock-groups of Arenig and even of Cambrian age. This rather unexpected result led to the examination of the country extending still farther eastwards from Pwll Strodyr towards Strmnble Head. It then appeared that, in this direction, an extensive area is occupied mainly by various members of the Cambrian System. The presence of these Cambrian and Lower Arenig rocks along the North Pembrokeshire coast, and in the adjoining inland district, was previously quite unknown. These discoveries were briefly mentioned in a paper read by Prof. Jones and myself,8 at the Birmingham Meeting of the British Association in 1913. Since the roek-~oToups found on the eastern side of the Pwll- Strod~ Fault are for the greater part quite distinct from the gTOUpS occurring on the west, the fault serves as a natm~l boundary-line ; and accordingly this account of the country lying west of that fault is presented separately. 1 , The St. David's Head Rock-Series' Q. J. G. S. vol. lxiv (1908) p. 273. ~- Proc. Geol. Assoc. vol. xxii (1911) p. 228. 3 A.H. Cox & O. T. Jones, ' The Geology of the District between Abereiddy Pen Caer' l~ep. Brit. Assoc. (Birmingham) 1913, p. 484. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

r~rt 2~i i ~rwzE~ ~ER~rDDY A~D ~BEROASTLE. 279

(b) Bibliography. ~. KIDD.~' Note on the Mineralogy of the Neighbourhood of St. David's' Trans. Geol. Soc. ser. 1, vol. ii (1814) p. 79. ;Sir HENRY J. DE LA BECHE.--' On the Geology of Southern Pembrokeshire ' Trans. Geol. Soc. ser. 2, vol. ii, pt. 1 (1826) p. 1. :Sir RODERICK I. MURCHISON.--' The Silurian System ' 1839, p. 401. H.I MACLAUCHT.AN.--' Notes to accompany some Fossils collected by the Author & Mr. H. S~ill, during their employment on the Ordnance Survey in Pembrokeshire '.Trans. Geol. Soc. ser. 2, vol. vii, pt. 2 (1842) p. 557. ~.~)ne-inch Map, Sheet 40, Old Series, 1st edition, published by H.M. Geological Survey, 1845, ~)ne-inch Map, Sheet 40, Old Series, last editi6n, ptlblished by H.M. Geological Survey, 1857. :H. HICKS & J. W. SALTER.--' Second Report on the "Menevian Group" & the other Formations of St. David's' Rep. Brit. Assoc. (NottSng- ham) 1866, p. 182. H. WYATT-EDGEr, L--' On the Arenig & Llandeito Groups' Geol. Mag. dec. 1, vol. ~ (1867) p. 113. H. HicKS.--' On the Arenig & Lland~ilo Rocks of St. David's' Rep. Brit. Assoc. (Bradford) 1873, p. 82. H. HICKS.--' On the Tremadoc Rocks in the Neighbourhood of St. David's' Q. J. G. S. vol. xxix (1873) p. 39. H. H~cKs.--' On the Succession of Ancient Rocks in the Vicinity of St. David's' Q. J. G. S. voI. xxxi (1875) p. 167. ,•. HOPKINSON 9 C. LAPWORTH.~-~ Description of the Grapto]ites of the Ar~nig & Llandeilo Rocks of St. David's' Q. J. G. S. vol. xxxi (1875) p. 631. H. H~cKs.-~" The Classification of the Eozoic & Lower Pal~oz(~ic' Pop. Sci. Rev. ms. vol. v (1881) p. 302. H. HICKS.--' On some Recent Researches among Lower Palmozoic Rocks of North Wales' Prec. Geol. Assoc. vol. vii (1882) p. 291. H. HIcKs.~"Evidences of Ice-Action in Nor6h-West Pembrokeshire' Gla- elalists' Mag. vol. i (1894).p. 191. T. ft. JEHU.--' Glacial Deposits of Northern Pembrokeshire' Trans. Roy. Soc. Edin. vol. xli (1904) p. 53. Miss G. L. ELLES.--' Graptollte Zones in Arenig Rocks' Geol. Mag. dec. 5, voh i (1904) p. 208. J. V. Er,SI)EN.--' The Igneous Rocks occurring between St. David's Hcad & S~rumhle Head' Q. J. G. S. vol. lxi (1905) p. 579. J. V. ELSDEN.--' The St. David's Head Rock-Series' Q. J. G. S, vol. lxiv (1908) p. 273. J. F. N. GREEN.--' Repor~ of an Excursion to St. David's' Prec. Geol. Assoc. vol. xxii (1911) p. 228. ~k. H. Cox & O. T. JONF~S~--' The Geology of the District between Abereiddy & Pen Caer' Rep. Brit. Assoc. (Birmingham) 1913, p. 484.

II. STR• The succession of the sh~ified rocks in this district is as -follows :~ Dicranograpf,~ts Beds .... ~ Castmll Limestmne = Mydrim Limestone...... [ Dicranograpt~s Shales. DidymograIJtusrmurchisoni Shales. Llanvirn ...... ~ Murchisoni Ash, & Llanrian Volcanic Series: [ Didymograpt~.s-bifidus Beds. [ Tetragraptus Shales. Arenig... ~ Perth Gain Beds." [ Abercastle Beds. Upper Cambrian ...... Linguta Flags. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

280 GEOLOGY OF NORTH-WEST PEMBROKESHIREo ~ToL 1z~L

General Structure of the District. Broadly speaking the structure of the district is determined by two main folds :--(a) a much broken syncline (the Llanrian Syn- dine) in the southern and central parts of the area; and (b) a much broken anticline (the Abercastle Anticline)in the northern portion. These two folds, however, are not complementary one to the other, as they are separated by a portion of a minor anticlinal fold (the Pwll-Olva Anticline). The axes of the folds retain a direction nearly east and west. Dips are usually steep, the strata of the middle limbs of the folds being nearly vertical, or even overfolded southwarda Strike-faults are numerous throughout the area, sometimes causing repetition, and sometimes cutting out groups of strata. Dip-faults are also exceedingly numerous, but of very small importance.

Description of the Rock-Suceeaion. (A) Upper Cambrian : The L/ngtda Flags. The main outcrop of the beds of the /s GrouF covers a narrow strip of ground extending almost continuously for a distance of 2-~ miles along the norther,~ flank of the Llanrian Syncline, from Pen Porth Eger on the west to Pwll Olva on the east. This outcrop is not quite continuous, since the sea has breached it to form the bays of Pwll Crocha, and Aberfelin. This leaves a gap of half a mile in the outcrop between Ynys Fach and Trwyn Llwyd. A further small outcrol~ occurs in the headland between Pwll Olva and Pwll Llong : that is, where the Pwll-Olva Anticline runs out to sea. Along the whole length of these different outcrops the Zingula Flags are faulted on the south side against Arenig (Porth Gain} Beds, and are cut off on the north side by the sea, so that only a small thickness of beds (some 500 feet at most) is to be seen. The rocks are accessible at numerous points, among which the headland of Pen Porth Eger and the eastern side of Porth Gain Harbour may be specially mentioned ; and they ha~,e been quarried for slates. on Trwyn Ellen and Trwyn Llwyd, but these quarries are now abandoned. The group consists of alternations of ar~llaeeous and siliceous beds. The argillaceous beds are now hard, rather flaggy, and often highly micaceous slates of a greenish hue, while the siliceous beds are laminated, white or grey, fine-grained 'quartzites' and felspathic grits. The quartzite or grit-bands are usually very numerous, and range in thickness from less than an inch up to 1 or 2 feet. Individual beds may show great and rapid variations in, thickness, such variation being accompanied by that peculiar contortion of the lamination-planes described by Prof. Fearnsides a~ ' curled bedding.' 1 These _Linyula Flags of the northern coast 1 Q. j. G. S. vol. lxvi (1,910) p. 150 ;. see also A~ R~ Andrew,~ Geoh Mag. dec. 5, vol. vii (1910) p. 165. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

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282 1)R. ~. H. COX ON THE OEOLOOY OF THs DISTIUCT [voL lIli, are exactly similar to the beds in the upper portion of the Linyula Flag Series as developed on the north side of Whitesand Bay. At the same time they are very different fa'om the lower beds of the series as seen on the south side of this bay. In general appearance, the Linyula Flags of the North Pembrokeshire coast are almost identical with the Ffestiniog Beds of North Wales. The differential effects of cleavage are well displayed in this alternation of harder and softer beds ; the quartzites remain practically una:ffected, whereas the argillaceous beds have been altered to true slates, although, as a l~le, the cleavage is not perfect enough to make these slates of much economic value. At certain levels, and especially near the highest part of the series as seen in this district, the laminated quartzites or 'ringers' are hot so persistent, since the siliceous material is distributed rather in the form of Ienticles and irregadar streaks than in continuous beds, and thus the group is not always easy to separate from" the PorCh Gain (Arenig) Beds. Locally in these beds the sandy material may increase in coarseness to such an extent as :to give rise" to quite coarse grits, which are sometimes pebbly, the p~bbles reaching a diameter of 2 inches. The pebbly layers run in seams up to 6 inches thick, separated by ordinary gritty!or argillaceous material, so that they never combine to form a massive bdd. The rather coarse grits are pale in eolom', weathering almost white, and the banding with argillaceous material still remains much more regular than in the striped arenaeeous Porth Gain Beds that will be described presently. As already stated, this type of deposit is most marked in the highest part of the Linyula-Flag Series, as developed on the northern coast; the phenomena mentioned are remarkably well displayed on the east side of Porth Gain Harhour. These arenaceous lentieles' are, however, occasionally developed among the more typical laminated 'ringer' quartzites. In both cases, the len~icles are probably due to current-action causing local redistribution of the arenaceous material. The rocks of the I_Angula-Flag Series are, as a whole, much harder and more massive than the majority of the other rock- groups in the district, and hence the strata have resisted the crushing movements which have so frequently obliterated the bedding in the neighbouring Ordovician shales. Instead of becoming crushed the Linyula Flags have given way by folding, and so we find the effects of folding much better displayed among the I_.a'n#ula Flags than in any of the other rock-groups. Folding and rolling may be observed in almost any of the cliff-sections. Special mention may be made of the fine anticline exposed in the first inlet west of Trwyn Ellen, where the fold has been very beautifully dissected by marine erosion. This fold pitches westwards at about 15 ~ (see Pl. XXII, fig. 1). A similar pitch is shown by a series of anticlines and synclines, which form reefs off Pen Porth Eger. The only fossil obtained from these beds is Zinyula davisii. It Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

pa~ 2] : BETWEEN .~BEREIDD~ AND ABERC!STL]~. , 283: is abundant at several localities, and is especially common in some of the res~larly laminated quartzitic bands. Tile shells are frequently found in a eomminuted condition, as though broken up by current-action. Although the characteristic Zingula.Flag trilobites have not been obtained, there can be no doubt, from a comparison. with the Lingula Flags of Whitosand Bay and of North ~Wales, that these beds on the northern coast are truly of gingula-Flag age. :Not more than 500 feet ot~ strata are seen, the upper portion being cut out by a fault which brings on krenig rocks, while any lower beds that may be present are submerged beneath the sea.

Ordovieian: (B) The Arenig Series. The Arenig rocks in this area consist of (1) a lower, somewhat arenaceous, group, and (2) an upper argillaceous group. The arenaceous members, which consist of sandy mudstones, sandy shales, and grits, are here subdivided and described under the terms (a) Abereastle Beds, and (b) Porth Gain Beds. It will be shown that these groups, taken together, represent the Tremadoc Beds of Hicks) The overlying argillaceous series is clearly equivalent to the Tetrayraptus Shales of other districts in South Wales. The base of the Arenig Series is nowhere to be seen, as the lowest beds exposed either come up along the crests of anticlines, or are faulted against Zingula Flags.

(*la) The Abercastle Beds.~These are the lowest beds of Arenig age to be seen in the area. ~ They are best exposed in a quarry on the west side of the valley at the head of kbercastle Harbour. They consist of a series of hard, sandy, grey-blue Ifiudst~nes, with numerous irregular streaks and patches of darker blue material, which is more argillaceous in character. As a result of weathering the beds become greyish-red; at the s~me time, the arenaceous character and the colour-streaks and mottling become more pronounced. Numerous large lenticular concretions occur, but hardly any difference between the material o~ the concretions themselves and that of the main mass of the rocks can b e detec~d. There are also present sOme rather irregular patches, filled with earthy material rich in limonlte : such patches represent weathered concretions: The quarry appears tobe excavated almost in the centre of an anticline, an4so 'the beds, each from 4 to6 feet thick, seem to be almost horizon~l. The dip is actually between 5~i and 15 ~ northwards. The bedding is crossed by an imperfect cleavage, the cleavage-planes dipping about 80 ~ northwards, so that large, cleavage-slabs may be obtained and such are, in fact, extensively used in the neighbourhood for walls. Certain bands are crowded with larger or smaller fragments

I Q. j. G. S. vol. xxix (1873) p. 39. 2 It may be necessary to except certain quartzites found east of PwU Strody r (see p. 335). Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

284 DR. A. H. COX ON THE GEOLOGY OF THE DISTRIC~ [voL |~1"1; of O:yyg.ia selwyni, as a rule highly distorted by cleavage. Some of the specnnens measure 4 inches across, not allowing for distortion. Fragments of Trinucle~s and of Orthoceras were obtained from similar beds exposed in a small quarry opposite the ford at Aber- castle. The same beds appear on the coast at the south side of Aber Yw, and here also Oyyyia selwyni was found. At this locality the dip averages about 70 ~ north-north-eastwards, but the angle is not very: constant. There appears, also, to be a small separate outcrop in the core of the Pwll-Olva Anticline.

(1 b) The Porth Gain Beds.--These beds consist of a lower series of micaceous sandy mudstones or hard arenaceous slates, passing gradually upwards into a coarse felspathic grit, which, in its turn, is succeeded by the Tetragraptus Shales. These Porth Gain Beds are well exposed on the northern side of the Llanrian Syncline, where they come up from below the Tetrayraptus Shales and occupy a narrow strip of ground extending from Pen Porth Eger on the west, through PorCh Gain and Ynys Fach, to a point immediately north of Trevine. They also occur in Pwll Olva, where they are brought up along the axis of an anticlinal fold. On the northern sides of these outcrops the beds are everywhere faulted against Zingula Flags, and so the base of the series is nowhere to be seen in the outcrops just mentioned. Around Abercastle, however, beds lithologically very similar to those at Porth Gain intervene between the Abereastle Mudstones and the Tetrayraptus Shales. At the type-locality, Porth Gain, the lowest beds noted are the mieaceous sandy mudstones exposed in the tramway-cutting which affords access to the diabase-quarries on Pen Clegyr. In this cutting the mudstones are only seen in a weathered condition : they are then light grey, with some surfaces strongly iron-stained, and often showing in a very marked degree the same concentric staining as the Tetrayraptus Shales. The rocks have been affected considerably by cleavage, which makes a fairly large angle with, and often obscures, the bedding. At a spot about half-way along the cutting, Orthis pr~ava 1 occurs in some abundance. A few yards farther west Linyula Flags appear. The fault which separates the two sets of beds is very obscure at the point where it crosses the cutting, partly on account of the extent to which the rocks have been broken up by weathering. By observation of the Point at which the ringers or hard siliceous bands of the IMnqula Flags terminate, the exact Position of the fault may, however, be readily found. Similar beds are well exposed along the sides of the Porth Gain inlet, and are easily accessible near the jetty on the eastern side of the harbour. A few ~yards north of the jetty the fault bringing on the .I~inyula Flags is clearly seen from the water-level, where it gives rise to a great crack hading south-westwards at 35 ~ On

I C. A. Mafley, ' Note on Oft,his carausil & O. cal~igramma var. proava' Summary of Progress for 1911, Mere. Geol. Surv. 1912, p. 78. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

]gart 2J BETWEENABEREIDDY AND iBERCASTL]~. 285 the south side of the fault appears a series of hard dark-blue slates or mudstones, somewhat arenaceous and micaceous, ]out otherwise very similar to typical Tetragraptus Shales. These micaceous slates, of which rather more than 60 feet are seen, rapidly become harder and more arenaceous, and eventually pass upwards into a coarse grit. This grit is bluish in colom-, weathering to grey, and is highly felspathic. Besides the quartz-grains of normal size, some considerably larger grains (almost small pebbles) occur, along with mudstone ~ragments. The grit also contains shaly inter- r measuring from a fraction of an inch up to 2 or 3 inches in thickness. Under the microscope it is seen [P 101] i that the quartz-grains are greatly outnumbered by felspars, mostly orthoclase, though plagioelase is not uncommon. Most of the felspar-grains are very fresh, but a few are largely replaced by a kaolin-like substance: this is seen in the case of the plagioclase, equally with the orthoclase. Grains and small fragments of mudstones mind of recrystallized rhyolites and rhyolite tufts are very abundant. Of less common occurrence are grains of some trachytic or andesitic rock. These igneous rocks were probably derived from pre-Cambrian (Pebidian) rocks, while some of the mudstone-grains may well have come from the Lingu~a Flags. The various grains are embedded in an abundant paste of chlorite, in which sericite has been largely developed. The grit is about 60 feet thick, and on its northern side passes up quite normally into the. Tetraqraptus Shales. A few feet of sandy slates, or of slates with sandy intercalations, serve to show that this passage is a conformable one. These transitional slates yielded Orthis proava and Callo.qrajatus sp., while the hard sandy slates below the grit yielded a very occasional I~ingula. All the beds are practically vertical. The coarse grit was observed in its proper position immediately south of the tramway-cutting. It may also be seen above the cave in Pen Porth Eger, where it is bent into a small syncline and faulted on the north side against Zingula Flags. A sequence exactly similar to that at Porth Gain is found on the north side of Aberfelin. The exposures at the lat~er locality are, however, not so readily accessible, since a small thrust, which brings the Porth Gain Beds and the basal members of the suc- Ceeding Tetragraptus Shales over higher horizons of Tetragraptus Shales, has given rise to a chasm that can only be crossed at low 9ide (see P1. XXIII, fig. 3). The thrust-plane is inclined at about 25 ~ south-eastwards. From the hard sandy slates below the grit at Aberfelin some badly-preserved Orthids were obtained. The sh-ip to which these outcrops at Pen Porth Eger, Porth Gain, and Abeffelin belong, comes to an end immediately north of 'Trevine, but the grit and its associated beds are well exposed on the eastern side of Pwll Olva, where the beds form the southern limb of an anticline: Here, again, there is a complete transition into Tetragraftus Shales above, and once more the lowest beds

1 Throughout this paper the numbers in square brackets refer to slides in ~ny ' Pembrokeshlre ' collection. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

286 ])a. A. H. COX ON THE GEOLOOY OF THE DISTRIC~ [voL of the Port& Gain Series are cut off by a fault which brings on Idngula Flags. The IMngula Flags are here very similar litholo- gieally to the PorCh Gain Beds immediately in contact ~dth them, and the fault is rather obscure. The remainder of the section is exactly similar to that at Porth Gain, save that the transition-beds between, the grit and the TetrayraTtus Shales are rather thicker, while the grit itself is correspondingly thinner. It would appear, then, that the arenaceous material was more evenly distributed in the east than in the west. The other limb of the Pwll-Olva Anticline appears in Pwll I~long, but on this northern side of the fold the junction of the Porth Gain Beds with the Tetrayraptus Shales is disturbed by faults, each with a northward downthrow (normal faults), and so the coarse grits are cut out. The hard sandy slates and mudstones present here a very striped appearance, the stripes being in places continuous, elsewhere discontinuous and lenticular in form. This striping is never so regular as in the Zn'ngula Flags, nor are there any of the thick ' ringers' which are so characteristic a feature of the latter formation. Where polished by tidal action convolutions of the stripes in these beds indicate very beautifully the enormous amount of compression and contortion to which the rocks have been subjected. Examples of Trinucleus sedgwicki Salter and Orthoeeras sp. were obtained from the striped beds, of which only a few feet axe preserved, the lower portion having been cut out by a fault which brings on the less arenaeeous shales constituting the lowest rt of the Porth Gain Series. In Pwll Llong these shales are not ag/Linst Zinyula Flags as in Pwll Olva, but they appear t~ be underlain by arenaceous.mudstones indistinguishable from the Abereastle Beds. The mudstones are not accessible from the shore, though they may be seen in small quarries on the cliff above t~wll Llong. The dip of the Porth Gain Beds in Pwll Llong is very variable in amount, but is always towards the Tetrayraptus Shales, though locally at ohly small angles. The faults which intersect the beds appear to be of little importance. On the south side of the Llanrian Syneline the Porth Gailt Beds occur in their proper position below the Tetragraptus Shales, but the beds are only well exposed around Llanveran Farm, west. of Pen Berry. Orthis Troava SMter, O. menaTiw Hicks, and Z)endrocrinus cambrensis Hicks, occur here in abundance. Hicks 1 recorded this locality as one of those from which he obtained ' Tremadoc ' fossils.

