121

THE OF THE DISTRICT AROUND ST. DAVID'S, PEMBROKESHIRE.

By J. FREDERICK N. GREEN, B.A., F.G.S.

[Read March 3rd, 191f.1 Advance copies of this Paper and of the Paper on Ramsey Island were printed and issued to Members in connection with the Easter Excursion of IgII. The two Papers are now reprinted with a few additions and alterations.

INTRODUCTION. ---HE Council of the Geologists' Association have done me 1 the honour of appointing me Director of an excursion to St. David's.'*' My experience of the district is not extended, and during my stay there I have devoted myself wholly to the struc• tural problems, leaving on one side the zonal paheontology and surface geology. Most of what follows must therefore be looked upon as a compilation from the papers mentioned in the Biblio• graphy (p. 136). Further, the area has been very insufficiently worked, especially in respect of its remarkable glacial geology, so that in these notes there must be many imperfections, the elimination of which may, I hope, be advanced by our excursion. The structure of this region, properly named" Dewisland," has occasioned one of the most severe disputes in the history of British geology. This controversy attained grave proportions owing to its bearing on other areas and on the generalisations drawn therefrom. I should not have ventured to assume in these notes the correctness of the interpretation given by myself in 19°8, which substantially upheld the main contentions of the late Henry Hicks, who made this corner of a classical district for the study of our oldest rocks, had I not been authorised by the kindness of Mr. H. H. Thomas and Professor O. T. Jones to say that, in a forthcoming paper on country some miles farther east, they would describe a closely similar succes• sion, viz., a series of tuffs penetrated by ancient crystalline intrusions and overlain unconformably by the basal . The stratigraphical interpretations of those who previously studied the district were very various in detail, but settled down to certain main points of difference: (I) The existence or non-existence of an unconformity above the ancient tuffs (Pebidian) which lie at the base of the stratified series. (2) The Pre-Cambrian or Post-Cambrian age of the St. David's granitoid rock (Dimetian). (3) The relationship of the quartz-porphyries; whether Pre-Cambrian and distinct (Arvonian) from the Dimetian or not. • This refer. to the Easter Excursion of '9". PROC. GEOL. AssoC., VOL. XXII, PART 3, IgIL] II 122 ]. FREDERICK N. GREEN ON

These structural questions are, however, not now of much interest compared to those presented by the two great groups of fossiliferous and igneous rocks. Although St. David's itself is difficult of access, once there almost all the main points of interest are within easy reach. A circle of three miles radius with the Cathedral as a centre con• tains nearly all the best exposures, including fourteen miles of magnificent coast-line and the fine sections along the Alan, Caerbwdy and Porthyrhaw Valleys. The Solva Valley lies close along the circumference. The roads and tracks are good.

GENERAL STRUCTURE. A fold of Cambrian and Ordovician rocks, with axis about east-north-east and west-south-west, exposes a core of Pre• Cambrian tuffs and intrusions, usually about a mile in diameter. St. David's itself lies on this core one-third of a mile from the southern boundary of the Cambrian. The fold is overturned south-west of the city, flattens eastward to a more symmetrical arch, but is overfolded again and strongly thrust when seen in the Upper Solva Valley, three miles east-north-east. South-east of the St. David's axis other overfolds and thrusts occur, one of the latter bringing the Cambrians over the Millstone Grit and Coal Measures at Newgale Sands, six miles east of the city. Newgale is situated near the seaward termination of another core of Arch~an rocks, parallel to the St. David's core, the intervening strip being, so far as yet ascertained, wholly occupied by lower Pal~ozoic beds with various intrusions. This general structure is, however, complicated by a multitude of faults, mostly small, but sometimes with a throw of two to four thousand feet. Many of these faults are reversed. In the neigh• bourhood of the city they have usually been flooded with basic material which has accumulated in considerable masses west and north of St. David's. When in Pliocene times the sea wiped out the traces of earlier denudation by planing down the country to a platform, now standing at a level of about 200 ft. above O.D., these larger intrusions with some other bits of hard rock were left standing out as rugged" stacks," which now constitute the most characteristic feature of the local scenery. It is now proposed to give a general account of the rocks and '3tructures of the district in their chronological order.

PRE-CAMBRIAN. Volcanic ROCRS.-The oldest rocks of the district are the Pre-Cambrian Volcanics or Pebidian, which, in the St. David's axis, consist entirely of tuffs, for the most part detrital. THE GEOLOGY OF THE DISTRICT AROUND ST. DAVID'S. 123