Relation of the Porth Gain Beds to the Abercastle B eds.--So far as the principal outcrops of the Porth Gain Beds are concerned, it has been shown that on the one side these beds pass conformably upwards into the TetragraTtus Shales, while on ..... 1 Q. 3. G. S. vol. xxlx (1873).p. 41. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

p art.2~ BETWt~,EI~ABEREIDDY AND &BERCASTLE.- 287" the other side they are everywhere faulted against Zingula Flags, with the single exception of the outcrop in Pwll Llong. Thus these outcrops yield little or no information as to the characters of the s~rata which should immediately underlie the Porth Gain Beds. Around Abercastle, however, the Abercastle Beds (that is, the sandy mudsfones with Ogygia selwyni) are succeededby various strata .which correspond lithologically almost exactly with the Porth Gain Beds as developed at the type-localities, These beds around Abercastle will now be described. It has already been remarked that the Abercastle Beds occupy the core of an anticline. On both sides of this anticline the Aber- castle Beds are succeeded first by shales or slates, but slightly arenaceous ; these in turn are followed by more arenaceous beds,- which are probably at about the same stratigraphical horizon as the Orthis-bearing mudstones of Porth Gain. Sections on the south side of the anticline.--The best section is perhaps that exposed in the road-cutting leading from Abercastle towards Mathry. Starting from the ford at Abercastle this road runs for a short distance in a south-easterly direction. Opposite the ford is a small disused quarry opened up in the Abercastle Beds. Immediately south of this point the mudstones are followed by a small thickness (about 30 feet) of dark-blue slates, very slightly arenace~ and exactly comparable with the lowest Arenig rocks at Porth Gain Jetty. The junction between the Abercastle Beds and these blue slates is well exposed, and close inspection shows that it is puckered up into a number of small corrugations. The general dip of this junction and of the axes of the puckers ~s north-westward at a high angle, whence one may conclude that there is a slight amount of overfolding. Succeeding these slightly arenaceous shales, but separated from them by a fault, are certain very sandy shales~ lithologically similar to those which immediately underlie the coarse grit at Porth Gain and elsewhere. The fault, which hades 60 ~ northwards, is seen in ~he bank near a small shed 9above the road. These very arenaceous shales exhibit, as a rule, a fairly regular: striping, but are sometimes (as, for example, by the shed mentioned above) indistinguishable from the Abercastle mudstenes.. Less than 60 feet of these beds are. present, the. remainder being: cut out, partly by the fault below (seen near the shed) and partly~ by a fault above (not seen), which brings on the slates of norms!: 7'etragraptus-Shale type. These Tetragraptus Shales are followed: i n their turn by a kcratophyre-flow (see below, p. 307).:

Sections on the north side of the anticline.,-A very: similar succession is seen on the north side of the Abereastle Anticline, where the various beds are exposed along the shores a~ the head of Abereastle Harbour (see P1. XXVI, figs, 3 & 4). :Now, however, partlyas a result of minor strike-faulting and partly, because of the lower inclination of the northern flank of the fold, : the width of outcrop of the various beds is much gr.eater than in" Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

288 ~)~.4. ~. cox on- THr. Gr.OLOGY dr Tn~. DIaTmcT [vol. bad, the southern limb. The beds also are in a dif[erent stage of weathering, and the cleavage now makes a large angle with the bedding, instead of being nearly parallel to it, as in the inverted southern limb. The result is that the rocks at the head of Aber- castle Harbour may, in places, appear ~omewhat different from those just described as occurring along the Abereastle-Mathry road. On flae west side of the harbour are seen the shales which overlie the Abereastle Mudstones. The exact junction of the two series is not well exposed, but exposures on the hill.~ide show that there is some minor faulting at this point. The shales or slates, when fresh, are, as usual, very similar to Tetragraptus Shales, but they are slightly arenaeeous; as seen near the boathouse, they have a very papery .type of weathering, and become then almost greyish white. Followed seawards, that is, up the succession, the arenaeeous character of these shales becomes more pronounced; but a fault intervenes, and brings on what are almdst certainly normal Tetrayraptus Shales. On the east side of the harbour the lowest slates are covered by alluvium, but the highly arenaeeomq shales which should succeed these slates, are well exposed. The beds are much striped, and become more arenaceous above. They then resemble the eolwesponding beds on the Math~ road and the sandy slates near PorCh Gain Jettyy. The upper portion of the arenaceous shales is cut out by the fault which brings on Tetrayrajatus Shales. This fault, in its course across the harbour from east to west, cuts out more and more of these striped beds, so that only a few feet of them are seen on the western side of the harbour. These various beds and faults appear on the coast at Aber Yw, where there is, in addition, some isoclinal folding. The axes of the folds dip, as usual, northwards. The cliffs here are rather inaccessible.

Thus, on both sides of the Abercastle Anticline, the junctions of the arenaeeous series with the Tetragraptus Shales are faulted, with the result that there is no indication of the occurrence of the coarse grit, so characteristic a feature at the top of the PorCh Gain Group at Porch Gain, Pwll Olva, etc. The only fossils that have been obtained from these beds are some Lingulcv from the striped arenaeeous shales on the west side of Abereastle Harbour, and an Orthoceras from the corresponding beds on the Mathry road. In the absence, therefore, of the characteristic fossil, Orthis proava, and also of the most distinctive stratum (the eoarse grit), there is only a strong probability, but not an absolute certainty, that these sandy beds above the Abereastle Mudstenes are the .equivalents of the PorCh Gain Beds. For the same reason, there rs a little uneertainty as to the proper position of the Abereastle sandy mudstones containing Oyyyia selwyni. There can be no doubt, however, that these mudstones belong to a slightly lower horizon in the Arenig Series than do the Orthis-yielding mudstones Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

pa~ 2] BETWEE~ ABEREIDDY AND ~BERCASTLE. 289

Of Perth Gain. Accordingly, although there is li~le doubt that the Abercastle Beds and the Perth Gain Beds belong to a single arenaceous phase of the lower par~ of the Arenig Series, it seems referable to introduce ~wo local names, rather than a single one, r the s~ta deposited during this phase. This course is the more necessary, since Orthis Troava is the commonest fossil in the higher part of the arenaceous series, while in the lower par~ Og~gia selwyni is characteristic. append a list of the fossils obtained from the Abercastle and Porth Gain Beds, the occurrences being denoted by a and b respectively :~ Orthis Troava Salter. b. T~inuc~eus sextgwlcki Salter. a, b. Orthis menapi~ Hicks. b. Ogyg~a se~wyni Salter. a. Lingu~a sp. b. Dendrocrinus cambrensis Hicks. b. Orthoceras sp. a, b. Ca~ograptus sp. b.

The Faulted Relationship of the Perth Gain Beds to the Lingula Flags. Returning now to the main outcrops of the Perth Gain Beds, I would point out that the fault which bounds them on the north side and brings on Lingula Flags, is noteworthy in several respects. In the first place, the rocks on each side of this fault are often very similar lithologically. This is especially the case with the less arenaceous bands of the Perth Gain Series which closely resemble the argillaceous bands in the I, ingula Flags, so that in those places where the fault-plane is not still open, its position can onl~r be found by noting where the felspathie quartzites, or 'ringers,' of the ~ingula Flags suddenly cease. This is the case, for example, in the tramway-cutting near PorCh Gain, on the west side of Perth Gain Harbour, and again on the east side of Pwll Olva. In such cases it might easily be imagined that there is a conformable sequence from the ~Lingula Flags up into the Perth Gain Beds, especially as the fault often keeps very closely parallel to the strike for considerable distances. This, indeed, is one oi the most remarkable features of the fault. But, after thus remaining strictly parallel with the strike over long stretches, the fault often changes its course with exceeding abruptness and swings away from the strike: there is one such deviation near the large diabase-mass of Pen Clegyr. Again, in its course of a few yards across Perth Gain Harbour, the fault must also change its direction very suddenly. Fortunately, although the fault-plane is sometimes closed in such a way that its detection may not be easy, the fault gives rise, as a general rule, to a marked gash in the cliffs. Along the gashes large caves have been excavated in several places, as for example, in Pen Porth Eger. Again, the islet of u Fach is tunnelled from end to end by a cave following the line of fault. It is interesting to notice how rapidly variations take place in the degree of development of the fault-gashes. On the east side of Porth Gain Harbour, for instance, there is a marked chasm Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

~90 ~R. A. H. COX O!~ THE GEOLOGY OF THE.DISTRICT [voL lx~,

along the fault-plane; while, only a few yards away aeross the harbour, the fault-plane is not seen without careful searching. Similar appearances are noted at numerous other points.

The F~quivalents of the Abereastle Beds and Porth Gain Beds in other Areas. The OrtMs-~elding beds at Llanvemn near Penberry were recognized by Hicks 1 as belonging to the same group as the beds on Ramsey Island, d~signated by him as 'Tremadoc,' and there .can be no doubt that, as regards both lithology and fauna, the Abercastle and Porth Gain Beds are also the equivalents of the ' Tremadoc' Beds of Ramsev Island. It has recently been shown by Mr. Pringle : that the Ramsey-lsland beds contain an Arenig fauna, also that the relationship of the beds to the YLingula Flags is a faulted one not, as was supposed by Hicks, a case of conformable passage. It has already been shown that, on the mainland, the Porth Gain Beds are also frequently fault~'d against f_Jnyula Flags on .one side, whereas, on the other side, there is .a complete upward passage into typical Tetrayraptu8 Shales. Further, it will be noticed that there is a very close resemblance in fauna and in lithological characters between the Abercastle and Porth Gain Beds and the lowest Arenig be&~ of the Whitland ,(Carmarthenshire) district described by Mr. D. C. Evans. 3 (2) The Tetrayraptus Shales (--Lower & Middle Arenig, Hicks, 1875).--These beds eompl~se a series of blue-black cleaved mudstones and slates, with occasional thin bands of ash or of ashy shales, the ashy beds being most prominent towards the top of the series. The mudstones and slates succeed the more arenaceous beds of the Porth Gain Series without any break, and, over the greater pal~ of the area, pass with scarceh~ any perceptible lithological change into the DiJy~wyraptus-b~Jus Shales above. The dark slates on weathering show the chocolate staining and frequently also the concretion-like banding that is so characteristic of the Tetragraptus Shales in other areas. ~ In the lower part of the series small, dark, phosphatic concretions are extremely plentiful. 5 They are usually rather oval in shape (presumably due to distortion by cleavage) with an average length of about half an inch. Similar concretions 2 or 3 inches in diameter may occasionally be found. These concretions may sometimes be seen to have fommd round a trilobite : less commonly, as at Pwll Llong, i Q~ j. G. S. vol. xxix (1873) p. 41. 2 Geol. Mag. dec. 5, vol. Viii (1911) p. 556. 3 Q. j. G. S. vol. lxii (1906) p. 603. D. C. Evans, ' On the Ordovichm Rocks of Western Carn~rthenshire' 9Q. J. G. S. voL lxii (1906) p. 603 ; see also T. C. Cantrill & H. H. Thomas, "Geology of the Country around Carmarthen ' Mere. Geol. Surv. 1909. s A~Llyses made by Mr. E. W. Scofield showed these concretions to contain an average percentage of 4"7 P.~O, For estimations of the phosphoric acid content of certain Cambrian rocks, see H. Hicks, Q. J. G. S. vol. xxxi (1875) ;:~. 368. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

part 2] BETWEENABEREIDDY AND ABERCAST~E:

larger concretions of chalybite, which attain diameters of 6 inches, may be obtained. These ferruginous concretions contrast markedly with the phosphatic concretions, in that they are more sharply differentiated from the surrounding slate. The aspect of the rocks, especially when unweathered, often varies 8lightly from the normal, owing partly to the variable relations between cleavage and bedding, and partly to the appearance of a well-marked but rather irregular rippling, developed in the neighbourhood of faults. These variations are especially noticeable in the upper portion of the TetragraTtus Shales as exposed towards the seaward end of Abereastle Harbour, where, apart from .a slight amount of rolling, the strata lie almost horizontally. In such cases where, despite the cleavage, the bedding becomes clearly apparent, the slates may take on a peculiar satiny lustre. Again, the coloration of the rocks may approximate rather to leaden grey than to dark blue: as, for example, in the slates near Dwyrain Sound at the entrance to Abercastle Harbour. 9The beds are everywhere strongly cleaved, so that fossils are difficult to obtain; and, owing to the very numerous large and small strike-faults, it is almost impossible to map with any certainty in this district the various zones which have been elsewhere 9establi shed. The localities at which the transition from the Porth Gain Beds into the Tetrayraptus Beds may be seen, namely, Porth Gain, Aberfelin, and Pwll Olva, have already been mentioned. At all these places the coarse grit which marks the top of the Porth Gain Beds is succeeded on the south by hard arenaceous shales, which soon pass upwards into the normal, well-cleaved, dark-blue slates characteristic of the Tetragraptus Beds. At other localities the junction between the two sets of beds is either inaccessible, as at Pen Porth Eger and Trwyn Ellen, or, as at Pwll Llong and Aber- castle, it is obscured by'faulting. The only fossils obtained from the lowest beds of the group are various trilobites, including Trinucleus ef. sedgwicki (from the slates immediately above the Porth Gain Beds at Aberfelin), and large specimens of Offygia selwyni Salter (from similar slates at Porth Gain). Tha best fossils from the higher beds were found in some slipped material in Pwll Olva, which yielded numerous specimens of 3)~dynwyraptus nitidus Hall and D. cf. extensus. The beds from which this material was derived are separated by a small fault from the slates immediately overlying the Porth Gain Beds. Slates near the south-eastern corner of Pwll Whiting yield Didymo- yraTtus cf. sparsus, while badly-preserved extensiform graptolites have also been obtained in Aberfelin from slates between the brook and the great diabase-sill. The many-branched graptolites, which are so abundant at one holSzon in Whitesand Bay, have not been obtained anywhere in the country around Trevine and Abercastle. These Tetragraptus Shales cecupy a very considerable portion Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

292 DR. x. H. COX O~~ THE GEOLOOY OF T~ msTmcl' [vol. lxxi, of the area. On the south side of the Llanrlan Syneline they crop out south of Llanrian Farm, and extend thence eastwards past Croes Goch, and westwards to Whitesand Bay and St. David's Head, as shown on Hicks's map) Around Porth-y- Dwfr, near St. David's Head, and at other points, the shales and mudstones have been considerably altered by large diabase intrusions, so that they now appear as hard, often flaggy, rocks, sometimes with little h-ace of cleavage. On the north side of the Llanrian Syncline the ouf~rol> of the Tetragraptus Shales forms a continuous band, extending from the north sxde of Traeth Llyf-n eastwards, through Porth Gain (where they have been quarried for slates),~ and past Trevine. North and east of the last-named village the beds are repeated by folding and faulting, so that they occupy most of the ground between Trevine, Longhouse, Abereastle, and Rhos-lanog. In the neighbourhood of the last-mentioned locality there may be small infolds of Didymograptus-bifldus Beds. North of the Abercastle Anticline the Tetragraptu8 Shales form a band extending from Pwll Wlliting, across Aber- castle Harbour, and on to Pwll Strodyr. No fossils have beer~ obtained from Pwll Strodyr, or from the slates along Abercas~le Harbour, except some fragments of trilobites (including Aynostus sp.). It was at one time thought that part of the slates occurring on each side of I)wyrain Sound belonged to some separate and distinct group, as they sometimes have a colour very. slightly different from the normal Tetragraptus Shales. Associated beds, also, have a peculiar satiny lustre, really due to the relations between bedding and cleavage, as previously mentioned. After slight weathering, however, these slates are indistinguishable from normal Tetragraptus Shales; and, since lh'dymograptus el. sparsus has been obtained from beds above Pwll Whiting, which appear to be at about the same horizon as the slates bordering Dwyrain Sound, there seems now no room for doubt that all the slates north of Abercastle really belong to the Tetragraptus Beds. It is not easy to arrive at any corTect estimate of the thickness of the Tetragraptus Shales in this area, owing to the numerous strike-faults which intersect the district. Judging, however, from the width of the outcrop along the north side of the Llanrian Syncline, there are only between 700 and 1000 feet of Tetragraptus Shales present. Contemporaneous igneous rocks occur in the Abercastle district at a high horizon in the Tetragraptus Shales. These rocks, kera- tophyric in composition, have been described as' lime-bostonites' by Dr. Elsden,a who regards them as true intrusive rocks. But, although frequently intrusive in mode of occurrence, they appear 1 Q. j. G. S. voL ~x~ (1875) pl. viii. 2 Since refuse from neighbouring quarries has been tipped into the large slate-quarry at Porth Gain, no reliance can be placed on fossils that have not. been found in situ in ~his particular quarry. 3 Q. j. G. S. vol. lxi (1905) p. 594. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

:part 2~ BETWEEI~ iBEREIDDY AND ABERC&STLE~ 293 tx) be almost contemporaneous in age with the beds in which they are found. The detailed evidence upon which a contemporaneous age is assigned te these rocks is adduced in a later part of this ~aper. (C) The Llanvirn Series. (1) Lower Llanvirn. -- The Didymoqraptus-bijeidus Beds (=Upper Arenig, Hicks, 1875).--The D.-bifldus Zone consists of a series of strata in which gritty, ashy, and flaggy beds predominate in the lower portion, while the upper portion consists almost entirely of bluish slates and mudstenes. The development of the beds in the area occupied by the Llanrian Syneline is rather different from that found north of the Abercastle Anticline: -accordingly, it will be best to consider the two areas separately. (a) The D.-bifidus Beds in the Llanrian Syncline.-- In the area occupied by this syncline the D..bifidus Beds succeed the Tetragraptus Shales without any break, and in fact without any very perceptible lithological change. In the absence of fossils, therefore, it becomes almost impossible to draw any exact boundary-line between the two groups. In general, the J~ifidus slates are darker than those of the TetragraTtus Group, and they .are frequently more like mudstones in appearance. The shales are often highly pyritous: they weather first light blue, with numerous red partings, and eventually tea silvery-grey. Development of a very pronounced cleavage results in a slight lightening of the colour of the rocks, which changes from almost black in the less-cleaved mudstones to a dark blue in the well-cleaved slates. The ashy beds are mostly blue, ashy, and flaggy slates, which weather very red, and o~en closely resemble slates that have been spored and rendered ilaggy by contact-action. They are, :further, very similar to some of the ashy bands near the top of the TetragraTtus Group. In the area occupied by the Llanrian Syncline the D.-bifldus Beds crop out in three distinct bands--two of the bands belonging to the faulted southern limb of the fold, and one to the northern Ymab. The southernmost band (a), which contains the full thickness of beds, is about 440 yams wide, and extends through Llanvirn .and Trevaccoon towards Pen-lan. The outcrop of the next band (l~), which extends from Trwyn Castell, through Bal~y Island, with 9a small detached portion around Llanriari, is much narrower, since only the upper portion of the D.-bifidus Series is here present. This band is nevertheless important, as it contains the strata thought by Hicks ~ to represent the ' Upper Llandeilo.' The third outcrop (~) forms a band on the north side of the syncline, one half fx) a third of a mile broad, stretching from Traeth Llyfn, past Henllys, to near Trevine. In this third band the full thickness of$he D.-bifidus Beds is usually present. In the area south

' Q. J. G. S. vol. xxxi (1875) p. i78. ~Q. J. G. S. No. 282. X Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

294 ~)R.A.H. COX 01( THE GEOLOGY OF THE DISTRICT [VOLl~i~ d Trevlne the three bands are apparently brought into juxta- position by faulting. It will be convenient to consider the strata in each of the three outcrops separately. In each case we shall see that the highest beds of the zone are the most fossil~ferous.

(a) The best section through.the southernmost band is the Llanvirn section described by ,Kicks~:a~ hter by Miss Elles. The diabase sill extending under Llan ".vij~=y-fran Farm appears to be intruded approximately along the junction of the Tetra#raptus Shales with the D.-bifldus Beds. The lowest beds of the D.-bifidus Zone are grey flaggy ashes exposed along the roadside south of the farm. Succeeding these ashy beds come the more typical 2).-b/fldus slates, which'are exposed in the old quarries lower down the hill, and from this locality Hicks 1 and Miss Elles~ obtained the numerous fossils recorded in their lists. I have found here

l)id~t,,s bi.f~dus (Hall). PU~ ~ Hioh. Mymo~tus nanus I~pworth. P~ z~d~ Hi~. /)/~/~raptue d~a~ (Brongnian). Ca~ymene sp. 6~maeoa~ph,, co,~fert~ Lapworth. A little below this quarry is a ' shatter-heir' which possibly represents the band coloured as 'felspathic trap:~.'on Hicks's map. ~ I could find no trace of 'trap,' either here, or'at~any of the other localities (as, for instance, Abet Pwll and Portli Tre-wen), where it should occur according to that map. Still lower down the hill, the (stratigrapbically) highest bed~ of the zone are seen underlying the Abereiddy Ash and exposed ia a small quarry west of the road. These uppermost beds are very dark in colour and highly fossiliferous, yielding wherever exposed abundant examples of D. bi./idus (Hall), D. stabilis Elles & Wood, 1). artus Elles & Wood, and Climaeograptus sp. The thickness- of the D.-bifidus Beds in the Llanvirn section is between 800 and 1000 feet. It is impossible to give more exact figures, since there is a certain amount of minor faulting involving considerable changes in the dip )o~: the beds, as is well seen i~ Aber PwU. 4 " The D..bifldus Beds of the Llanvirn outcrop appear on the- coast be~-ween Porth-y-Raw 4 and Aber Dinas 4; but in the coast- section only the highest beds, quarried near Abet Dinas, am readily accessible.

(~) The Trwyn Castell-Barry Island and Llanria~ outcrops.--Along the entire southern sides of these outcrops

i Q. j. G. S. voL x~i (1875) p. 177. 2 GooLMag. dee. 5, vol. i (1904) p. 208 ; see also J. F. N. ~ Prec. GeoL Assoc. vol. xxil (1911) p. 227. s Q. j. G. S. vol. YT~i (1875) pl. viii. These points all lie beyond the western boundary of the map (P1. XXV)~ Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

part 2] B~.TWEEZ~AB]~R~rm)Y X_~ AB]~RCASTT.]~.... 295

the D.-bifldus Beds have been faulted against ])icranograTt.us Shales, for which reason the lower beds of the D.-bifidus Zone are not seen. The faulting was not observed by Hicks, 1 who assumed that the succession was normal, and accordingly referred the Bifldus Beds to the Upper Llandeflo. The fault presents certain peculiar features which wilt be described in detail later (p. 306). The best section is seen in the large quarry on the north side 9of Abereiddy Bay. The lowest-exposed beds of the D.-bifidus Zone, lying immediately north of the Castell Limestone and of the fault, consist of a few feet of slates, full of phosphatic concretions and having a very rusty and papery type of weathering. It is these beds that yield the numerous trilobites mentioned by Hicks (o2. cir. p. 179). I have obtained from them Ogygia buchl (Brongaiart). Canulariael. homfrayl (Salter). Ca~ymene sp. Bd~erophon perturbatus (J. do C. Barrandla sp. Sowerby). Orthoceras sp. Monobob~na p~umbea (SaIter). Succeeding these' trilobite-beds'is a considerable thickness of somewhat iiaggy slates or mudstones,, sometimes slightly striped and of a lighter blue than the beds below. These slates, which form the eastern wail of the quarry, have only yielded l~rono- bolina plu~2bea. They pass upwards into highly-fossfliferous dark blocky mudstones, which form (in part) the northern wall of the quarry, and are splendidly exposed in the cutting leading to Trwyn Caste]]. These fossiliferous mudstones are overlain by the Llanrian Volcanic Series, and themselves contain various small, with a few large, ash-bands ; the largest=of these ash-bands (occurring a little above the keratophyre sill) forms a prominent feature in the lower part of Trwyn Castell, as viewed from Aber- eiddy Bay. The dip varies between 50 ~ and 60~ to a little east of north, and the cleavage makes a small angle with the bedding. The fossils obtained from the blocky mudstones include D~dymograptus bifat~s (Hall). Didymograptusstabib~'Elles &Wood. Didymograptu8 murchisoni vat. Didymograptu~artus Elles & Wood. geminu~ (Hisinger). O~ossograptus of. hinc~i Hopkinson. Didymograpt~ ~tanus Lapworth. Ogygia buchi (BrongnJart). The various species of JDidy~nograptus are extraordinarily abundant; but it is noteworthy that Dilologral0tids, which are so abundant in the highest beds of the D.-bifldus Zone on the southern (Llanvirn) side of the bay, are relatively scarce on this northern side. This factlis of some significance with respect to the exact age and correlation of the t~vo volcanic bands~the Murchisoni Ash and the Llanrian Volcanic Series. The advent of the Llanrian volcanic conditions did not immediately affect the D.'bifldus fauna, as the gral0tolites am still abundant in the shales above the lowest of the Llanrian lavas. This agrees very closely

Q. J. G. S: voL xxxi (1875) p. 178. X2 Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

296: DR. A, lI. COX ON THE GEOLOGY OF THE DISTRICT [VOI.]rr~ with observations made among the Hope Shales and Stapeley Ashes of Shrepehire. 1 All the beds, from the fossiliferous mudstones down to the ' trilobite-beds,' are well exposed, and evidently form a continuous series about 250 feet thick. There is nowhere evidence of any important sh'ike-faulting, although a few minor dip-faults may be seen. The dip is also fairly uniform throughout, 55 ~ to 60 ~ northwards, though it is tree that uniformity.of dip is without much significance in this district. The whole series of beds must, however, belong to the top half of the Hifldus Zone, so that the list of trilobite species given by Hicks stands in need of revision. The fossiliferous mudstones are again seen to dip under the Llan- rian Volcanic Series along the drive leading to Barry-Island Farm, and they are further exposed in r separate outcrop which extends through Llanrian village. At 1}he last-mentioned exposure the fossils cbllected included Didymo.qraTtus bifidus (Hall), ~). ~nurchisoni, var. geminus (Hisinger)i Hellerogahon sp, and various trilobites. The exposures in Llanrian do not, however, offer so favourable a collecting-ground, since the slates are in a very weathel~d papery condition.

(7) The Didymograptus-bifidus Beds in the northern limb of the Llanrian Syncline: the Traeth Llyfn- Trevine outerop.--In the northern limb of the fold the whole of the beds of the D.-bifldus Zone are present. The strata are everywhere vertical, or nearly so; but the width of the outcrop varies, owing to the effects of faulting. The average width is about a third of a mile. The lowest beds of the zone and the upward passage from the Tetrayraptus Beds below is much better seen in this northern limb of the fold, as the beds appear in coast-sections at Aberfelin and elsewhere. In Aberfelin the junction of the D.-bi~dus Beds with the Tetrayraptus Beds seems to lie near a small diabase-sill (not marker on the map). There may be, however, a slight amount of faulting here. Slates between the large diabase-sill a~d the brook yielded fragments of extensiform graptolitcs ; while the dark-blue mudstones in the isouth-eastern corner of the bay yielded Didymoyragotus cf. bijfidus (Hall), D. stabilis Elles & Wood, and Hellerophon sp. Immediately following on these fossiliferous deposits come flaggy ashes, exposed in the read above the cliff, and exactly similar to the beds exposed in the higher (geographically) of the two Llanvirn quarries. A comparison of the Llanvirn and Aberfelin sections makes it clear that there is a larger development of ashy beds in the southern limb of the fold than there is in the northern limb. There appears also to be a complete transition from Tetra- yral~tus Beds to Hi fldus Beds in Traeth Llyfn, so much so that, I C. Lapworth & W. W. Watts, ' The Geology of South Shropshire' Proc. Geol. Assoc. vol. xiii (1894) p. 317. 9.... Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

par~,2] IBETWEE~ A~BEREIDDY AND A_BF_,ROKSTI,Eo . 29~7 owing to the scareit?r of fossils and the similarity of the llthology, it is difficult to draw any definite line of demarcation between the zones. The difficulty of delimiting the extent of the two zones is ~ceentuated by the disappearance westwards of the ashy band of Abe~elin and Pwll Crochan. The mudstones near the path leading into Traeth Llyfn yielded ~ylina sp., ~lacoTaria cambrensis Hicks, Dil~lograptus sp., and various other obscure graptolites: they appear to be near the base of the D..bifidus Series. Slates, nearer the southern wall of the bay, yielded JDidymograptus cf. nanus Lapwor~h, 1). qnurchisoni var. yeminus (Hisinger), and Diplograptus sp. An ash-band in these ~ifidus Slates shows very clearly the shearing effects of the cleavage- stresses, and has in addition a peculiar nodular sh~chlre. The ashy lnaterial in the nodules is partly replaced by carbonates. The dip of this ash-band is 80~ southwards, while the cleavage- planes are inclined at about 75 ~ northwards. The highest sla~es of the zone and the beginning of the Llar~- rian Volcanic Series in the northern limb of the syncline can be seen in the yard at Henllys Farm, where ashy slates immediately underlying the massive igneous rocks yielded numerous examples of DidymograTtus stabilis Elles & Wood.

(b) The D.-bifidus Beds of the Abercastle Anticline. ---On the northern side of this anticline D.-bifldus Beds occupy all the ground between Castell Coeh and Aberhenow. Over this area the S~Ta~a have been thrown into a shallow, but much faulted, syncline, with the result that the outcrops are repeated more than once (see P1. XXVI, figs. 2 & 3). A small transverse fault also repeats *~he beds on Ynys Castell. Throughout this northernmost outcrop only the lower beds of the D..bifidus Zone have escaped erosion by the sea, and their development is rather different from tha~ of the corresponding beds in the Llanrian Syncline. The difference is mainly due to the development at, or very near, the base of the D..bificlus Beds of a group of massive ashes with a conglomeratic basal grit. It will be convenient to describe these beds under the term 'Ynys-Castell Ashes,' so as t~ distinguish them from the various other ash-bands of the district.

Ynys-Castell Ashes: (i) The basal grit.--The lowest bed of this group is the grit, or breccia, well-exposed at the southern end of Ynys CasteU, where it overlies Tetragraptu8 Shales. The junction of ash and slates is very well shown, and, although quite shal]~, it is evidently a natmul junction and not due to faulting. The junction dips very gently north-eas#wards at the same angle as the lamination in the grit itself and in the silicified ashes above, so thati the grit evidently represents a true basement- bed to the ashy series. On Ynys Castell the erosive action of the sea-waves brings out very clearly the grlt~y conglomeratic nature of the bed, which on weathering is seen to be distinctly laminated. The constituent fragments vary considerably in size, from small Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

298 DR. A.m cox o~ THE e~.O~OeY O~ TmB DraT~tC~ [voL Ixxi, sand-groins up to blocks 18 inches or more long. The composition of these grains and fragments, which are embedded in an abundant argillaceous matrix, is as diverse as their size, and will be the subject of further mention. It may be stated, however, that the biggest pebbles are of hard felspathic mudst0nes, greenish-grey in colour, and Possibly deiSved from Cambrian rocks, since they are quite unlike any of the Ordovician strata in this area. When but partly weathered the grit resembles a red hrff, and when quite unweathered, its true nature (especiallyin the higher part) is not at all apparent, since it then appears as a hard, compact, bluish-green rock, with only a few reddish spots and streaks to indicate its true nature. The thickness of the markedly gritty beds is about 18 feet. Since this grit forms the basement-bed of a series of fine-grained ashes, and is only of very local extent, it is Possibly volcanic in origin. An exami~a~On under the microscope of a specimen from the lower part "of the bed [P 76] shows that the grains consist of more or less augatar quartz-fragments, along with felspars of various kinds. In additio~ there are composite fragments and pebbles of a grest variety of rooks, inoluding 9dark-blue shales or mudstones, impure felspathio grits or greywaokes of different types, fine-grained siliceous rooks (P cherts), cherty sandstones, and, less commonly, a trachytic "rook. Owing to pressure, the individual grains and pebbles are often dis~rted, and their boundaries against the abundant .shtty matrix may be consequently rendered obscure. In the higher parts -of the grit-bed [P 72] the matrix becomes less argillaceous but more mliee~us, and consis~ of an almost cryptoorystaliine aggregate of quartz and felspar, in which the distribution of a small amount of iron-ore serves '~o show up the lamination. The pebbles and composite fragments have :now almost disappeared, but numerous grains of quartz and of ~nr~]i-felspars, ,both orthoclmm and plagiocl~e, still remain. Round the larger felspars there :is fr~luaUtfly an c~growth of secondary felspar, extending in~ the ground- mass. These secondary outgrowths may be observed, both in the case of ipl~doclase and in that of orthoch~se. In this rock we have clearly a t~ausition from the coarse grit below to the silieified ashes above, a transition 'which may be seen in the field as well as under the microscope. The basal grit is found again in its proper position, at the base iof the ashes, on the mainland at the east side of Abercastle Harbour. Owing to the proximity of a small fault, however, the relations between the beds are not so clear as on the island. There is still another small outcrop exposed in an old quarry above the bay east of Perth Gwynnon-mawr ; but here, again, the relations of the grit to the neighbouring beds are not clear, Partly owing to the presence of a diabase sill, and Partly as a result of faulting. The whole coast-line west of this point is, in fact, determined by a fault.

(ii) The silieified ashes.--The basal grit passes up into a series of fine-grainedsilieined ashes, which are well expo~i on Ynys Castell and on the mainland opposite. The rocks have a aplintery conchoidal fmc~are and a semi-vitreous lustre. They are commonly bluish-green, weathering to a reddish-green, and often exhibit a development of thin white films on exposed surfaces, Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

l~ar~ 2] BETWEEN ABEREIDDY AND ABERCASTLE. 299

comparable with those seen in rhyolites. Many of the beds are quite massive, having a thickness of from 4 to 6 feet, while otl~er beds may be only a few inches thick. Some of these thinner beds resemble cherts, in consequence of the extreme sil[cification Eaat they have undergone ; while the thicker beds are often closely :similar to acid igneous rocks. On Ynys Castell the dip is very gently north-eastward, and in the thinner beds here a vertical cleavage is developed. The cleavage-planes, as is usual in this ~listrict, strike in an east-and-west direction, and, by crossing a well-marked system of jointing, cause the rocks of these thinner beds to split into blade-like splinters. Across the harbour, on the mainland, the dip is about 15 ~ towards the sea, and the cliffs are there honeycombed by a number of caves (see P1. XXIII, fig. 1). At both localities only the lowest beds of the Ynys-(~astell Ashes are seen, the higher bands being cut out by a normal fault which crosses Aberhenow and lets down .Bifldus mudstenes, here caped by a diabase sill, against the lowest ash-bands. The highest ashes, and their gradual passage into the overlying mud- Stones, may be studied in the north-western limb of the shallow syneline, where the beds appear in the northern and western cliffs of Castell Coch, and on the coast about a quarter of a mile north- west of Aberhenow. As seen at these points, the ashes are very similar to those of Ynys 0astell; but, towards the top of the series the beds become less massive, and at the same 6_me less siliceous and more argillaceous. The cleavage also becomes more pronounced. As a result, the beds change to hard, bluish-green, slaty rocks, in which the colour gradually darkens as the rocks pass into the overlying blue-black mudstones. The main change takes place within a few feet of sediment. In this upper portion of the Ynys-Castell Ashes, and to a less extent throughout the series, large ellipsoidal concretions are of ~requent occurrence. The concretions are sharply differentiated ~om the surrounding rocks, and they may attain diameters of 3 feet. Microscopic examination showed that the concretions con, ~sist of dolomite in small idionaorphic rhombs, all traces of olqginal itexture having been obliterated. The beds containing these concretions can be seen low down in the cliff east of Castell Coch, and ~also at the extreme north point of the coast opposite Ynys Daullyn. Since the southern limb of the small syneline shows only the lower portion of the ashes, while in the northern limb the upper part of the beds is alone exposed, one can but estimate the minimum ~hickness, which amounts to 200 feet, for the series. It is probable, however, that the total thickness is not very much greater. Microsoopical examination of specimens from the harder bands shows that ?dmse ashes vary between rather narrow limits. Specimensfrom the flinty 9rocks just above the basal grit present an appearance and composition very similar to that shown by specimens taken from the top part of the grit ~tself (see above, p. 298), except Eaat the larger clastic grains, such as were, ~present in ~ae grit, are quite wanting in the fine-grained ashes. The ground- mass [P 45 & 461 again consists of a micro- to cryptocrystalline mosaic of ~lkaliofel~pars and qua~z~ the former predominating. Lamination is shown Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

300 DR. ~.. t[. COX ON THE O~..OLOGY OF THE DISTRICT [vol. Lui,

by the distribution of a small amount of granular or stringy ob)oritio material, or in other cues by the pl~esnee of a small amount of limonite. In the fine-grained ground-mass are some very small but less fine-grained patches of quartz and of felspars, some of the crystals evidently being of detrital origin, but mainly appearing to be the result of recrystallization. Such rocks present an appearance almost identical with that of adinoles, although there can be no question in this case of any alteration by a basic intrusion. Other specimens from the ashes have a porcellanous rather than an almos~ vitreous appearance. These porceKanous rocks are seen, when examined under the microscope, to resemble very closely the rocks described above ; but they are [P 47 & 48] even finer~ed and more regularly ]~mln~ted, while granular argillaceous material, accompanied by faintly pleochroic ' sericite,' makes its appearance. In examples taken from the higher beds of the northern side of the syneline this argillaceous material has largely increased in amount, and the rocks consist [P 49 & 50] of little irregular patches of quartz and numerous tiny flakes of esricite involved in a paste almost too fine-grained to be resolved, but appearing to consist of both quartz and felspar. Some of these rocks present almost the normal appearance of a slate, except that they are lighter in colour and far more transparent than a slate, in consequence of the reerysballi~ation which they have undergone. Thus the Ynys-Castell Ashes show s transiticm from massive compact tufts, which may easily be mistaken for igneous rocks, to rocks that are almost normal clay-slates.

The mudstones overlying the Ynys-Castell Ashes.-- These beds consist of well-cleaved dark (almost greyish-black) mudstones, often highly pyritous. This series was referred to as the Castell-Coch Beds in an earlier publication. 1 These mudstones ocem" in regular beds 2 to 4 feet thick, with occasional laminated sandy intercalations, and they are lithologically very similar to the ~9/fld~s Mudstones of the Llanrian Syneline. The beds are well seen on Castell Coch, also in Aberhenow, where they have been folded into a small subsidiary anticline pitching at about 5 ~ in a south-south:westerly direction (see P1. XXIII, figs. 1 & 2). A diabase-sill caps the mudstones at the latter locality. :In all phces where these beds are seen the dips are fairly gentle, never exceeding 30 ~ and usually less, so that the cleavage crosses "the bedding almost at right angles. The rocks are well jointed, and where the strata lie almost horizontally, as at Aberhenow, quite vertical cliffs have been formed. In other places this prominent jointing may, at a distance, be easily mis~ken for bedding, The beds seen have a thickness of about 200 feet, but the top of the series is nowhere reached. :No fossils have been obtained from these mudstones, their apparently-unfossiliferous character being doubtless largely due ~ the very unfavourable relations that obtain between cleavage and bedding. Yet, while no fossils have been obtained from the mudstones above the Ynvs-Castell Ashes, and but scanty forms from the slates below, there is not much doubt that the ashes occupy an horizon at or very near the base of the D.-bi/-'idus Beds. Closely ,similar fine-grained ashes occur at this level in Ramsey Island,

1 A. It. Cox & O. T. Jones, ' The Geology of the District between Abereiddy & Pen Caer' Reu. RT.it Agaric,. (Birmingham) 1913, p. 484. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

par~ 2~ BXTWEV.>rAB~,~mDmr ~ ~nERCAST~..' 801 where their position has been accurately proved by Mr. Pringle 1 ; while it is noteworthy that almost identical beds are found at localities so far apar~ as the St. Clear's district in Carmarthen- shire ~ and Hope Dingle (Shropshire). 8 At none of these three localities, however, does there appear to be anything comparable with the conglomeratic basement-bed that is so prominent a feature on Ynys Castcll. 9Presumably, the Ynys-Castell Ashes of the Abercastle Anticline are represented in the Llanrian Syncline in Par~ by the lithologically quite different ashy slates of Llanvirn and Pwll Crochan, already described as occurring near the base of the J~ifldus Shales of that area. Thus, although ashes of Ynys-Castell type are found in numerous widely-separated districts, the beds themselves are only of quite local extent, and do not seem to be all on the same horizon. (2) The Llanvirnian volcanic rocks.--Volcanic rocks of Llanvirnian age are represented on the south-west side of Abereiddy Bay by the D.-mureMsoni Ash, and by the Llanrian lavas and associated pyroclastic rocks on the north-east side of the bay. The D.-~nurchisoni Ash marks exactly the top of the D.-bifidus Zone, whereas the eruption of the Llanrian volcanic rocks began at a slightly earlier period and before the close of the JD.-bifidus time, so that the two volcanic bands are only in Parb synchronous. . '(a) The D.-murehisoni Ash.--This follns a well-marked band, which can be followed almost continuously from a point near Llanon on the east, westwards through Trevaccoon and Porth Eiddy to Abereiddy Bay, whence it determines the coast:line for over a mile by way of Aber Pwll to Aber Dinas. The ash-beds follow qui~ suddenly on the dark fossiliferous D.-bifidus Shales ; but the junction with the 1).-murchisoni Shales above is hardly so well defined, as there is some alternation of ashy material with the lowest fossiliferous shales of that zone. At Abereid4y Bay the ash is about 150 feet thick. It is well bedded and cleaved, and its colour is usually greenish, mottled with whiter patches. Under the microscope it is seen to be largely composed of pumiceous lapilli, but there are also present shale-fragments measuring up to 3 inches in diameter. Some of the ashy bands which alternate with the lowest JD.-murehisoni Shales are of 9distinctly com~e material. Among the highest beds, seen in the j. Pringle & others, ' Final Repor~ on the Geology of Ramscy Island" Rep. Brit, Assoc. (Australia) 1914, p. 111 ; see also Geol. Mag. dec. 6, vol. i (1914) p. 522. 2 D. C. Evans, ' On t~he Ordovician Rocks of Western ~armarEaenshire' Q. J. G. S. vol. lxii (1906) p. 613 ; see also T. C. Cant.~ill & H. H. Thomas, ' The Geology of the Counta-y around Carmarthen' Mem. Geol. Surv. 1909, pp. 24 & 35. 3 C. Lapworth & W. W. Wa~s, ' The Geology of South Shropshire' Proc. Geol. Assoc. vol. xiii (1894) p. 317. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

~02 ])~.X.,H. 00X ON ~HE O~,OIJOOI O~ ~HI I)IfflIG'T [vol.hn~

quarry between Abereiddy and Porth Eiddy, are some that are much more bluish, and at the same time more massive, and present a less perfect cleavage than in the more typical strata of the band. It is possible that there are actually thin flows near Porth Eiddy, while about a mile and a half farther east there are certainly some felspathie lavas accompanying the ash-band.

(8) The Llanrian Volcanic Series.--This series crops out in three distinct bands--one between Llanrian and Bryn-hal, a second between Trwyn Castell and Barry Island, and the third south of Felindre and Henllys near Llanrian. The first two outcrops belong to the southern limb of the Llanrian Syneline, and in each ease the volcanic rocks are underlain on the south side by very fossiliferous shales apparently high up in the zone of J)idymoyraptus bifidus. The third outcrop belongs to the northern limb of the synellne, and is underlain on its north side by the JD.-bi.)qdus Shales of ~[ennys. The second and third outcrops are almost in line one with the other, and this apparently misled Hicks to conclude that there were two continuous and distinct bands occupying slightly different horizons. Actually, however, the s~rueture between Llanrian and Barry Island is extremely complicaf~:4 by faulting; and it is forCm~ate that fairly-numerous exposures serve to place the main facts beyond doubt. It is noteworthy that, where the outcrop of the Llanrian lavas approaches most closely to that of the JD.-murchisoni Ash, these lavas begin to appear in the ash-band. The Llanrian Volcanic Series is only about 400 to 500 feet thick. It consists largely of pyroclastic rocks, of which there is a great 9variety, along with a considerable number of rhyolite flows, and also some interstrafified bands of dark shale (often quite free from pyroclastic material, and yielding a .D.-bifldus fauna). There are also present some hard silicified sediments, with a consistency approaching that of cherts. The various rocks are best exposed on the headland of Trwyn Castell and around Cam Llwyd (see P1. XXII, fig. 2). There are also numerous small exposm'es between Llanrian village and Hen.llys, and along the road from Llanrian to Llanon, this road running for some distance along the strike of the beds. The lavas are found mainly in the lower part of the series. The 9individual flows are usually rather thin, and do not appear to have any. considerable lateral extension ; so that, although the volcanic series as a whole is well marked, and persists over a considerable distance, the sequence within the series is very variable as it is followed from point to point. The flows may be separated by more or less thin partings of tuff-like material, or they may rest directly one upon the other, giving rise to a considerable thickness of igneous rock, which appears fairly uniform in character. Some of these lavas are very dark--almost black--in eolour, owing to the abundance of iron-ores present. Such rocks weather very red, but may show also the white crust characteristic of Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

par~ 2] BETWEEN ABEREIDDY AND ABERCASTLE. 303 rhyolites. Of commoner occurrence, as, for example, round . Llanrian and among .the higher flows of Trwyn Castell, are g~eenish-wh!te rhyolites of very 'flinty' appearance. Fluxion- banding is not usually observable microscopically, but may be seen in some of the rocks on Cam Llwyd. The lavas are non- vesicular and non-porphyritic, while nodular, spherulitic, and perlitic s~ruc~ures are entirely absent. Xenoliths of slightly- hardened shale may be occasionally observed. The thinner flows may be more or less cleaved, in which case it becomes no easy task to distinguish such flows from some of the cleaved ruffs. Even when they are perfectly massive, the separa- r in the field of lavas from ruffs is o~en a maY~er of some difficulty; while microscopic examination often fails r yield definite evidence, in consequence of the silicification and recrystallization that the rocks have undergone. Further, shear-zones in the massive lavas may simulate tuff-partings be~-ween different flows. The pyroclastic rocks, which form so large a portion of the volcanic series, show considerable variation, owing partly to differences in the size of the constituent par~cles and partly to the frequent admixture with normal sedimentary material, l~ost of the tufts are very fine-grained, and (like the lavas) may be free from any porphyritic constituents, or they may contain a fair abundance of small, broken, porphyritic crystals. Both cleaved and uncleared examples of such rocks may be found. Towards the top of the Llanrian Series the rocks are entirely of pyroclastic origin, and include massive fine-grained bands closely :resembling felsitic lavas, along with coarse-grained agglomeratic beds, in which the fragments are often rounded and include in their number many mudstone pebbles, giving to the beds the appearance of a conglomerate. These higher beds form the southern wall of Traeth Llyfn, and, by decrease in the amount of pyroclastic material, they pass up into normal, dark, highly-cleaved and sheared slates, which form the most easily accessible par~ of this :southern wall, the more ashy beds being only accessible from the shore at very low tide. The upward succession appears to be broken by a great fault, which determines the position of the southern corner of the bay. This fault brings the slates immediately overlying the volcanic :series against lithologically similar slates, which really belong to a low horizon in the D.-bifidus Shales of the northern limb of the Llanrian Syncline. Thus the D.-murchisoni Shales, which might be expected to follow shortly on the volcanic rocks, are not seen in Traeth Lly~n. The same is the case at other localities, since everywhere the upper portion of the volcanic series is faulted, the posit~on of the faults being probably determined by the line of weakness between the resistant igneous rocks and the soft shales. 9Consequently, the upper limit of the volcanic series cannot be 9definitely fixed. That the presence of D.-bifidus Shales on each side of the vol- ~eanic rocks of Trwyn Castell and B~r .ry Island is actually due to Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

304 D~. A. H. COX O1( THE GEOLOGY OF TI~ DISTRICT [voL lxxi,

faulting, and not solely to folding, becomes clear from an examination of the succession in the volcanic series itself. There, as pointed. out above, lavas predominate in the lower part of the series, and agglomeratic i~s in the upper part; so there is evidently no symmeh~ in the arrangement, and consequently no mere folding- back of the volcanic band upon itself. (3) Upper Llanvirn.--The JOidymograptus-mureki- 9oni Shales (=Lower Llandeilo of Hicks, 1875).--These consist of a series of shales which succeed the D.-murchisoni Ash 9without any break, and pass up also without a break into the DicranograTtus ,Shales above. The D.-~nurehisoni Shales are of still darker eolour than the yD.-b~dus Shales, and of more flaggy appearance, with a very rusty papery mode of weathering. The quarry on the beach at the southern comer of Abereiddy Bay has been a classical coUecting-ground since the descriptions give~ by MacLauchlan, and later by Hicks. Swarms of graptolites may be obtained, including Didymogra~tus murchisoni (Beck). ] Cryptograptus t~ornis ( Carruthers)~ Didymogvaptus murchisoni, vax. ] C~imaoWraptussp. geminus (Hisinger). ] Dendrograptus sp. Orth~ & Wood.Calearatua'vat. ~scus ]

The Dendrograptids are most abundant in the lowest beds of th~ series. Apart from graptolites, other fossils, such as trilobites and brachiopeds, are exceedingly scarce, as is commonly the case i~ other areas where beds of similar type are found. A number of graptolites from this locality are figured in the Pal~eontographieaL Society's Monograph. The beds dip about 40 ~ northwards. The cleavage is practically parallel with the bedding, and the graptolites are often mucl~ distorted. Upwards the beds become softer and less flaggy, and pass gradually into the 1)icranoyraTtus Shales. The thickness of: the ~).-murchisoni Shales is only about 100 feet.

(D) The Dicranograptus Beds. (=Middle Llandeilo, Hicks, 1875.) (1) The 1)ieranograptus Shales.--The junction of these shales with the D.-murchisoni Shales below them is well exposed in the cliff towards the south side of Abereiddv Bay, and there is clearly a gradual passage between the t~vo sfets of beds, wit~ no indication of any separate representative of the Asaphus Ash and Asaphus Limes~ne of Carmarthenshire. 1 The JDicranograptus Shales are a series of black shales and mudstones, weathering to various shades of brown, red, and buff,

D. C. Evans, 'On the Ordovlcian Rocks of Western Carmarthenshire" Q. J. G. S. vol. lxil (1906) p. 624 ; see also T. C, Cantrfll & H. H. Thomas~ ' Geology of the Country around Carmarthen' Mere. Geol. Surv. 1909, p. 39. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

part 2] BETWEEN ~_BEREIDDY AND ~BERC&STLE. 805 exactly as do the similar beds of 0armarthenst~re. 1 When partly weathered, the mudstones present a very earthy appearance, and eventually become quitc soft and papery. Such beds occupy the entire cen~al par~ of Abereiddy Bay, passing up on the north side of the bay into the limestone. The outcrop of the shales is, however, divided into two parts by the alluvium-filled valley (mentioned on p. 387) which leads round to Porth Gain. This valley probably follows for a large part of its ~ourse the east-and-west repeating fault which separates the :Llanrian and Trevaecoon outcrops: of the Llanvirnian volcanic rocks. In this case there would also be more or less repetition among the 1)icranograptus Shales. The achlal thickness of shales exposed .is about 800 feet, the remainder of the sh~ta being buried under alluvium. Fossils arc not very abundant, as a rule. The lowest beds of the series yielded Orthograptus calcaratus, var. priscus Elles & Wood, Ogygia buchi (Brongniart), and fragments of Orthis. Higher up in the series the shales near some old lime-kilns are full of badly- preserved and very fragmentary graptolitcs, mostly forms of _DicranograTtus with also ?Zeptograptus. The very similar beds which immediatcly underlie the Castell Limestone yielded Trinucleus sp., Amplexograptus perexcavatus Lapw., and Belle- rophon sp. These shales are evidently the equivalents of the /-Iendre Shales of Carmarthenshire. The lithologieal similarity between the beds at Abereiddy Bay and the corresponding beds in Shropshire (the Rorrington Flags)~ is particularly notable; ~but the hard limestone-bands common in the lower part of the Rorrington Flags are absent from the Pembrokeshirc strata.' The proper position of these Dicranograptus Shales was already .assigned to them by Hicks, who compared them with the occur- fences at YM:usclewick Bay and Lampeter Velfrey, etc. 9 (2) The Castell Limestone.--This limestone, which suc- ,ceeds the Dicranograptus Shales, is exposed around the entrance to the large quarry on the north side of Abereiddy Bay. In places there is a little dishlrbance along ~he line of junction with the shales below. The limestone bands are rather variable in character, some being fine-grained, massive, and very uniform; others are more shaly in appearance, while others aguin" are very arenaceous, the latter type on weathering resembling a buff-coloured sand- ~stone. When unweathered, the limestones are of a dark grey- blue. The limestone includes bands of shale measuring as much as 3 feet in thickness. These shale-bands contain some calcareous partings, but otherwise are exactly similar to the Dicranograptus Shales below. The whole series is about 50 feet thick, the dip of ithe beds being on the shore about 50 ~ north-north-westwards, bu~ nearer 60 ~ at the quarry entrance. 1 D. C. Evans, op. cir. p. 629 ; T. (L Cans & H. H. Thomas, op. cir. p. 45. C. Lapwort~ & W~ W. Wa~ts, ' The Geology of SouCh Shropshire' Proc. ~eol. Assoc. vol. xfii (1894) p, 318...... - Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

~)6 DR. A. H. COX ON THE GEOLOGY OF THE DI6T~ICT [voL The calcareous bands ~eld a fair abundance of various forms, including T~ew ~loltdi Murchison or I L/nga2a. Tr. gol,dfu~i Barrande. , Hatysites. Plec~araboctitessoricea (Sowerby), var. I 8t~. and also some ill-preserved graptolitos ; while the shaly layers have yielded Tric~lmts. I Amptea~ra~tus ~vatus Iap. Bt~richia. I wort~ Or~h/s. D/c,d/og~a,ptm serif,aria (11:[~11) or D. di- Ol~tograptus ter~us~utus varicatus (Hall).

The graptolites fix the horizon of this limestone with fair certainty, as about the same as that of the Mydrim IAmestone of Carmarthenshire, with which it compares very closely in lithe- logical character. This limestone, with the underlying black shales, constitutes the ' Middle Llandeilo' of Hieka

The fault above the Castell Limestone.--In the quarry- section the limestone is followed by a few hard black shales, which have only yielded some traces of ,horny brachiopods. A large repeating fault then brings on the 'trilobite-beds' already described as belonging to the zone of Didymoyraptus b~fldus (p. 295). 9The fault is marked by a fault-breccia occurring in one main and several subsidiary bands, which, where exposed, keep quite parallel with the bedding-planes. The larger breccia-band is about 6 inches thick. It is a rather curious coincidence that the beds on eac.h side of the dislocation dip at almost exactly the same angle (about 60 ~ north-north-westwards), and that both sets of beds are crossed and about equally affected by a cleavage dipping about 40 ~ north-north-westwards, so that the importance of this fault is not at all clear until the fossils are collected. This is still more the case, since similar lii~le bands of breccia may be seen among the .Dieranoyraptus Shales in places where there has obviously been no sliding moment at all, but merely a little crushing. The fault was first observed by Mr. ft. F. N. Green, and the fossils proving the presence in the quarry of a large dislocation were discovered by Prof. 0. T. Jones. 1 The fault and the Significance of the graptolites were overlooked by Hicks, ~ who accordingly assumed a normal sequence, and assumed the 1)idymograptus-bi./idus Beds on the north side of the fault to the 'Upper Llandeilo,' although he remarked on their slmila~ity to the beds in the ' Lower Llandeilo '--that is, the 1)i moyraptus-murchisoni Beds It is only just to mention that, at the time when Hicks examined the ground, the fault- plane itself may not have been exposed, while the quarry had probably not been so far cut back as to expose the higher and more 1 Prec. Geol. Assoc. vol. xxfi (1911) p. 228. Q. J. G. S. vol. xxx; (1875) p. 178. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

part 2] BETWEEN ABEREIDDY AND ABERCASTLE, 80~ fossiliferous beds of the Did~t~grapt~s-b~d~s Zone, in which case the detection of the repetition would have been ex~Temely difficult. The Dieranogral~tus Shales (with the overlying limestone) represent, then, the strat~graphically highest beds now to be seen in the dist-rict, and they only occur in the valley opening int~ Abereiddy Bay. All other parts of the area under consideration are occupied by older beds, and the same appears to be ~rue for the very large area extending east~vards towards, if not achlally up to~ Strumble Head and Fishguard. 1

III, PETRO~06Y~ (A) The Keratophyres o~ Pwll Whiting and Abercastle. (i) Field-relations.--These rocks were first mentioned ant1 an account of their petrology given by Dr. J. V. Elsden, ~ who described them under the term of ' Iime-bostonites.' He considered them to be inh~sive rocks, and possibly older than the intrusive diabases. The examination of the stratigraphy of the area has now shed some further light on the mode of occurrence and probable age of the rocks. The keratophyres occur in two bands, north and south respectively of the Abereastle Anticline. The southern band extends from a point near Abercastle eastwards for a distance of over a mile towards the farm of Carnachen Wen, near which point its outcrop appears to be truncated by the Pwn-S~rodyr Fault, as already indicated on the Geological Survey map and mentioned by Dr. Elsden. The northern band extends from the headland between Pwll Olva and Pwll W~niting eastrwards for about 2 miles to Pwll S~rodyr, passing near Cwm Radau, across Abercastle ]:[arbour to Aber Yw. The outcrop of this northern band is, however, by no means continuous, as it is broken across by several faults near Cwm Radau and Abercastle Harbour, while between Aber Yw and Pwll St~odyr the band has been entirely removed, Partly by erosion of the sea, and Partly by the normal fault which runs parallel with the coast between tJaese two bays. This'outcrop is also terminated by the PwI1-Strodyr Fault. When ~lly developed, the thickness of the bands is about 250 feet; but at certain point, as in Abercastle Harbour, and in Pwll Strodyr, the thickness rapidly decreases to such an extent that the rock can hardly be mapped. These rapid variations in thickness cannot be en~h~ly explained by faulting. The keratophyres are unaccompanied by normal ~fl~s, and in most cases they appear to act as in~usive rocks, slightly hardening the shales in contact with them on each side. But the exact relations between sedimentary and igneous rocks are very peculiar and extremely complicated, and I can only explain the phenomena by supposing that the igneous magma was intruded' under i A. H. Cox & O. T. Jones, Rep. Bri~. Assoc. (Birmingham) 1913, p. 484. 2 Q.j.G.S. vol. lxi (1905) p. 594. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

~08 DR. A. H. COX ON THE GEOLOGY OF THE DISTRICT [VOL ]~r;, a very shallow cover, or in other words that these rocks represent what were practically lavas which burrowed among the mud of the sea-floor rather than actually flowed over that floor, and, at times, the escaping vapours completely fractured the almost solidified rock, converting it into a breccia.

In the first place, these keratophyres appear, so far as one may ~udge, to occupy a fairly-definite horizon in the Tetragrapt~ Beds, and very near the top of the group. This is most clearly seen in Pwll Llong, and on the north side of the Abercastle Anti- eline. Unfortunately, owing to the scarcity of fossils and to the abundance of minor ~au/ts, definite p~oof of the constancy of the horizon is wanting. In this connexion, too, one must not lose sight of the fact that the diabase sills of the district often keep to one definite horizon over very considerable distances. At both the upper and the lower boundaries of the masses there is usually a very considerable amount of brecciation and intermingling of sedimentary and igneous material. Farthest away from the main masses the breccia consists of angular frag~nents of igneous rocks lying in an abundant slaty matrix. The breceiated character was noted by Dr. Elsden, 1 who compared the rocks to the brecciated ~dt-like intrusions described by Kilroe & McHenryS from the South-East of Ireland, and suggested as an alternative that the breceia~ion might have been produced by earth movements, such as those that gave rise to the Pwll-Strodyr Fault. I find, however, that the breceiation has a much wider distribution than is suggested from Dr. Elsden's account, and that it occurs at localities, as for example in Pwll Llong, a considerable.':distanee from the line of the Pwll-Strodyr Fault. Further, there is no regular relation to be observed between the angularities of neighbouring fragments, w~ile such breeeiated rocks prove, under the microscope, to be very .fresh and to show no signs of having been crushed. Therefore, this does not appear to be a case of brecciation ~'n situ. Nearer the main igneous mass the amount of slaty material in the breccia decreases, until there are only a few small veins or patches and streaks of sedimentary material in a practically massive ~gneous rock. Consequently, there are rocks in which the matrix is of sedimentary material and the fragments of igneous material, along with rocks in which the reve~e is the case. Here is, then, to all intents and purposes a transition from a true sedimentary to a true igneous rock. The transitional rocks have a thickness of sometimes 20 to 80 feet, sometimes only a few inches. So far, the breccia resembles the ' intmsion-breccias ' which are not uncommon along the margins of intrusions, and of which numerous examples have been described by various authors. Now, however, certain other features must be noted. Wherever the amount of igneous material considerably outweighs that of the

1 Q. j. G. S. vol. lxi (1905) p. 597. Ibid. voh lvii (1901) p. 479. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

9I~art 2] BETWEEI~~BEREIDDY AND ABERCASTLE. 309 sedimentary rock, there the shale has been hardened by contact- alteration. As the proportion of sedimentary material in the breccia increases, the hardening becomes less apparent, and is eventually no longer to be observed, while at the same time the shales may not possess any cleavage. But, with a still gn'eater increase in the amount of argillaceous material and while no trace of contact-alteration is ~ be seen, yet the sedimentary portion of the rock, and in extreme cases the igneous portion also, takes on a well-defined cleavage. Thus the igneous rocks are clearly 'pre- cleavage' in age. In such cases, examination under the microscope shows that the direction of fluxion in the igneous rock is quite independent of the boundaries between igneous and sedimentary material, indicating that the igneous rock was already quite solfd before being brought into contact with the sedimentary material (see P1. XXIV, fig. 4). Although solid, yet the fragments of igneous material were often still so hot that distinct chilled margins were developed on a microscopic scale (see P]. XXIV, fig. 3). Sometimes, then, the igneous material was so cooled that it caused :no alteration in the adjacent sedimentary material, while, at other times (or, rather, in other places), it still retained enough heat to bring about a certain amount of hardening of the neighbouring, or included, shaly material, itself becoming chilled in the process. The amount of hardening naturally increases proportionately to the increase in the amount of igneous rock present in the breccia. It is necessary, then, to take into account the following facts :-- (i) The occurrence of the rocks apparently at a fairly-definite horizon; (ii) the occasional signs of a slight cont~et-alteration of the beds above; (iii) the breccia~ion of both top and bottom of the bands which is not due ix) brecciation in situ; (iv) the appearance and disappearance of contact- alteration in the sedimentary portion of the breccia ; and (v) ~he occasional occurrence of small fragments of igneous rock in slates many yards away from any parent mass of igneous material.

These together can" only be explained by supposing, as previously stated, that the keratophyres represent laves which tended to burrow in the mud of the sea-floor; also, that the covering of mud w~s often so slight that the escaping gases caused a brecciation of the igneous rock at a time when it was practically solid, occasional fragments being thrown a considerable distance. The shattered fragments would come from the outer and cooler portion of the mass, and, where comparatively few in number, would not retain enough heat to cause any sensible alteration in the mud in which they came to rest. Where the fragments were more abundant and of larger size, some slight amount of hardening .would be caused, while at the same time:sedimentary material would sink far down into the cracks and gpaces formed in the main mass of igneous rock, and would there often be intensely baked; just as in the well-known cases of the sediments in the cracks of the Old- Red-Sandstone and Carboniferous laves of Scotland, described by Q. g. G. S. No. 282. r Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

310 DR. ~. H. COX ON THE GEOLO0rY OF THE DISTRICT [VO1.'lx~i~

Sir Archibald Geiklel and others. In the Pembrokeshire examples the included seclhnent was often so baked, and at the same time so intensely silicilied, that it; is now represented by what is practically chalcedony. There is no reason why, provided the lava-mass was not too thick, a similar (but perhaps slightly less intense) brecciation should not have taken place at the bottom of the mass as well as at the top. Where the igneous material was burrowing in the mud, and no brecciation occurred (perhaps because the cover was too thick or too compact), there the overlying sediments would be baked and hardened as in the ease of a normal sill. Since the resistance of the mud towards the passage of molten material would vary from place to place, it is only to be expected that the thickness of the igneous band would vary considerably, especially so if the magma were very viscous, as seems to have been the case in the presen~ instance. It is, indeed, a well-known fact that ~achytic lava- s~reams frequently show great variations in thickness within a small lateral distance. That vapours were abundantly given off from these rocks is shown by their often very vesicular character : this is especially noticeable in the upper part of the band which juts out into Pwll Whiting. The extremely viscous nature of rocks of similar type has been mentioned by Dr. H. H. Thomas. 2 The alterai,~on of the sedimentary rocks was due mainly to silicification resulting from pneumatolytie action, rather than to a mere baking effect by direct contact-action. This is proved by the fact that the greatest change in the sediments takes place above---and in the upper portions of--the igneous bands, where the sla~es are now represented by chert-like rocks passing over into chalcedony. The change is most pronounced, precisely in those places where the igneous rock is most vesicular. On the other hand, silicifieation never affects the breccias at the base of the igneous bands. It is notewol~hy that (loeally) the brecciation seems to have taken place at a somewhat earlier stage than was normally the case: therefore the igneous fragments are cemented, not by a matrix of shale, but by a matrix of igneous origin, thus giving rise to a SOle of flow-breccia of the type so common in lavas generally. This appearance may be observed in Pwll Strodyr. Instances have been recorded in other areas, as, for example, in East Lothian, 3 of igneous masses forcing their way underneath a very thin cover of wet sediments, in such a way that, although of contemporaneous age, they act as intrusive bodies. Again, an example of a submarine lava-stream having been changed into a tuff-like mass through the disruptive action of its own escaping i , Ancient Volcanoes of Great Britain' vol. i (1897) p. 283. ' The Skomer Volcanic Series' Q. J. G. S. vol. lxvii (1911) p. 195. a G. Barrow,' The Geology of East Lothian' Mere. Geol. Surv. 1910, p. 69 ; see also W. G. Fearnsides, ' The Geology of Arenig Fawr & Moel Lly~ant' Q. J. G. S. vo]. lxi (L905) p. 629. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

part 2] BETWEV.~iBV.REIDDY Am) iBERCiSTL~. 311 vapours is recorded by Prof. W. S. Boulton in the case of the pillow- basalb near Weston-super-lVfare. 1 There is, however, this difference in ~he Pembrokeshire rocks, that the breccias do not themselves show any fluxion-s~uctures such as are seen in the breccias described by Prof. Boulton. Further, there is never the slightest aign of pillow-structure in the Abercastle keratophyres, but this s~'ucture is hardly to be expected in such comparatively acid rocks. The effect of the subsequent cleavage on these rocks deserves some mention, since there are various apparently contradiqtory appearances :-- (i) Where the adjacent shales have been considerably hardened, they have" hardly been affected by the cleavage ; (ii) where in the breccia the amount of igneous material is large, the shale may remain uncleaved, even though it has not been hardened at all ; (iii) on the other hand, with a decrease in the amount of igneous material present, cleavage makes its appearance in the shales, and may, or may not, affect the igneous rock as well. These various appearances depend apparently on the protective influence of the numerous hard fragments of igneous rock; (iv) similar variations are found in the shale-inclusions of the main igneous masses. When the inclusions are small, they are uncleared, and may or may not be hardened; but, when the shale-inclusions are of larger size, they are often perfectly cleaved. Thus we have the apparent anomaly of the igneous rock including at one and the same time fragments both of cleaved and of uncleared material. It is evident that, while the massive igneous rock serves to protect the smaller inclusions from the effects of cleavage, it is unable to protect the larger inclusions. The igneous rock itself is usually uncleared ; but sometimes the breccias, and less commonly parts of the larger masses, have been unmistakably affected. Apparently, then, the hard massive rock may transmit the cleavage-stresses, although itself remaining unaltered. This phenomenon may, of course, be observed in cases where (as in the L/ngu[a Flags) a series of harder and softer beds have been subjected to cleavage ; but it is far clearer in the case of these keratophyres and their shale-inclusions. The apparently-contradictory appearances described above "show the need for caution in drawing inferences, as to the relation of any igneous rock-mass to the cleavage, merely from a limited number of observations.

(ii) Petrography.--These rocks were described by Dr. J. V. Elsden. 2 He writes : ' The rock decomposes rather easily, and shows a tendency to present a vesicular appearance from the weatherlng-out of irregular elongated cavities, formerly occupied by a dark material, resembling chloritic matter, which spots the unweathered portion of the rock ...... The general appearance is that of a greenish.grey trachytic rock.' The rocks are practically non-porphyritic, and no original dark minerals are seen. Occasionally, the rocks are full of the little opaque white spots about 2 ram. in diameter, mentioned by Elsden as occurring in the very similar rock of Cwm-y-Craig near lVlathry. 1 , The Igneous Rocks of Spring Cove' Q.J.G.S. vol. Ix (1904)p. 158; ~ee ~lso W. W. Watts, in the discussion on that paper, ibid. p. 169. : Q. J. G. S. vol. lxi (1905) p. 594. Y2 Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

312 ~R. Jk. H. COX ON TIlE t~EOLOGY OF .THE DISTRICT [vol. 1~, Under the microscope the rocks are seen to have a somewhat ta-aehytic texture, and to consist mainly of felspar-laths together with more or less abundant interstitial chlorite in large patches, in irregular gl~nules, or sometimes in fibrous aggregates. By a decrease in the amount of interstitial chlorite the texture becomes almost pilotaxitic [P 85]. The felspars occur as laths, or as forked and skeletal microlites, measuring from 0"5 down to 0"1 ram. in length. A flow-struchtre is frequently visible (see P1. XXIV, fig. 4); but it is not usually continuous in direction over any distance, even within the limits of a microscope-slide, and it often "disappears altogether [P 56], attesting the great viscosity of the cooling magmas. The felspar-laths commonly show both Carlsbad and albite- ~-winning, symmetrical extinctions of the albite-lamell~e ranging up to 18 ~ while the refractive indices are always lower than quartz, and mostly below 1"541, indicating a felspar which is almost pure albite. In unweathered examples these felspars are water-clear. Most of the rocks sliced proved to be non-porphyritic, though sometimes felspars of larger size than the normal laths are to be seen [P 56]. Some of these larger felspars are albites; but the majority, which (despite their relatively-larger size)are rather ill-formed, appear to consist of a soda-orthoclase. In some specimens a certain amount of ill-defined, powdery, dark material has separated out round the felspar-laths where they are embedded in chlorite [P 178]. This dark material is particularly prominent at the end of the laths, in which direction the main growth of the felspar naturally took place. The powder presumably contains the ferromagnesian oxides eliminated from the magma as the felspar formed. The appearance is exactly similar to the darkening of a tachylytic glass round felspar-microlites, and it bears out Dr. Elsden's suggestion that some of the chloritic material in these keratophyres possibly originates from the hydration of an original glass. Against slate fragments there is sometimes a thin zone, about 1 mm. thick, of rock fairly free from this dark powder, but otherwise perfectly normal, showing that a very slight amount of chilling took place, with the result that the dark material had not sufficient time for its differentiation (see P1. XXIV, fig. 3). In some slides VP 85] a certain amount of quartz in small irregular grains and aggTeg-ates is present ; but, in nearly all cases, it is clearly secondary, and in places it can be seen that the rock has ,been partly silicified, so that even the felspars are in part replaced by quartz. Oceasionallv the replacement has been so perfect, that the lines of the original albite-twinning can still be traced in the quartz-pseudomorph. The rounded or irregular vesicles are infilled by chlorite, together with some calcite and quartz. Calcite may also replace the chlorite of the ground-mass in a pcecilitic manner. In one of the slides [P 178] are several pseudomorphs after a pyroxene, which, jud~ng from its shape, was very probabiy one of Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

~al~ 2] BETWEEN~BEREIDDY AI~D A_BERCASTLE. ~13 the rhombic pyroxenes. The original substance is now replaced entirely by chlorite, along with a little calcite. The pseudomorphs, which in basal sections have a diameter of about 0'5 ram., enclose an occasional felspar-microlite. It seems probable that cel~ain rocks exposed at low tide near the causeway loading ~o Ynys Castell, and also on the opposite (that is, western) side of the harbour, are closely connected with the keratophyres : perhaps they represent offshoots from" the main keratophyre mass. These rocks only occur in patches too small to be mapped. The neighbouring slates are slightly hardened, and a certain amount of brecciation of the igneous rock may be observed. The rocks, which are highly vesicular, are very decomposed, the minerals, both felspar and chlol~te, being largely replaced by calcite, and so the rocks are hardly in a fit state for microscopical examination. The texture is somewhat coarser than in t~pical examples from the larger keratophyre-masses; othel~ise, there does not appear to have been any important difference. The slate which forms the matrix of the breccias (see p. 308) is full of little chips of the keratophyre, and even of isolated felspar- microlites. There is commonly a little darkening of the exh'eme edge of the shaly material, but this is frequently the only sign of any contact-action. The cleavage-lines can be seen winding round the fragments of igneous rock, while the orientation of the felspar- crystals in the igneous rock bears no definite relation to the boundary between the igneous and the sedimentary materials [P 178]. This latter is the case, even in examples whel~ the shaly material is much smaller in amount, and is so completely enclosed by the igneous rock that the sediment has been silicified and rendered harder than steel [P 56]. The slate contains the remains of sponge- spicules, and probably also of radiolaria [P 52 & 56], now rdplaced by chalcedony. These spicules occur, even where ~he slate has not been markedly silicified. The detection of these spicules is due to Dr. Herbert H. Thomas. As a result of his examination of the &bercastle rocks, and of an analysis made by him, I)r. Elsden 1 compared them with the m~enaites, or lime-bostonites, of the Christiania district. 2 It was afterwards suggested by Rosenbusch, a who likened the Abercastle rocks to the ' Lahn porphyries,' that they should rather be placed among the keratophyres. The close petrological resemblance of the keratephyres of Skomer Island to the Abercastle rocks was noticed by Dr. Thomas. a Further, some of the andesites described and figured by Mr. Cantrill & Dr. Thomas from Llangynog 1 Q. j. G. S. vol. lxi (1905) p. 596. W. C. BrSgger, ' Die Erup~ivgest~ine des Kristianiagebietes ' pt. iii--' Das Ganggefolge des Laurdalits ' Christiania, 1898, p. 207 ; see also ' Basic Eru1 - tire Rocks of Gran' Q. J. G. S. vol. 1 (1894) p. 23. a 'Mil~roskopische Physiographic der Mineralien & Gestdne' 4~h eJ. vol. ii, p~. 2 (1908) p. 944. ' The Skomer Volcanic Series' Q. J. G. S. vol. lxvii (1911) F. 194. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

314 D~. _~. H. COX ON THe. 6EOT.O6Y OF ~HE DIS~aICT [VO1.Ixxi,

(Carmarthenshire), 1 appear to be very similar to the Pembrokeshire rocks. Both the Skomer Island and the Llangynog occurrences are lava-flows, probably of Arenig age. Since it has now been shown that the Abercastle rocks are really contemporaneous rocks, also of Arenig age, the close resemblance of the rocks of these three localities is readily undel~tood.~ -Dr. Elsden's analysis of the Abercastle rock is tabulata~l on p. 315 (No.. I). From that analysis he calculated the albite and anorthite percentages as 60"26 and 16"12 respectively. Using his analysis, and working (so far as was possible)with minerals actually seen in the rock, I have calculated the mineral composition as follows :~ Orthoclase ...... 1"3 Albite ...... 60'2~ corresponding to 63"7 per cent. of plagio- Anor~hite ...... 3"5 ~ clase, of the composition Ab 95, An 5. Ilmenite...... 1"7 Magnetite ...... 1"5 Amesite 3 ...... 13"8 Hypersthene ...... 9"8 Quartz ...... 4"3 Calcite ...... 4"6 Total ... 100"7

i The values for the felspar (Ab 95, An 5) ca]culat~l from the analysis by this method agree almost exactly with the values (app. Ab 96, An 4) as determined from a consideration of the extinction-angles and refractive indices. It will be noticed that the calculation shows a considerable percentage of a rhombic pyroxene, whereas in the rock itself but very little pyroxene is ~ be seen, and that only in pseudomol]~hs. Possibly, most of the pyroxene is in the form of very small microlites (or of crystallites), which are lost among the chlorite itself, and among the dark undifferentiated material embedded in the chlorite. Or, still more likely, judging from the pseudomorphs, all the pyroxene is now in the for~t of chlorite. The presence of free silica is indicated by the calculation; but most, at any rote, of the quartz now seen in the rock is of secondary origin. 1'On the Igneous & Associated Sedimentary Rocks of Llangynog' Q. J. G. S. vol. lxii (1906) p. 240 & pl. xxv, fig. 2. Yet another example of petrographically-similar lavas occurring in the Arenig is furnished by the volcanic rocks of Trefgarn Bridge, some 8 miles south-east of Abercastl~, recently described by Dr. H. H. Thomas (' Geology of the South Wales Coalfield' pt. xi : ' Geology of the Country round Haver- fordwest' Mere. Geol. Surv. 1914, p. 21). I am indebted to I)r. Thomas for the opportunity of examining that memoir when in proof. In making the calculation, the small amoant of adherent water--0"48 per eent.--was neglected, while 0"43 per cent. of magnesia and 0"39 per cent. ~nore of combined water were used than are warranted from the figures of the analysis. By making these almost negligible adjustments, the calculation was greatly simplified, and all the constituents were used up in the simplest possible manner. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

part 2] BF~TW~.~ ABEREIDDY AND ABERCASTLE. 315

The greater part, if not the whole, of the calcite has been introduced into the rock from external sources. This is borne out by the fact that in the freshest rocks hardly any calcite is to be seen; while the proportion of calcite steadily increases with intensity of weathering, until almost the entire rock is replaced by calcite, the change affecting both felspars and chlorite indifferently. So much calcite cannot possibly have come merely from any original lime in the felspars, as the result of any albitization that may have taken place. Thus, unless account is taken of the entirely- external origin of this calcite, any calculation of the norm would be vitiated. This makes it difficult to surmise the original nature of the felspar, although the general appearance of the rock under the microscope would suggest that the felspars have suffered little if any change, and that the albite character is original in this case, 1 It is difficult to find other rocks the analyses of which compare really well with that of the Abercasfle keratophyre, but analyses of somewhat similar rock-types are tabulated below :--

I. I I. I II. IV: V. VI.

Si02 ...... 55"38 54,'20 56"50 60"13 61"8 58"47 TiO2 ...... 0"90 2"60 0"85 trace 2"17 Al20a ...... 18"34, 21"0 18"14 20"47 16"5 18"60 Fe203 ...... 1"13 2"6 3"12 1"04 } 6"7 1"92 FeO 5"86 4,'3 2"86 @72 4"77 MgO ...... 3"4,7 2"0 1 "22 1"15 1"2 0'94 CaO ...... 3"25 3"8 3"38 2"59 4"5 0"99 Na20 ...... 7"12 6"5 5"28 9"60 7"2 5"52 K20 ...... 0"22 0"4 1"60 1'06 1'4 3"30 0"50 H20 at 110~ ...... 0"48 1"26 H~O above 110~ 2"39 3"44 0"6 2"19 CO2 ...... , 2"00 1"5 5"11 } 0"04 P205 ...... ]trace O'G 0"45 Other constituents m 0"29 Totals ...... 99"32 100"20 99"9 L. Elsden. traube. Schmelck. ington. Acton. l~dley.

I. 'Lime-bostonite,' Abercastle. J. V. Elsden, Q. J. G. S. vol. lxi (1905) p. 595. IL ' Lime-bostonite,' Kilbride. C.I. Gardiner & S. H. Reynolds, ' Palaeozoic Rocks of the Kilbride Peninsula' Q. J. G. S. vol. lxviii (1912) p. 93. III. 'Lime-bostonite' (myenaite), M~ena (Southern Norway). W.C. BrSgger, ' Die Eruptivegesteine des Kristianiagebietes--III : Das Ganggefolge des Laurdalits' Christiania, 1898, p. 207 ; also ' Basic Eruptive Rocks of Gran' Q. J. G. S. vol. 1 (1894) p. 26. IV. Keratophyre, Fair Haven (Connecticut). E. O. Hovey, Am. Journ. Sei. ser 4~ vol. iii (1897) p. 291; analysis recorded by H. S. Washington, ' Chemical Analyses of Igneous Rocks' anal. 1, p. 204,. V. Andesite, Cam. Boduan (North Wales). A. Harker, ' Bah Volcanic Series of Carnarvonshire ' 1889, p. 69. V[. Soda-traehyte (lava), Skomer Island (Pembrokeshire). H. H. Thoma% ' The Skomer Volcanic Series' Q J. G. S. vol. lxvii (1911) p. 192. 1 See g. S. Flett, in the discussion on ' The Skomer Volcanic Ser:es Q. J. G. S. vol. lxvii (1911) p. 213. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

316 ])R. A. H. COX ON THE GEOLOGY O]~" THE DISTRICT [vol. lxxi,

It will be seen that only in the case of the Kilbride rock is there a fairly exact correspondence. In the other rocks, the analyses of which are quoted, there is a larger amount of potash, but usually less carbon dioxide and combined water. In the :Norwegian m~enaite, and in bostonites generally, the ratio of soda to potash is somewhat different from that in the Abercastle rock, as was recognized by Dr. Elsden. It is true that the same statement would seem to apply to a good many keratophyres. 1 In the quartz-keratophyres, however, the soda-potash ratios are, as noted by Elsden, freqnently similar to that in the Abercastle rock: the same is the case with the spilites. Judging from the distinctly-albitic character of the felspar, it would seem advisable, as suggested by Rosenbuseh, to regard the Abercastle rocks as keratophyres, and this quite ilTe- spective of their mode of occun'ence.

(B) The Rhyolites of tile Llanrian Volcanic Series. (i) The field-relations of these rhyolitic rocks have already been considered (see p. 302). (it) Petrography.--In the field there appear to be two rather different tyl~.s of rock among the lavas belonging to this group; these differences noted in the field correspond to two different ~'pes of microscopic structure. In the first place, we find rocks which macroscopically have a colour varying from pale cream to a dark, rather bluish, green These rocks present a very 'flinty' appearance, with a conchoidal fracture and an almost glassy lustre. They occasionally show a contorted fluxion-banding, but are more usually without any such banding, presenting then an ext;remely-uniform appearance, since they are non-vesicular and quite destitute of porphyritic constituents. The examples of such rocks were mostly collected from the seaward end of Trwyn Castell, and also from close to the summit of Carn Llwyd. The featureless character of these rocks is borne out by microscopic examination [P 20 & P 108~, when little but cryptocrystalline felsitic material, without any trace of original structure, is to be seen. Spherulitic and perlitic structures are entirely wanting, nevertheless it is highly probable that such rocks represent devitrified obsidians. Phenocrysts are non-existent, and there are no traces of any dark mineral ever having been present. The small amount of scattered iron-oxide is now mostly in the form of limonite. Occasional con-oded xenoliths of a rhyolite or rhyolite- tuff with a slightly different microscopic structure may be observed; and in places also numerous, but very small, xenoliths of shale or mudstone are present.

1 For analyses, see H. Rosenbnseh, ' Elemente der Gestelnslehre' 3rd ed. (1910) p. 343. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

part 2] BETWEEN/~BEREIDDY AND ABERCASTLE. 317

The second type of rhyolite, which often occurs interbanded with, and shows occasional transitions to, the very 'flinty' type just described, consists of rocks having a stony or ' felsitic' rather than a 'flinty' appearance, and a fracture that is irregular or platy, though not conchoidal. The two types may be present in a single flow, but are generally found in separate occurrences. The colour may be a creamy white, but more commonly is reddish green, and may even be black. These rocks weather more readily than the flinty type, probably in consequence of their less fine- grained texture. On weathering they may, like the 'flinty' type, become filmed over by the thin white clmst characteristic of rhyolites generally; but more often the), assume a reddish-green colour. Where rocks of this type alone are found, flow-structure is not usually to be seen in the field. The rocks are again non- vesicular and practically non-porphyritic. Rocks of this type are of much commoner occurrence than are those of the flinty type, and may be seen at almost every locality where the Llanrian lavas are exposed. Two somewhat weathered examples of these reddish-green rocks [P 16 & 17], collected near the old tower on Trwyn Castell, show under the microscope a slightly irregular fluxion-structure in natural light. The ground-mass is again cryptocrystalline, but now with a considerable development of tiny flakes of sericite resulting from the decomposition of felspar. There are also present scattered microporphyritic and fairly clear felspars, mostly albite ; but some of the crystals are untwinned. Many of these small felspars are irregular in outline, instead of being idiomorphic, and they do not necessarily lie parallel with the fluxion-structure. These features may be observed in modern glassy lavas. Dark minerals are again absent, the red eolour being due to the abundance of iron oxides, which helps to show up the fluxion-structure. One or tnvo tiny xenoliths of shale were observed. These rocks must originally have been thoroughly glassy. A section taken from one of the black rocks on the eastern, or landward, side of Trwyn Castell [P 15] presents a different appearance, and somewhat recalls the 'aschensh,uktur' of Miigge. 1 In a cryptocrystalline, almost microcrystalline, ground-mass showing fluxion-sh~cture and densely charged with magnetite dust, are embedded very numerous patches of felspar (P1. XXIV, figs. 1 & 2). The patches often have concave outlines, or they may be more or less elongated and lath-like in shape ; but they are mostly composed of irregular aggregates rather than of single felspar-c~stals. Some of this felspar is albite, and occasional suggestions of 'chequer- structure' are to be seen : ; but much of the felspar is untwinned, 1 Neues Jahrb. Beilage-Band viii (1893) p. 648. ~" F. Beeke,' Zur Physiographie der Gemengtheile der Krys~]linen Sehiefer' Denkschr. K. Akad. Wissensch. Wien, vol. lxxv (1906) p. 28; see also J. S. Flett, in' The Geology of the Country around Newton Abbot' Mem. Geol. Surv. 1913, p. 58. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

318 DR. k. H. COX ON THE GEOLOGY OF THE DISTRICT [vol. IT~ and appears to be an orthoclase rich in soda. There is no direct relation between the orientation of the felspar aggregate as a whole, and the orientation of the felspars composing the aggregate ; while the felspar of the ground-mass is often in optical continuity with the felspar of the larger patches. The whole structure appears to be largely secondary, and due to the recrystallization of patches and streaks formed originally as the result of flow in a highly-viscous mass. A pale greenish rock from the Treneved-Road quarry [P 23] shows an almost exactly similar appearance, save that magnetite dust is far less abundant (hence the much paler colour), and that recrystallization of the streaks and patches has taken place to a greater degree, so that the structure is to some extent lost when viewed between crossed nicols. There is, again, no regularity to be made out in the orientation of the secondary felspars in the patches. The few scattered original microporphyritic felspars, which are mostly albites, stand out quite clearly from, and are much more cloudy than, the secondary aggregates, the felspar of these latter being often water-clear. Occasionally, the porphyritic felspar has grown slightly through the addition of clear secondary felspar, and cracks in the phenocrysts have also been healed with this secondary felspar. Streaks and patches, in shape exactly similar to the felspar patches, may be produced by movement of oily or greasy material floating on water; and it may be noticed that a fairly rapid or irregular motion tends to give rise to streaks presenting concave outlines, and that the streaks may even divide or fork. Con- siderable variations in the appearances will naturally be produced by differences in surface-tension, which will partly depend on the mutual solubilities of the water and oily material. If, now, we imagined a magma, while flow-movements are ceasing owing to increased velocity, separating into two partly immiscible portions, one portion probably richer in felspathic molecules, and the other richer in silica molecules, and the elongated and m~her irregular patches so produced undergoing a later recrystallization, we should find exactly the appearances presented in these rhyolites. Such textures seem to be not uncommon in Ordovician rhyolites of other areas. Very similar appearances may be observed in various occurrences from ~Torth Wales. The result of such a texture is that the rock presents almost, if not absolutely, the same appearance as many rocks having the ' aschenstruktur' of ~Miigge, a structure or texture found in those acid pyroclastic rocks which are largely composed of tiny fragments of devitrified pumice, the devitrified fragments being set in a more or less abundant cryptocrystalline matrix. These glass-wisps often present concave outlines, are largely replaced by secondary felspar, and it is often claimed that such a structure is quite distinctive of a tuff. An examination of the Pembrokeshire rhyolites, however, shows that the presence of concave outlines in such felspar- Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

part 2] BETWEEN iBEREIDDY AND A~BERC/kSTI,E. 319 aggregates is by no means a proof that the rock containing them is pyroclastic in origila. In this connexion, it may be noted that one of the l~orth Wales examples, described and figured by Miigge i as showing typical 'aschensia-uktur,' is from a rock described as a rhyolite by Dr. Harker. 2 As the flow-structure in the rocks becomes more regular in direction the felspar-patches diminish in number and size [P 25] ; evenhmlly, they disappear altogether, and the rock either passes into the 'flinty' type already described, or into a rearm-e- less confusedly-cryptocrystalline rock [P 13 & 27] which cannot, by microscopical examination alone, be distinguished from an extremely fme-grained recrvstallized tuff. The original nature of many of the Pembrokeshire rocks has, in fact, been so obscured by recrystallization, that field-evidence is usually more reliable than that obtained by microscopical examination, and in cases where field-evidence is obscure the microscope rarely yields any definite information. This point will be further elaborated in the description of the pyroclastic rocks. An interesting circumstance is the close relationship be~ween the textures of the lavas, and the extent to which the rocks have been affected by cleavage. It is only the very fine-groined ' flinty' type that has completely escaped the effects of the cleavage-stresses. All the other types, which have a rather coarser texture, have been more or less affected, often very considerably so, and accordingly th.ey may present in the field the appearance of flaggy tuffs. Since the two types--the ' flinty' and the ' felsitie '--behave thus differently, it is evident that the main recrystallization had already been effected before the cleavage-stresses became active.

As regards the composition of these lavas, they evidently belong to that division of the rhyolite family which is often spoken of as the quartz-keratophyres. The high soda-content is shown by the prevalence of albite-felspars, both in the scanty phenociTcsts and among the secondaiT felspar which forms in the patches described above. That the felspars of these Pembrokeshire rocks are not exclusively sodie,however, is shown by the frequent presence of untwinned crystals, which seem in most eases to represent an orthoclase luther rich in soda. It appears probable that the rocks are, on the whole, closely allied in composition to the rhyolites recently described from Skomer Island, 8 and from the Dolgell)~ district, a and they belong to a type which appears to have a very wide distribution among the Ordovician rhyolites of Wales.

Neues Jahrb. Beilage-Band viii (1893) p. 713 & pl. xxvii, fig. 41. ' The Bala Volcanic Series of Carnarvonshire ' 1889, pp. 13, 19, & 21. 3 H. H. Thomas, ' The Skomer Volcanic Series' Q. J. G. S. vol. lxvii (1911) p. 189. 4 p. La]~e & S. H. Reynolds, ~The Geolcgy of Myr,ydd-y-Gader ' Q. J. G. S. ~o]. ]xviii (1912) p. 360. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

820 :DR. /L. It. COX ON TItE GEOLOGr OF THE DISTRICT [vOL ll~

(C) The PyroclasticRocks. In the Llanrian Volcanic Series there is a great variety of acid pyroclastic rocks, the variation being due in some cases to the incoming of larger or smaller amounts of argillaceous material, and in. other cases to considerable differences in the size of the constituent particles, which vary from lapilli with diameters up to 2 inches, down to the finest dust. Thus, in the field, may be seen all transitions from massive fine-grained tufts to ashy slates on the one side, and to agglomeratic beds on the other. The commonest types, especially prevalent in the lower portion of the series, and occurring frequently between the val~ous flows, consist entirely of the finest rhyolite-dust; they am almost destitute of any lapilli, while even fragments of microporphyritic crystals are quite scarce. These rocks are usually reddLuh green, and, as a rule, they are much more cleaved than are the associated lavas. Under the microscope such tufts are seen to consist almost entirely of a cryptoerystalline base, in which numerous shreds and grains of chlorite are often so distributed as to impart a laminated character [P 6 & 19]. In other cases, however, there is no trace of any lamination. The chlorite, of course, accounts for the green eolour of the fresher specimens. In the more weathered examples oxidation of the iron results in the staining, first of the chlorite, and eventually of the whole rock, which then becomes red. Secondary white mica, occurring in small irregular shreds with a general orientation in common, is extremely abundant in the small well- cleaved tufts, diminishing with decrease in the degree of cleavage [P 5], and disappearing altogether in the massive uncleared varieties [P 2]. In many cases, not a vestige of original structure can be made out, and the whole rock appears to have consisted only of the very finest dust: it is precisely such very homogeneous and featureless rocks that are most affected by cleavage. In other cases, where the rocks are less cleaved, or not cleaved at all, indications of the original structure are still apparent, and suggest an ' aschensh'uktur ' on an extremely fine scale [P 2 : also P 80 & 31 from the top of the Murchisoni Ash at Portheiddy]. In consequence of the very complete recrystaUiz~tion, however, all trace of these original shnlctures disappears when the rocks are examined between crossed nicols. These rocks with the faint indications of olqginal structure may or may not show, similarly to those described in the preceding paragraph, a slight lamination picked out by chlorite, and simulating a regular fluxion-structure. In either case, it becomes impossible in many examples to distinguish such tufts, by microscopical examination alone, from some of the flows [for instance, P 18, 20, 27]. The separation in such cases can only be made Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

l~ar~ 21 BETWEEN ABEREIDDY &ND A_BEBC&STLE. 321 when the field-relations of the various beds are clear. The distinction in the field may be rendered difficult by the fact that some of the massive tufts on weathering develop a hard white crust with a rough surface resembling that presented by a semi-nodular rhyolite ; and on this white surface there may be slight ridges simulating flow-banding, but really due to lamination. & good example of such a tuf is seen in the thin massive band [P 2] which forms the topmost bed at the first, or easternmost inlet on Trwyn Castell (see P1. XXII, fig. 2). This thin band overlies a thicker bed of the cleaved red type [P l]. Many of the tufs, both cleaved and massive varieties, contain no material other than the fine dust, but the majority of them include in addition small and scarce broken quartzes and felspars, and sometimes also small fragments of foreign rocks. The quartzes were evidently derived originally from acid lavas--as the crystals show crystalline form, and are corroded in the manner so characteristic of quartz-phenocrysts [P 1]. It is noteworthy that, although porphyritic quartz is not uncommon in the tufts, yet it has not been noted in'any of the undoubted lavas assoctated with the tufts. 9Such a relation is by no means infrequent in the Ordovieian lavas and tufs of other districts. The porphyritic felspars are mostly untwinned, and they may show an irregular fringe of later material extending into the ground-mass, but in optical continuity with the original felspar. The composite fragments that are occasionally to be seen include small lapilli of pumiceous tufts or of other tufts, tiny flakes of some slaty rock, and also of a rock which is either a tuft or a fine- grained grit [P 2 & 110]. Quite diferent from all the types described above are those tufs that consist mainly of pumiceous lapilli. The Murchisoni Ash furnishes a good example of this latter type. Under the microscope a specimen from this ash [P 67] is seen to be largely built up of pumice or glassy material, with numerous vesicles drawn out into long tubes outlined by dark matter. The tubes are now filled principally by an almost colourless and isotropie chlorite, along with a smaller amount of a fibrous ehloritie substance possessing strong birefringenee. As the result of devitTifieation, numerous crystals of felspar and of quartz have formed, the latter often in small spherulitie aggregates. This secondary crystallization is independent of the original run of the tubular vesicles, as it cuts across them quite irregularly. Magnetite and pyrite in tiny euhedral crystals are abundant, together with a small amount of calcite now partly replaced by radial aggregates of quartz. In the ~'eat majority of the pumiceous lapilli the amount of solid material must have been very small, the tubular vesicles occupying the greater portion of the space. Such lapilli are practically destitute of any original crystals. In a small minority of the lapilli, however, the relative proportion of the vesicles decreases, thus increasing the amount of original gn~und-mass, which then contains numerous, Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

322 ~)R. A. H. COX ON THE GEOLOGY OF THE DISTRICT [voL lg.xi, but small, skeletal felspar-rods. Besides the pumiceous lapilli, shale-fragments and a few broken felspar-crystals may be seen. Such pumiceous tufts are not very common in the Llanrian Volcanic Series. A good example, however, is furnished by the pale tuff-band which occurs in the D.-bifidus Shales close above the keratophyre-sill exposed in the eastern quarry, Abereiddy. This tuff [P 68] is mainly composed of fragments of pumiceous material, so packed together that the boundaries of the individual fragments can hardly be made out. Here, again, the vesicles are infilled, and the walls apparently in part replaced, by an absolutely colourless and isotropie cblorifie malarial. There are also present numerous laths and skeletal rods of felspar along with small particles of traehytie lavas, and of shales, as well as bits of tufts exhibiting quite a different microscopic structure. The ashy slates nearer the keratophyre int~asion are crowded with somewhat similar pumiceous fragments, in which the results of devitrifieation can be well studied [P 78]. A few pumiceous lapflli may also be found among the coarse agglomeratic tufts [P 107], which form the upper portion 'of the Llanrian Volcanic Series. Most of the pebbles in these agglomeratie beds, however, are derived from tufts of the very fine-grained and more featureless types described earlier. Besides the various tufts of which an account has been given, there are many types transitional between slates and crystalline tufts, but such types hardly merit any special description.

(D) The Diabases. (1) Field-relations.--Sills of diabase are very numerous throughout the area, occurring in all the formations of the district. Many of the occurrences have ah.eady received notice at the hands of Dr. J. V. Elsden, 1 who showed that there exist two main types of diabase in North-West Pembrokeshire:-- (i) Normal ophitic diabases without quartz, largely developed in the Fishguard district, and (ii) quartz-enstatite-diabases passing over into quartz-norites, etc., of which there is a great development in the St. David's district. Dr. Elsden further proved that in the Abcreiddy-Abercastle area, which lies about half-way between Fishguard and St. David's, there is an overlapping of the two types, so that both are to be found, lie inclined, therefore, to the view that there was a gradation in a westward direction from the ophitie-diabase type to the quartz-norite type, with an indefinite zone of enstatite-diabases in the intermediate district with which this paper is mainly concerned. So far as my own observations go, I have not been able to prove that the quartz-bearing rocks actually pass into the .quartz-free types, since the two rock-types appear always to occur in separate

Q. J. G. S. vol. lxi (1905) p. 579. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

part 2] BETWEEN ABEREIDDY iND ~kBERCASTLE. 823 masses. Nevertheless, I am quite in agreement with the view that both types were intruded at much the same time, and that they were in all likelihood very closely connected magmatically. As to the ac~l date of these intrusions, Dr. Elsden, while admitting that the evidence was imperfect, was disposed to consider that the intrusion of these basic rocks took place lafer than the faulting which gave rise to the Pwll-Strodyr Fault. In suppol~ of this view, he mentioned the absence of any signs of shearing or breeciation in the rocks. I have, however, been so fortunate as to obtain further evidence, which points to the strong probability that all the basic rocks were intruded prior to the main folding movements, and accordingly the intrusions antedate both the cleavage and the faulting (see p. 888). It will at once be clear firm a glance at the map (P1. XXV) that the diabases occur, almost without exception, as thin int~'usions which usually follow the bedding very closely for considerable distances, with, however, occasional slight transgressions from one horizon to another. It is significant that nowhere was any definite evidence obtained of faulting along the lines of diabase inh~sion. On the other hand, there is a considerable amount of evidence pointing to the fact that intrusions were not injected along fault-planes, l~oreover, the faults, wherever they can be seen or be mapped, prove to be free from any igneous rock, except in one single instance--the Pwll-Strodyr Faulfr-and in this ease the fault-intrusion is not a basic rock (see p. 881). So far as the majority of the basic intrusions of the Abereiddy- Trevine district are concerned, it is very difficult to obtain any satisfactory evidence as to the date of intrusion. This is due to the prevailing vertical dip of the stratified rocks, and to the fact that cleavage and bedding frequently coincide. Accordingly, it is not easy to determine whether the intrusions are true sills anterior to the folding, or more dyke-like in nature and intruded along cleavage-planes which were established subsequently to the folding. The intrusive rocks certainly follow the cleavage very closely, with, however, occasional sharp l~reaks right across the cleavage-planes. There is not, as a rule, any sign of the cleavage swinging round the igneous mass or away from the normal direction of cleavage, though occasional examples of the latter case may be found. Both cases are seen in the neighbourhood of a small diabase-silli on the west side of Aberfelin beach. In contact-altered slates the cleavage gradually decreases ; but, whether this is due to a subsequent baking of previously-cleaved rocks, or to the protective influence of the massive diabase, it is very difficult to decide. Neither is the evidence from the l~ther scarce included fragments any more conclusive. It has already been shown in some detail, in the case of the keratophyres, how very various are the effects that an igneous mass may exert on the smTounding shales.

1 The sill is too small tm be cotoured on the geological map (P1. XXV). Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

824 Dl~. ~t. H. COX O~ THE OEOLOGY OF T]tE DISTRICT [VO1.lxxi,

Thus, with regard to the age of most of the diabases, the evidence is inconclusive, appearing to favour, if anything, a post- c]eavs~e date for the intrusions. In one case, however, namely: the diabase of Aberhenow, the field-evidence is fortunately quite clear. This intrusion, one of the ophitic quartz-free diabases, obviously occupies the core of a shallow syncline which is situated immediately north of, and is complementary to, the Abercastle Anticline. The intrusive mass has followed closely the bedding of the 1).-bifidus Shales which, at this point, lie almost horizontally (see P1. X•VI, fig. 3). The diabase masses at Ynys Daullyn, Ynys Castell, and Pwll Strodyr, are probably all faulted patches of this same sill, which in its course eastwards does gradually transgress on to continuously lower horizons. But, as seen in the Aberhenow section, the mass is clearly a sill remaining horizontal where the strata are horizontal, and tilted where the strata are tilted--as, for example, west of Pwll Strodyr, where the sill dips northwards at about 30 ~. A mile or so farther east, between Abermawr and Aberbach, 1 several diabases of the same ophitic and quartz-fTee type are 9clearly seen to have been folded with the surrounding strata,: leaving not the slightest room for doubt that the intrusion of this set of diabases took place before the folding. To settle the question for the quartz-diabases, it is necessary to proceed westwards to Porth-y-Dwfr, 3 a narrow inlet on the west side .of Pen Berry.4 At this locality three small diabase-masses are seen to rest upon flaggy-looking beds, which dip north-eastwards 9or northwards at angles ranging between 10 ~ and 50 ~ Although these patches of diabase are now quite small, yet the adjacent sedimen~ry rocks (which probably belong to a high horizon in the Tetragraptus Shales)have been so gTeatly altered, that they are now spotted, very hard and flaggy, and show no real cleavage. They thus resemble almost exactly the highly-baked beds (also belon~ng to the Tetrayraptus Shales) at St. David's Head. At Porth-y-Dwfr there is a thickness of at least 100 feet of these .strongly baked beds, and there may be still more submerged beneath the sea. It is, therefore, fairly certain that the various isolated -patches of diabase around this inlet originally formed part of a single intrusion, which, judging from the extent of contact-alteration in the beds below, must have been of large size. The upper portion of this intrusion has been completely denuded: the lower portion, however, must have acted as a sill, which in this case has not subsequently been tilted into a vertical position. Further, it is highly probable that the great mass of Pen Berry itself is only

1 Bays situated beyond the eastern limits of the map (P1. XXV). The Aberbach Quartzites--see A. H. Cox & O. T. Jones, Rep. Brit. Assoc. (Birmingham) 1913, p. 484. :~ See sketch-map, fig. 1, p. 275. a Marked on Hicks's map, Q. J. G. S. vol. xxxi (1875) pl. viii. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

part 2] BE~WESrr~.BEREIDDY AND ~.BERCASTLE. 825

a continuation' of this same sill, which has now been broken by several minor faults, some of them north-So-south faults, others perhaps east-to-west faults, in the manner shown in the section (fig. 8). This large sill has 1:1 almost completely protected the underlying strata from the influence of the cleavage-stresses.1 a" At Porth-y-Dwfr, then, where the strata dip but gently, the quartz-diabases are seen to ,act as sills inclined equally with the e~ strata. At other localities, as, for instance, Traeth Llyfn and Abeffelin, where the strafa have been tilted into a vertical position, the quartz-diabases still follow the bedding, and accordingly themselves stand verr Sinee all these diabases probably belong to a single set of intrusions, it is thus fairly certain that they, similarly to the quartz-free dia- l~ases, were inh~ded before the main folding movements; and that the occurrences at Traeth Llyfn, etc. simply represent sills ~ that have been tilted into a vertical position, so that their outcrops appear on the map as long, thin, and dyke-like. The same state- e~ ment presumably holds good for ~ ~ the large masses of St. David's Head and Cam Lidi 2 nearer St. ~2--. David's. The quartz-diabases of I Northern Pembrokeshire are, therefore, comparable as regards mode of occurrence with the famous Whin Sill, rather than with the east-to-west dykes of Scotland. That some of the Pembrokeshire sills have a very I In this connexion it is interesting to note f~hat, according to the most recent account of the petrologically-similar Penmaenmawr int-rusion, the evidence likewise shows that the in~sion is older fJ~an the cleavage (H. C. Sargent, 'The Penn~enmawr Intrusions' Geol. Mag. dec. 6, vol. ii, 1915, p. 17). See J. V. Elsden, Q. J. G. S. vol. lxiv (1908) p. 27~. Q.J,G.S. No. 282. z Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

326 v~. 4.,5. cox o~ ~HE GEO~0OY O~ TmB l)xsT~r [voL lxxi, considerable extension is shown by the length of their outcrope. It is, in fact, quite possible that the masses of Pen Berry, Tremynydd, and Llanvirn on the south side of the Llanrian Syn- cline, and the Traeth Llyfn-Abeffelin intrusion on the north side, originally all formed par~ of a single sill intruded at, or near, the top of the TetrayraTtu8 Shales. The structure of the ground between Traeth Llyfn and Pen Clegyr suggests that some slight local folding had taken place before the diahases were inta-uded, but the main folding movements were undoubtedly subsequent to the intrusions. The evidence of other districts shows that the main folding in Northern Pembroke- shire was pre-01d Red Sandstone in date. It has already been mentioned that a diabase of the ophitic type occupies the core of a shallow syncline at Aberhenow, near Abercastle. A similar mode of occurrence is seen in the case of the Llech-Dafad I intrusion, where a quartz-bearing diabase occupies the centre of a syncline in Cambrian strata. $ Such occurrences suggest that both types of diabase were ihhmdod while some ~olding was actually in progress; but, whether these movements were merely forerunners of the main pre-01d Red Sandstone movements, it is not as yet possible to state with any certainty. The detailed examination of the Pen-Caer district may, perhaps, shed some further light on this subject.

The large mass of Pen Clegyr, near Porth Gain, consists of a quart~-diaba~ which is petrologically somewhat different from the other quartz-diabases of the district. The mode of intrusion of this mass is not easily determined, since the cliff-sections are rather inaccessible, while the section along the tramway-cutting leading into the large quarries is unfortunately at present somewhat obscured by Drift at the critical point. From the cliff-sections, however, it would appear that the igneous rock transgresses the bedding of the .Linyula Flags with which it is brought into contact. It is possible, though not probable, that the apparent r is due to faulting. It seems more likely that the intrusion is a laccolite of somewhat irregular form, owing to the rather massive character of the surrounding strata. The fault which throws Arenig rocks against the .Linqula Flags had to change its course, because of the proximity of this large diabase mass. It should be remembered that the throw of'this fault mas~ not be very great.

(2) Petrography.--The diabases need but little description, since the majority of them clearly conform to types well-known and fully described from other districts. Further, the principal occurrences in North-Wes4 Pembrokeshire have already been

1 Ll~h ~ ~ about a mile and a half n~-eut o~ll S~x~vr. 2 j. V. Elsden, Q. J. G. S. vol. lxi (1905) p. 588. A. H. Cox & O. T. Jones, Proc. Brit. Assoc. (Bh~ingham) 1913, p. 484. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

pal'~ 2] BETWEENABEREIDDY AND ABERCAS~'LE. 327 examined by Dr. Elsden 1; wherefore only the main points will be noted here. (i) Ophitic diabases without quartz.--The principal development of rocks of this type is in the eastern part of the area, m the ground north of Abercastle, although more westerly exposures occur at Caer Haled and Cam Llwyd. These occurrences on the mainland yielded rather fresher specimens titan that on Ynys C:tstell, described by Dr. Elsden,~ otherwise they agree closely with his description. The ophitie etmracter is well-m_arked, the augite occurring in large irregular crystals which are usually almost colourless, but sometimes of a very pale brown. There is no trace in these crystals of any structure parallel to (001) except in a few rare instances, where evidently it is merely a fracture caused by pressure. Rather obscure traces of enstatite are occasionally ~ound [P 65], the mineral now belng pseudomorphed by bastite and chlorite; but the pseudomorphs are usually so indistinct, that the original presence of a rhombie pyroxene cannot in all cases be satisfactorily determined [P 66], Chlorite is abundant, sometimes in irregular aggregates giving indigo-blue interference- colours, sometimes markedly fibrous and more strongly bire- fringent. Pleochroic haloes round small inclusions of sphene are of frequent occurrence. In view of the very fresh appearance of much of the augite, it seems improbable that all the chlorite replaces augite. At the same time the general appearance of the aggregates does not suggest that they are pseudomorphous after enstatite-erys~ls. The felspars are strongly zonal, and in the best-preserved examples EP 97] are shown by refractive indices and extinction-angles to have the average composition of a medium labradori~e, Ab 2 An 3. There is naturally some variation on either side of this point, the actual composition varying from that of an acid to that of a basic labradorite. In one rather weathered example [P 74] a certain amount of water-clear albite was found; but, as a rule, the albitization in these rocks is merely incipient. Another example EP 66] shows the presence of a small amount of interstitial orthoclase. Frequently, however, the felspars are too decomposed for accurate determination. As to other minerals, both biotite and primary hornblende are absent, although, in the more weathered examples ~P 741, a considerable amount of tremolite is often associated with the chlorite. Quartz is absent, except in cases where the felspal~ are completely decomposed, and then it appears only as a secondary constituent. In one example however, taken from near Pwll SVrodyr [P. 1.131, there may actually be a little quax~z that is of primary origin. In general, these rocks are evidently very similar to many of the diabases described by Dr. Harker from North Wales.3

1 Q.j.G.S. reel. lxi (1905) p. 579, &/b/d. eel. lxiv (1908) p. 273. /b/d. eel. lxi (1905) p. 593. 8 , The Bala Volcanic Series of Carnarvonshire' 1889, p. 80. z2 Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

328 j)R. x. H. COX O~ THE e~.OLO6I OF THE DXSTRXCT [vol. lxxi,

(ii) Quartz-enstatite-dlabases.--These rocks, of which the long sills running from Aberfelin to Traeth Llyfn furnish good examples, in their turn conform, for the most part, to such types of quartz-diabase as have frequently been described from many different areas. There are, in addition, some occurrences of more exceptional type which will be described later. Rocks of the normal type vary considerably in grain, often becoming exh~mely coarse and gabbroid in character. They differ from the ophitic diabases described above in several important respects. The augite is again colourless or brown, but it occurs usually in long-bladed prisms, and only very rarely encloses some felspar in subephitic fashion [P 73]. It frequently shows a basal striation, which becomes more obvious in partly-weathered examples [P 61]. , Twinning parallel to (100) is very common, and an hour-glass shmehtm is not infrequent. This structure, however, is unaccompanied by the strong dispersion of the optic axes that is associated with hour-glass structure in the augite of tesehenites and monchi- quites. The augite is sometimes bordered by a little hornblende. Eusta~ite is often fairly abundant in idiomorphic prisms, now completely replaced by serpenbine. In some specimens the pyroxenes become very subordinate, and may even be all but completely wanting EP 63], giving rise to a rock composed essentially of plagioclase and micropegmatite. This applies to rocks forming the main mass of the diabase, and not merely to aplitic veins. The plagioclase-felspars give large rectangular sections and prove, by refractive indices and extinction-angles, to be almost pure albite. They are full of small inclusions of secondary minerals: namely, calcite in small patches, ehlo14te in both large and small areas, and shreds of sericite, sometimes regularly oriented, sometimes in radiating sheaves, or, again, quite irregularly dispersed. This rather spongy appearance seems characteristic of a certain tyt~e of secondary albite. 1 Many of these large plagi0clases are surrounded by a thin zone, about "05 mm., of material which is much freer from inclusions, and occasionally is faintly zonM: this represents original albite or albite-oligoc]ase felspar. Round some of the larger of the chlorite-inclusions is a zone of water-clear albite, with a peculiar streaky t~inning recalling, at first sight, anorthoclase EP 73]. Micropegmatite is extremely variable, both in amount and in coarseness of grain. Often the felspar-rods grow out at right angles from one of the large plagioclase-crystals, while at other times the arrangement in the pegmatite is rudely centric. The felspar in the mieropegmatite is usually untwinned, but occasionally shows indications of cross-hatching, wherefore it possibly approaches anorthoclase in composition. Quartz is also frequently found in graphic intergrowth with the large idiomorphic I E. B. Bailey & G. W. Grabham, ' Albitization of. Basic Plagiochse- Felspars' Geol. Mag. dec. 5, vol. vi (1909) p. 254 ; see also J. S. Flett, in ' The Geology of the Country around Newton Abbot' Mem. Geol. Surv. 1913, p. 60 & pl. ii, fig. 4. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

part 2] BETWEENABEREIDDY AND ABERCASTLE. 329

plagioclase-felspars, and it further occurs in large irregular grains more or less surrounded by micropegmatite. Among minerals of minor importance is chlorite, which occurs abundantly in large masses, partly replacing ensta~ite and partly replacing the idiomorphic felspal~ as also the felspar of the mieropegmatite. Chlorite further occurs as irregular veins cutting through all the other constituents o]~ the rock. It contains numerous inclusions of sphene, which give rise to pleoehroic haloes. Altered ilmenite is plentiful, and there is often much secondary calcite. Except for the greater degree of alteration, these rocks bear a remarkably-close resemblance to the quartz-dolerites described by Mr. Tynrell from the Kilsyth-Croy district, 1 and to other well-known quartz-dolerites, such as the Whin Sill ~ and the Ratho intrusion near Edinburgh. a The Pen-Berry intrusion, mentioned on p. 324, belongs to a rather different type. This rock [P 174J has been fully described and analysed by Dr. Elsden, 4 who has pointed out its very close resemblance, both in appearance and in composition, to the well- known Penmaenmawr rock, so that it need not be further described here. Specimens from the various little masses round Porth-y-Dwfr [P 128, 129 & 1301 closely resemble this Pen-Ben'y type, except for occasional slight variations in the degree of coarseness, and thus strengthen the evidence that these occmTences were originally part of one continuous sill (ante, p. 324). A third variety of q tmrtz-diabase is found in the large mass of Pen Clegyr near Porth Gain. This is the peculiar (quartz-)albite- enstatite rock which, again, was described by Dr. Elsden) To his description I have nothing to add. On account of the high silica- percentage--61"45--and the texture of the specimen figured by Dr. El sden, Rosenbusch 8 suggested that the rock partook most probably of the nature of an aplitic vein. This, however, is not the case, as this particular type occurs in very large mass. All my specimens, taken from various parts of the extensive quarries in the years 1911-1913, belonged to this particular type. Since the rock is quarried at an average rate of 400 tons per week, it is clear that the mass is no mere vein, but is of very considerable extent. Thus, the main differences between the quartz-diabases on ~he one side and the ophi~ic diabases on the other, are (i) the pronounced tendency in the quartz-diabases to idiomorphism of, and the prevalence of sahlite structures in, the augites ; (ii) the greater 1 Geol. Mag. dee. 5, vol. vi (1909) p. 305. J. J. H. Tea]], Q. J. G. S. vol. xl (1884) p. 640. a Id. ' British Petrography' '1888, p. 190 ; and J. S. Flet-~, in ' The Geology of the Neighbourhood of Edinburgh' Mere. Geol. Surv. 1910, p. 306. 4 Q. j. G. S. vol. lxi (1905) p. 588. 5 Ibid. p. 592 & pl. xxxix, fig. 4. ' Mikroskopisehe Physiographie der Mineralien & Gesteine' 4th ed. vol. ii, pt. 1 (1907) p. 585, & vol. ii, pt. 2 (1908~ p. 1259. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

830 DR. I. H. COX ON THE GEOLOGY"OF THE DISTRICT [voL tlt3r

frequency of enstatite; (hi) the abundance of primary quartz, largely in micrographic intergrowth with felspar; and (iv) the plagioelases of the quartz-diabases are in celCcain cases markedly albitized, whereas in the ophitic quartz-free diabases the basic plagioclases tend to remain as such, and any albitization is of quite minor importance. This latter feature is in marked contrast to the behaviour of the otherwise very similar rocks of North Cornwall, in which region the quartz-free diabases are often completely albitized, while the quartz-diabases themselves have remained in this respect unaltered. 1 In Scotland, on the other hand, the Permo- Carboniferous quartz-dolerites are, often found in an albi~zed condition.~ It is usually noted that in any one district the less'basic rocks are more susceptible to albitization than are the more basic rocks. 3 The Pembrokeshire quartz-diabases are certainly less basic than the ophiti~.diabases, and therefore it appears only natural to expect them to be more distinctly albitized, always provided, of course, that the two types Were magmatically related one to the other. The contact-action of the diabases on the adjacent slates is limited in both the main types of diabase to a hardening effect, accompanied by a certain amount of bleaching and sometimes, but not invariably, of spotting. The slates then eventually assume a itaggy appearance, with only indistinct traces, or even with no trace whatever, of cleavage. Nowhere has anything in the nature of an adinole been found.

(E) Other Igneous Rocks. There remain one or two further occurrences of igneous rocks to be considered. The most interesting of these is the variolitic keratophyre exposed in the large quarry on Trwyn Castell, Abereiddy Bay. The rock is clearl~ intl~sive, and takes the form of a sill which is apt to transgress the adjacent strata--the Upper D.-bifldus Beds. I have not, however, been able to find it actually transgressing into the rhyolites of Trwyn Castell, as is stated to be the case by Mr. J. F. N. Green. 4 On the contrary, it appears invariably to be separated from the rhyolites by an appreciable thickness of D.-bifidus shales and ashes. The rock is fine-groined and very vesicular, the vesicles being infilled with chlorite. When fairly fresh, the rock is green in colour--owing to the large amount of disseminated chlorite; but it becomes very red on weathering, in which condition it sometimes exhibits a spheroidal jointing like a diabase. This type of weathering and jointing is never seen in the Abereastle keratophyres previously described. I H. Dewey & J. S. Flett, 'Some British Pillow-Lavas & the Rocks Associated with them' Geol. Mag. dec. 5, vol. viii (1911) p. 207, s E. B. Barley & G. W. Grabham, 'Albitization of Basic Plagioe]ase- Felspars' Geol. Mag. dec. 5, vol. vi (1909) p. 255. 3 Id. ib/d. p. 252. 4 Proc. Geol. Assoc. vol. xxii (1911) p. 228. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

part 2] BETWEEI~ /kBEREIDDY AND ABERCASTLE. ~31 Under the microscope, the rock is seen to be made up of about equal parts of felspar and of ground-mass [P 12]. The felspars, which appear to approach albite in composition, are more or less lath-shaped, and usually present very irregular terminations. They average 0"5 mm. in length by 0"1 in breadth, thus giving rise to a rock of distinctly coarser texture than that of the contemporaneous keratophyres already described. The grcmnd- mass is dark, and consists of a mixture of chlorite and .unindi- vidualized opaque material shot through in all directions with hair-like tibias of felspar- there is but little regularity in the arrangement of these felspar-fibres. Clear patches of chlorite represent infilled steam-cavities. In a specimen taken from the lower end of the sill, the felspars of the first generation are smaller, are frequently skeletal, and often have forked terminations (P 54, & P1. XXIV, fig. 5). At the same time, the ground-mass is more abundant and the hair-like fibres are more prominent. The latter now tend to assume a sub- radiate arrangement, the growths often springing from one of the larger felspars. The fibres are, as before, embedded in cMorite and accompanied by the ill-defined opaque material, which is sometimes dispersed quite irregularly, and sometimes is obviously pushed outwards during the growth of the felspars of the first generation. Elsewhere, however, the opaque material may grow out in thick clusters, springing from a central rod quite inde- pendently of the arrangement of the surrounding felspar-fibres. Well-individualized iron-ores are not at all abundant, except at the actual chilled contact, where small crystals of magnetite and pyrite suddenly appea~r in large amount. This marginal segregation of the iron-ores furnishes a small-scale example of the apparently- limited miscibility of sulphidic and silicate magmas) There can be no doubt that this variolitic keratophyre in the ~D.-bifidu8 Beds of Abereiddy is a n~re basic rock than the contemporaneous kera~phyres in the Tetra~qraptus Beds of the Abercastle area, and essentially different from them. The Aber- castle rocks never show any approach to a variolitic character, and, as remarked above, they never exhibit spheroidal weathering such as is found in the Abereiddy rock. It may be observed that there is no resemblance, either in the macroscopic or in the microscopic characters, between the varioIitic keratophyre of Abereiddy and the variolit~c "tachylytes' described by Dr. Cowper Reed 2 from Garn Fawr on Strumble Head, 8 miles north-east of Abereiddy. In view of the isolated nature of this one small intrusion, the rock does not, at present, seem deserving of a special chemical analysts. Pwll-Strodyr Fault-intrusion (felsite).--A felsitic rock has been intruded into the zone of rocks affected by the Pwll- 1 A. Harker, ' Natural History of Igneous Rocks' London, 1909, p. 141. Q. J. G. S. vol. li (1895) p. 183 & pl. vi, figs. 6-7. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

832 DR. A. H. COX ON THE (~EOLOGY OF THE DISTRICT [voL l~xi, Strodyr Fault (see p. 335), the intrusion probably taking place along a plane of weakness developed during one of the periods when the fault was active. The intrusion is about 50 feet thick, and behaves as a sill running parallel to the main structural direction in the surrounding slates, and likewise parallel to, but a foot or two below, the junction between slates and a crush- breccia. 9Macroscopically, the rock is compact and remarkably uniform. It is of a pale blue-grey, becoming cream-coloured on weathering, and breaks with a rather splintery fracture along pronounced master-joints. Microscopically, this rock presents little interest. It [P 111] consists almost entirely of quartz and felspars, forming a sort of mosaic, the texture of which varies between cryptocrystalline and mierocrystalline. Between crossed nicols numerous irregular patches of quartz and felspar stand out from the finer-grained base, but all the structures appear to be entirely secondary, true phenocrysts being absent. The only original structure is a rough flow-structure, picked out by thin wavy strin.gs of ehloritic material. This rock shows, both in the hand-specmaen and in thin section, a remarkable resemblance to some of the "silieified tufts of the Ynys-Castell Ashes (p. 299); but the mode of occurrence ig naturally quite distinctive. The exact age of the rock is at present uncertain. Further considerations of the petrological relationships of the rocks described above may be with advantage post~ned, until the still more varied types in the ground farther east have been described in greater detail. In that ground towards S~mnble Head, there is a greater development of rhyolites 1 and diabases 2 than in the Abereiddy district; and they are accompanied by more exceptional types, such as the variolitic taehylytes described by Dr. Cowper Reed. Still more recently, the existence of a great series of spilites has been recorded. 3 It would appear more oppor- tune to consider the igneous rocks of North-West Pembrokeshire as a whole, when the stratigraphical relations of the spilitic series have been more definitely determined.

IV. THE T~CTONICS. (A) The Folding. The folding is of the type usually found in Northern Pembroke- shire and adjacent pales of Carmarthenshire. The main structures range between east to west and east-north-east to west-south-west. The limbs of the folds are, as a rule, steeply inclined, and the middle limbs are generally vertical, or slightly overfolded to the south: hence it is evident that the pressures come from the north. 1 F. R. C. Reed, Q. J. G. S. vol. li (1895) p. 149. J. V. Elsden, ibid. vol. lxi (1905) p. 579. A. H. Cox & O. T. Jones, Rep. Brit. Assoc. (Birmingham) 1913, p. 495. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

part 2] B~.TWE~ ABEI~EIDDY AND ~BERCASTLE. 333

The main folds, the Llanrian Syncline on the south and the Pwll-Olva and Abereastla Anticlines on the north, have already been mentioned. The course of the~ main folds was probably determined by the hard resistant mass of the St. David's pre- Cambrian ridge, which lies immediately south of the district here described. In this connexion it is noteworthy that only in the extreme north of the district, around Aberhenow: that is, in the area farthest removed from the pre-Cambrian massif, is any considerable extent of horizontal, or gently inclined strata to he found (see section, fig. 2, p. 281). That the pressures were fairly intense may be inferred from the fact that it is only the most rigid strata, such as the Zin#ula Flags and the Porth-Gain Beds, that show in cliff-sections any signs of contortion and of minor folding. All the more argillaceous beds have been so squeezed together that, as a general rule, no plication is visible. The pitch of the minor folds is, where seen, always to the west, or west-south-west, at angles of from 5 ~ to 15 ~ This westerly pitch may be observed at several localities, including Pen-Porth Eger and.Aberhenow ; and it is particularly clear in the case of the anticline in the Zingula Flags, so well seen at one of the inlets on Trwyn Ellen (P1. XXII, fig. 1). As regards the major folds, the effect of any pitch is obscured by the numerous faults; but it is probable that the Abercastle Anticline, at any rate, pitches gently westwards in the same way as the minor folds. In this case the effect of pitch in determining outcrops is increased by the faults.

(B) The Cleavage. Cleavage has affected all the rocks of the district, with the exception of some of the igneous rocks. It strikes generally parallel with the main folds, and dips northwards at angles of 40 ~ r 80 ~ In the more argillaceous rocks the bedding has often been completely obliterated, while in the more resistant beds, such as the Porth-Guin Grit or some of the tufts, a lenticular jointing has resulted. Slates have been worked from many diffm~nt horizons :-- ~Lin#ula Flags, Tetragraptus Shales, JBij~dus Shales, and MurcM- soni Shales, but apparently nowhere with any great success, and all the quarries are now abandoned. Slates from strata other than JLingula Flags weather much too readily to be of any real value ; while the rocks of the Zinyula-Flag Group are too flaggy, and contain too frequent interbedded grit-bands. The cleavage is obviously later than the folding, and earlier than the faulting. Away from the faults ~hemselves signs of any folding subsequent to the cleavage were seen only at one or two localities, among which the east side of Pwll Olva may be mentioned. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

334 Dm A: H. COX ON THE GEOLOGY OF THE DIST]~ICT [voL l~

(C) The Faulting. The common faults are (1) the west-south-west strike-faults, and (2) the north-north-east transverse faults. There are also (3) certain other faults that take other co~lrses, but only a few examples of these last have been observed.

(1)--Much the nlost important are the strike-faults, which are so exceedingly abundant throughout the area that only the more important of them can be shown on the map. Their course is usually between east to west and east-north-east to west-south- west; but, as will be seen from the map (PI. XXV), they may bifurcate and swing about and intersect one another so as to form sometimes a perfect maze. They usually bade northwards at angles up to 60 ~ sometimes causing repetition, at other times cutting out whole groups of strata, ac.cording to the particular fold-limb in which the fault is situated, and also according to the inclination of the beds. The greater number act as normal faults. Usually these east-and-west faults are very clean-cut features. Where they pass through slates there may be some crumbling of the cleavage-planes, accompanied by the formation of a small thickness (often a few inches only) of powdery material along the fault-plane. Otherwise there may be no brecciation, even in the case of faults of large throw. In consequence, they have practically no effect on the inland topography, although in the cliffs they may give rise to planes of weakness along which the waves excavate caves and small inlets (see P1. XXIII, fig. 1). The fault which separates .Linffula Flags and Arenig beds probably belongs to this set of faults, although it often hades southwards instead of northwards. This fault, as also the one that bounds the Llan- deilo rocks at Abereiddy Bay, has already been described in some detail (pp. 289 & 306).

(2)--Transverse or dip-faults, with a north-east or north-north- east direction, are exceedingly prevalent, but almost invariably of such minor importance that their effect can hardly be marked on a map. Numerous examples can be seen in Traeth Llyfn and on Trwyn Castell. The north-to-south faults, which give rise to little inlets, also possibly belong to this series ; though, as a rule, 9the true north-north-easterly faults have no effect on the relief of the ground. Although the result of a single fault is usually too small to be mapped, yet the combined effect of a number of faults on any particular outcrop may be considerable. The general tendency is for the north-north-easterly faults to cause outcrops h> take a west-south-westerly course, instead of one almost due east and west.

(3)--Of a different type is the north-westerly fault that has given rise to Abercastle Harbour, and probably aiso to the valley which runs up from the harbour in a south-easterly direction past Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

part 9_-] BETWEEN ABEREIDDY AND ABEIICASTLE. 335 Trellyn. It is a fault of small throw which is later than the east- and-west strike-faults. The direction of hade is not seen. In fact, ~he fault is everywhere buried beneath alluvium, or is under the sea ; but its presence is clear from the occurrence of the gently- dipping Ynys Castell Ashes on both sides of the harbour. 9Another fault which Cakes a similar direction has already received mention, namely, the small fauit in Aberfelin which thrusts Porth-Gain Beds over Telra~Traptus Shales. The fault, which is clearly seen at sea-level as an apparently gently inclined crack, hades about 40 ~ north-eastwards, and has given rise to a deep chasm between the rocks (see P1. XXIII, fig. 3). Like the Abereastle Fault, it is later than the strike-faults ; but it does not appear to extend for any noticeable distance in a landward direction.

(4) The Pwll-Strodyr Fault and the rock-gro.ups tG the east of it.mThis fault is the largest dislocation in the distlSct. It has given rise to a narrow Drift-filled valley which takes a general north-westerly direction. The Pwll-Strodyr Fault thus differs from the common east-and-west faults, both in direction and in its marked effect on the surface-features. A further difference is that, instead of its being a clean-cut fault, it is accompanied by a wide zone of shattering, along which a fault- breccia has been formed and an igneous intrusion has made its way, while at a later date numerous quartz-veins have developed. It is evidently comparable with the great north-west lines of disturbance which a~ect the South Wales Coalfield. The fault is well seen where it runs out to sea on the eastern side of Pwll Strodyr. At this point the centre of the valley does not lie immediately over the fault-plane, but a little to the south thereof. As one goes north towards the fault, the Arenig slates are seen t~ be crumpled, contorted, and sheared, until they are almost in the condition of phyllites, while thick quartz-veins are very numerous. Then comes a 50-foot band of felsite intruded into slates, which, curiously enough, are not so much crumpled. A foot or so north of the igneous band, and coming on very suddenly, is what appears to be a fault-breccia, composed of a mixture of quartzite and of' slate: this breccia-band forms the eastern extremity of Pwll Strodyl'. The felsite has not invaded the accrual fault-brecciai and this, as also the fact that the zone of maximum quartz-veining is situated some distance from the fault-breccia, suggests that movement was repeated several times along almost, but not quite, the same line.

The formations north-east of the Pwll-Strodyr Fault.--Immedia~ely north of the breccia-band appears a series of quartzites interbanded with mudstones, which is well exposed in the bay immediately east of Pwll Strodyr. The beds dip steeply north-east towards a thick series of Lingula Flags, which extends thence for some considerable distance, again with a ' prevalent north-easterly dip. It is probable that the Lin.qula- Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

886 DR. A. If. COX ON THE GEOLOGY OF THE DISTI~ICT [VO]. IXXi,

Flags are faulted against the quartzite series by an of[shoot from the PwU-Strodyr Fault, and it is possible that all these strata immediately east of the fault are inverted. The age of the quartzites has not yet been determined, as no similar beds have ever been found in the country west of the Pwll- Strodyr Fault. Very similar quartzites, however (the Aberbach Quartzites), are well developed between Abermawr and Aberbaeh, 1 a mile east of Pwll S~odyr, and probably the Pwll-St-rodyr Quartzites belong to the same group. This quartzite group is possibly of Lower Arenig age, in which case it is certainly older than any of the Arenig beds seen west of the fault. The whole district east of the Pwll-Strodyr Fault, however, awaits further investigation.

V. Tmz PHYSICAL FEATURES. The plain of marine denudation.wThe topography throughout the district is dominated by the plain of marine denu- clarion, whichrises from a height of about 200 feet near the coast to over 400 feet farther inland (Pls. XXII & XXIII). This 400-foot plain is of very wide extent in Western Pembrokeshire, and is possibly of Plioeene age. -~ It has been considerably dissected by a valley-system which has long since reached maturity; the valley-sides have, therefore, gentle slopes, while the bottoms of the valleys are choked with alluvium, or sometimes with Drift. This valley-system seems quite inconsequent with regard to the solid qeology, except in so far as a valley sometimes follows a line of fault. Since all previous features have been obliterated by the formation of the plain, and since the valleys are inconsequent, it can be readily understood that the surfaee-featm.es are largely independent of geological structure. It is true that, where fhere is a considerable development of hard beds, as in the Lingula- Flags, a small feature rising slightly above the neighbouring shale- ground may be formed, but such features are of little if any help in mapping. Smaller masses of hard rock, such as the diabase-sills, often make not the slightest feature on the plain. Only very rarely are there any elevations rising above the general level of the plain. Occasionally, however, along the course of the igneous bands, stack-like masses project, exactly as do the stacks of[ the present shore-line. Examples of this may be noticed along the course of the keratophyre-bands near Longhouse and Abercastle. Such stacks, viewed from a distance, present a deceptive resemblance to small volcanic necks. Similar masses project occasionally also from the diabase-siUs, as, for example, near Llanvirn, and on Cam Llwyd near Traeth Llyfn. Farther west the hills of Pen Berry, Cam Lidi, etc., near St. David's, are well known as presenting similar examples, but on a larger scale.

I A. H. Cox & O. T. Jones, Rep. Brit. Assoc. (Birmingham) 1913, p. 484. J. F. 1~I. Green, Proc. Geol. Assoc. vol. xxii (1911) p. 130. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

part 2~ BETWEEI~ IkBEREIDDY XND ABERCASTLE. 337

Glaciation.--The whole district has been overridden by the Irish Sea ice-sheet, which has filled some of the smaller hollows with Drift. As a result, pebbles of various distincLive rocks from North Wales and from Scotland, such as the Ailsa Craig rock, may be collected on the beaches. The glaciated rocks on Pen Berry show that the ice-sheet reached up to a height of over 450 feet. Some account of the glaciaL'ion of the hills nearer St. David's has been given by Hicks. 1 while a paper by Prof. Jehu ~ contains a description of many of the glacial deposits of Northern Pem- brokeshire.

Drowned valleys.--The effects of a subsequent depression are seen in the formation of Abereastle Harbour, which is simply a drowned river-valley exactly comparable with the small harbours at Perth Clais and Solva, z near St. David's. The peculiar depression running from Perth Gain to Abereiddy has probably a similar origin. It is evidently part of a valley, of which both the head and the mouth have been lost owing to erosion by the sea. Except that it is filled with alluvium instead of being under water, it is in every way similar to the valley opening into Abereastle Harbour (see P1. XXIII, fig. 2). In fact, if it were not for a storm-beach at Abereiddy, the sea-waves even now would gain access to a part of the valley. The valley-slopes are well graded, and do not show any signs that the hollow owes its existence to erosion by the sea during a period of greater depression, as is suggested by Prof. Jehu. There is no evidence to show in which direction drainage originally took place along this valley.

VI. SUMMXRY iND CONCLUSIONS. If we now summarize the results obtained, it will be foun4 that in the Abereiddy-kbercastle district :-- (1) The Ordovici~n strata do not succeed one another in a simple upward sequence, but that they have been thrown: ' into great folds which are sometimes overfolded, and have subsequently been broken by extensive strike- faulting. The limbs of the folds increase in steepness as the pre-Cambrian ridge of St. David's is approached. (2) The folding' even brings up strips of Cambrian strata. Nevertheless, no true Tremadoc rocks occur. (3) There is a complete sequence of Ordovician rocks from low down in the Arenig Series to high up in the Glenkiln. (4) The lowest Arenig rocks seen are a series of arenaceous strata (the Abercastle and Perth-Gain Beds), .which correspond to the ' 2gesuretus Beds' of Ramsay Island.

1 Gl~.eialis*s'Magazine, vol. i (1894) p. 191. Trans. Roy. See. Edin. vol. xli (1904) p. 53. * J. F. N. Green, Prec. Geol. Assoc. vol. xxii (1911) p. 133: Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

~38 DR. A. H. COX ON THE GEOLOGY OF THE DISTRIC~ [voLLui, (5) These arenaeeous beds stand in faulted rehtionship to Cambrian rocks (Linyula Flags), so that the base of the Arenig ~ nowhere to be seen. (6) Only the higher beds of this arenaceous series are usually exposed, and these beds pass upwards without break into TetrayraTtus Shales. (7) The Didymogral~tus-b~dus Beds vary somewhat when followed from south to north, since the masSive ashes (Ynys-Castell Ashes), which form the lower part of the zone as developed in the northern part of the district, are feebly represented in the central and southern parts. (8) Llanrian volcanic rocks are represented on the west side of Abereiddy by the ~D.-murchisoni Ash, and on the east side by the Llanrian Volcanic Series. This volcanic series begins high up in the zone of D. bifidus, whereas the YD.-murc&isoni Ash marks exactly the junction of the zones of .D. b/.fidu8 and D. murchisoni. (9) Save that there is no distinct development of the .4saphus Ash and Asaphus Limestone, the Llandeilo rocks compare exactly with those of Carmarthenshire, and do not contain any contemporaneous igneous rocks. (10) Contemporaneous igneous rocks occur at two horizons in the district--(i) Keratophyres high up in the Tetra- yraTtus Shales, and (ii) quartz-keratophyres (soda- rhyolites) high up in the JD..bifldus Beds. Thin bands of ash also appear at various other levels in the Tetragraptus Shales and in the D.-bigCldusBeds. (11) The intrusive rocks (diabases) belong to two types :-- (a) Subophitic quartz-diabases, and (b) ophitic diabases" without quartz. Both types were intruded before the folding, and consequently previous to the cleavage and to the fault~ing. (12) :A great north-west line of disturbance--the Pwll-Strodyr Fault---cuts across all other structures in the district, and brings on entirely different groups of strata.

In conclusion, I desire to express my great indebtedness to Prof. O. T. Jones (whose observations at Abereiddy were originally the detemnining cause of the inception of this work) for the identi- fication of most of the fossils recorded in this paper, as well as for his invaluable help and advice in the field. I have also to thank Dr. Herbert H. Thomas for the interest that he has taken in this work, for advice on various points, both stratigraphical and petrological, and further for his help in the production of the map. To Mr. J. Pringle I am much indebted for showing me over the Ramsey-Island sections, and for his help in fixing the position of certain beds--the Ynys-CasteU Ashes. I am also indebted to Mr. N. G. Blackwell for numerous photographs of tl~e district, and ~o Dr. F. Barrow and Mr. (now Lieut.) E. W. Scofield for their companionship in the field and for help in fossil-collecting. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

part 2] BETWEEN ABEREIDDI" AND .~_BERCASTLE. 889 I have also to thank Mr. Perkins (of Llanvim) for his very kind hospitality. Finally, I desire to express my thanks to Dr. W. T. Gordon for reading part of the manuserip~ of this paper, as well as for help wi~h ~he phof~mierographs.

EXPLANATION OF PLATES XXII-XXVI.

PLATE XXII. Fig. 1. Folding and faulting in the L~tg~t~a Flags, Trwyn Ellen : view lool~ing west towards Perth Gain. The fold pitches westwards at about 15 ~ (seen in the lower part of the picture). The less massive beds (seen on the left) show cleavage, but not the more massive beds (seen below). See pp. 282 & 333. N.G. Blackwell, photo. 2. Rhyolites, turfs, and shales of the Llanrian Volcanic Series, Trwyn Castell.--The small cairn in the middle distance "is Cam Llwyd, formed by a diabase-sill which is continued in the rocks in the sea. The bay is Traeth Llyfn, bounded on the far (north) side by a diabase-sill which has bleached the adjacent slates. The beds are there vertical. See pp. 302 & 321. N.G. Blackwell, photo.

PLATE XXIII. Fig. 1. The entrance to Abercastle Harbour : view looking north- westwards from Ynys Castell (seen in the foreground). The thick- bedded rocks above the caves are the Ynys-Castell Ashes dipping towards the observer, and repeated in the foreground owing to faulting along the harbour. The structure along the coast is synclinal, despite the prominent little anticline. The ashes accordingly reappear north of the syncline, and form the cliffs with marked jointing opposite the distant island--Ynys Daullyn. The central portion of the fold is occupied by the overlying B~fidus Mudstones, capped by a diabase- sill (which forms the top part of the cliff beneath the wire fence). Ynys Daullyn is formed by the same diabase faulted down to the north, the fault giving rise to the gap between the island and the mainland. See pp. 298 & 324, and P1. XXVI, fig. 3. lq. G. Blackwell, photo. 2. A b e r c a s t I e Har b our : view looking south-eastwards. In the foreground are horizontal B~fid~s ]Kudstones, capped by a diabase-sill forming the grassy slope.. On the left is the island of Ynys Castell, the eastern half with the rugged cliffs being formed by the same diabase faulted down. The smoother part of the island is occupied by horizontal Ynys-Castell Ashes. The harbour is formed by a drowned valley along a transverse fault. The two gashes on the i~ght-hand side of Ynys Castell are due to small strike-faults. See pp. 298 & 334, and P1. XXVI, figs. 3 & 4. N.G. Blackwell, photo. 3. Aberfelin : view looking north-eastwards. The headland is formed by Perth-Gain Beds dipping steeply towards the observer, and passing conformably under cleaved Tetragrapt~s Shales. Near the sca-level a small thrust is visible. See pp. 285 & 335.

PI~T~ XXIV. Fig. 1. Rhyolite [P 15], Trwyn Castell, Abereiddy Bay, showing ttow- structure and felspar-patehes sometimes with concave outlines. Natural light. • 22. See pp. 302 & 317. 2. The same, viewed between crossed nicols ; but few traces of the original texture remain visible. X 22. See pp. 302 & 317. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

~0 DR. ~t. H. COX ON THE GEOLOGY OF THE DISTRICT [VOl. 1XXi,

Fig. 3. Keratophyre [P 178], basal breccia, Pw]] Llong, showing fragments enclosed by slate, which itself is full of small chips of igneous rock and of isolated felspar-crystals. The extreme edge of the slate is darkened, owing to the separation of iron-oxides. The large f~ag- ments of keratophyre show a chilled margin, which is not as dark as the unchflled central portion. Natural light. • 22. See pp. 308 & 312. 4. The same (a different part of the came slide), viewed between r nieols, showing the almost pflotaxitic texture with a flow-structure quite independent of the present boundaries of the fragments. It is only a coincidence that the flow-structure in both the fragments is similarly oriented. • 22. See pp. 308 & 313. 95. Variolitic keratophyre [P 54], intrusive, Trwyn Caste]], Aber- eiddy Bay. This rock is quite different from the contemporaneous keratophyre illustrated in the previo~ figures. The photograph shows skeletal felspars, surrounded by variolitic aggregates of felspar-fibres. The clear areas a~e patches of chlorite. • 160. See p. 330. PI~lnl XXV. Geological map of the district between Abereiddy and Abercufle, on the scale of 4 inches to the mile, or 1 - 15,840.

PI~nB XXVI. [Sections on the scale of 6 inches to the mile.] Fig. 1. Section from Berea to Pen Clegyr. 2. Section from Trevine to Casts]] Coch. 3. Section along the west side of Abereastle Haxbour. 4. Section along the east side of Abereastle Harbour.

DISCUSSION.

The SECRETARY read the following letter received from Mr. J. PRINGLE :-- ' I regret my inability to be present this eveu;ng, particularly as I am specially interested in the area described 'by the Author. I am familiar with thedistrict, and have the additional advantage of having seen most of the sections under his guidance. Moreover, I have been able to compare his conclusions with the results of my researches on Ramsey Island. ' The two areas are almost identical, and all the points brought forward can be confirmed by the sections on the island. 'The Author is to be congratulated on having established the age of the LlaniSan lavas. They clearly belong to the same period of ~aleanicity as the lavas and ashes on Ramsey Island. In each instance they rest on the same hGrizon--" the "f~re~a~s" beds, and therefore their correlation with similar beds in other districts is now a comparatively easy matter. One notable absentee On the mainland, however, is the quartz-porphyry which forms so characteristib a feature on the island; but it is worth noting that, like the diabasee, the porphyry is a folded sill. The coarseness of the tufts on Ramsey Island suggests that they were deposited much nearer the focus of eruption th~n the beds at Abe~iddy. ' ...... ' The base of the Llanvirui~ at Abercastle, and at Abermawr (m the iRland, is simi~ly forms ~ of flue-grained silioeous t~s, Which are lithologisally ii~dis~;~ble ~from the "china-stones" near tl~e base of the Hope Shales. The presence of the cgnglomerate in t~e Arenig Series at Abercastl e cannot be correlated, so far as I am aware, in West Wales, and it may be of p~a~ely local occfir~ence.' Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

par~ 2] BETWEEN ABEREIDDY AND ABERC~STLE. 841

Prof. O. T. JO~rES, in congrahllating the Author upon the accomplishment of his work in a difficult region, wished to call attention te a paleontological question which was involved. Since it had been proved that the Upper Llandeilo of Hicks was the Upper Arenig or Lower Llanvirn of the same Author repeated by 9 strike-faulting, it became necessary to re-examine the fauna of the Upper Llandeilo, which was maintained by Hicks to show considerable differences from that of the Lower Llanvirn. ' Comparison between the structures in the Abereiddy and those in the North Cardiganshire areas showed that the ~olding and strike- faulting belonged te the same general movement, and in Pembroke- shire there was abundant evidence that this preceded the Old Red Sandstone and belonged to the Caledonian. The continuous change in the strike of the rocks from approximately north and south in :North Cardiganshire to east and west in West Pembrokeshire, proved how unreliable a guide the mere direction of movement might be in determining its age. In connexion with De la Beche's suggestion that the east-and-west strike of Pembrokeshire Was due to the superposition of the later Armoriean upon an earlier Caledonian movement, the speaker believed that there was little evidence in support of the idea, and that it was more probably due to the influence upon the folding of rigid masses of rocks such, for instance, as the pre-Cambrlan cores of St. David's and neighbouring areas. Dr. J. V. ELSVE~ said that the Author had worked out an area of considerable Cbmplexity, both from a stratigTaphical and from a petrological point of view. With regard to the Llanrian rhyolites, he-hoped that it might now be possible te 'correlate those of the 9Prescelly district, the horizon of which was as yet unknown. With regard to the rocks termed by the Author keratophyres, some at least of these appeared to the speaker to be intrusive, but the Author regarded them as flows. There were several exposures of the brecciated variety, one occurring at Priskilly Fawr, 4 miles south of l~[athry. He hesitated to accept the Author's conclusion that the sills of diabase had been folded with the s~Tata. These bore little or no evidence of crushing, and appeared to him ~o be of post- movement age. He congratulated the Author upon the success of his labours in a difficult area. Dr. HE~BERT H. T~o~s expressed the opinion that the Author's work had done much to simplify the classification of those beds that made up the central portion of the Ordovieian System. The realization of the fact that the Upper Llandeilo of earlier writers was non-existent, and that the remarkable lithology and fauna ascribed to these beds really belonged to an older formation, cleared the way for a complete and harmonious correlation with the Llandeilo rocks of Llandeilo and the rest of Carmarthenshire. With regard to the Porth-Gain and Abercastle Beds, the speaker thought that there could be very little doubt as to their Arenig age. In Carmarthenshire, Arenig beds containing Ogy.qia selwyni and Phyllo.qraTtus were followed downwards by similar deposits containing O. selwyni, Peltura, and Parabolinella ruqosa. These Q. J. G. S. :No. 282. 2 x Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 16, 2016

342 e~.oLoox OF NORTH-WEST PEMBROKESHIRE. [VOl. lxxi, lower beds were of Tremadoc age, and might be correlated with the upper part of the Ueratopyye Limestones. There was no trace of deposits yielding such a Tremadoc fauna in Pembrokeshire, and this absence pointed to unconformable relationship between the Axenig deposits and the Lingula Flags. The AVTHOR, in reply, mentioned that the presence of the conglomel~tie grit of Ynys Castell at the base of the Bifidus Beds was quite unexpected, since no similar bed was known at that horizon elsewhere. Comparison with the Ramsey-Island sections, however, left no room for doubt as to the correct position of this bed. He was much indebted to Mr. Pringle for showing him those sections. The Author was in entire agreement with Prof. O. T. Jones, that the main folding was, despite its direction, due to the movement usually described as Caledonian. With regard to the rhyolites described by Mr. Parkinson from the Prescelly Hills (20 miles east of Abereiddy), their age had never been definitely established, owing to the.scarcity of fossils. They might be on the same horizon as the Llanri~n lavas, but it was also possible that they occupied some lower horizon in the Arenig. The presence of keratophyres towards Math.ry, farther south- east than the bands shown on the map, was preb~bly due to repetition by folding, and was not in any way connected with the presence of the neighbouring Pwll-Strodyr Fault. In the area between Abercastle and Mathry there was a widening of the Tetra- 9raptus-Shale outcrop owing to the pitch; and in this area the keratophyre was found on both sides of a line which appeared to be a continuation of the anticlinal axis bringing up Porth-Gain Beds in Pwll Olva. The fresh condition of the brecciat~