They are probably of the same age as the Bangor Volcanic series, the Uriconian, and perhaps the Charnwood Rocks. A thickness of over 3,000 ft. is exposed. Excellent sections me seen close to the hotel, but they are best studied to the south-west, especially about Porth-henllys and Treginnis-isaf. Table I (p. 134) gives a conspectus of the succession, but for a more detailed description I must refer to my paper of 1908.* The compositicn is very varied, including pebbles ranging from acid rhyolite, through trachyte and hornblende• andesite to augite-andesite, with acid and basic glasses, bits of halleflintas and other tuffs, and broken crystals of quartz, felspar, augite, apatite and magnetite. Among the most interesting points are boulders of red fluidal rhyolite scattered through the Treginnis series, andesite with ferruginous pseudomorphs after hornblende in the peculiar quartz-felspar tuff at the top of the Penrhiw series, bedded halleflinta quarried in the Caerbwdy Valley, and the sheared tuffs of Porthlisky. At Pointz Castle rhyolite flows are intercalated. Intrusive Rocks.-Special attention must be given to the quartz-felspar-porphyry sill, mimicking an augen-gneiss, which, except in the extreme west, comes in above the Penrhiw series. Its schistosity appears to be due to pressure while still hot and viscous; and it can be traced from Treginnis-uchaf into the porphyritic margin of the "Dimetian" near Rhoscribed. The sill, being hard, is often exposed, but its junctions are rarely seen. The roof can, however, be observed at Porth-henllys. The Dimetian granophyre, the true nature of which was first detected by Sir A. Geikie, is itself of interest owing to its Pre-Cambrian age and to the alteration which it has undergone. It covers an area immediately south-west of the city two miles in length and about a mile in greatest breadth. The rock is much jointed, often lesembling a rudely-bedded rock, especially when the joints are filled by slicken-sided calcite or ferruginous material. It is traversed by bands of brecciation, formerly mistaken for tuffs. The ferro-magnesian elements (biotite and perhaps horn• blende) are replaced by chlorite. The margin is delicately granophyric, with some felspar and many well-iormed bipyramidal quartz phenocrysts. It is best seen near Rock House, where it is cut by an elvan; but no exposure of the actual junction with the tuffs is as yet known. The quartz-porphyries associated with the granophyre are best developed in and north of the city, but are also found at St. Non's Bay, Pen Pedol, Rhodiad, Newgale Sands, LlanhowelI, etc. Beautiful spherulitic structures are developed in the finer grained varieties, and have been frequently described. More usually the base is micropcecilitic. The Cathedral and neigh• bouring buildings are founded upon a coarser rock with micro- " Quart. Journ. Geol. Soc., vol. lxiv, p. 363. 1 2 4 ]. FREDERICK N. GREEN ON gramtlc matrix. Sometimes the quartzes are spherical and weather out like pebbles.

PALlEOZorC.

Lower Palaozoic.-The actual junction of the Cambrian and the Pebidian is exposed on the south coast on both sides of St. Non's Bay, and on the west coast at Maen Bachau and Ogof Go1chfa (Whitesand Bay). The basal conglomerate contains a variety of rocks, including jasper, porphyry, many quartzites and boulders of tuff. A point of interest is the red staining which affects rock immediately underlying (in a stratigraphical sense) the conglomerate. The stained tuffs show a replacement of epidote and chlorite by hematite, and of felspar by a mixture of ferruginous and transparent products. This alteration, which seems similar to that which occurs under the Torridonian in the Highlands, usually extends about 15 feet from the junction and is clearly older than the folding. It has not affected the Granophyre. The relation is well shown at the Natural Arch in St. Non's Bay (PI. XX), where stained Pebidian tuffs with Porcellanite bands, belonging to the highest part of the Caerbwdy series, are thrust over the basal Cambrian conglomerate, which is seen in the lower part of the cliff resting unconformably on the tuffs. The conglomerate is again thrust against the (stratigraphically) overlying green flaggy sandstone, and the tunnel has been excavated along the band of shattered rock. This section has been described and sketched in the Quarterly Joumal ofthe Geological Society by Geikie (vol. xxxix, p. 287), Hicks (vol. xl, p. 525), and Lloyd Morgan (vol. xlvi, p. 246). The only place where a normal junction of the Cambrian and "Dimetian" is known is near Porthclais, the little port of St. David's, where a trench dug by Mr. George Barrow and my• self exposed felspathic grit resting on decomposed granophyre. The rocks are very finely exposed about this spot, which has been described and mapped several times in connection with the controversy as to the age of the Dimetian rocks, the numerous junctions being variously claimed as intrusive or faulted. The accompanying table (No. II, p. 135), which has been compiled from the publications of Henry Hicks, gives a con• spectus of the Cambrian and Ordovician formations, as developed near St. David's. The lowest, viz., the Caerfai Beds, can be studied at Caerfai Bay and at other points on the coast south of the city. They are of interest, not only for the they contain, but for the thin bands of felspar-tuff which occur in red Lingulella primava-shale. These tuffs have been claimed both as contemporaneous and detrital, without, unfortunately, any PROC. GEOL. Assoc., VOL. XXII. PLATE XX.

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NATURAL ARCH IN ST. !\ON'S BAY, PEMBROKESHIRE (FROM THE WEST). (Replodllced by kind pe11Jlissio" of MI'. W. Lt. Eva"s.)

10 file P. l~..j.. THE GEOLOGY OF THE DISTRICT AROUND ST. DAVID'S. 125 statement on either side as to the differentiating criteria. They are composed chiefly of well-shaped felspars without fragments of older consolidated tuffs, and the absence of any sign of attrition points to aerial carriage and quiet sinking. It is probable, how• ever, that the matrix of the green and purple sandstones consists mainly of fine muddy detritus from the Pebidian. Above the Caerfai Beds come the Solva Beds, well developed round the lovely village of that name, three miles east of St. David's. They build a considerable part of the southern coast• line of our area, and are nearly 2,000 ft. in thickness. The most fossiliferous part is the yellow flagstone at the base (zone of Paradoxides harknessi) which has yielded some remarkable organisms, notably Plutonia and Protospongia. The Menevian or Upper Paradoxidian fauna was first dis• covered by Salter at Porthyrhaw, about two miles east-south-east of St. David's. The lower part (grey flags with P. hicksii) resembles the Solva beds lithologically, and may be separated from the typical Menevian black slates by a non-sequence. The fossils include some rare trilobites. At the top comes a sand• stone, which separates the Menevian from the great Lingula Flag group, over 2,000 ft. thick. Very little seems to have been done towards the zonal subdivision or mapping of the Lingula Flags in Dewisland, though most of the divisions determined in North Wales are known to exist. They will be seen at Porthyrhaw and Whitesand Bay, where they are overlain by Tremadoc* Beds. Only the lower part of the Tremadocs will be seen at Whitesand Bay, as the rest is faulted out, but there is a more fossiliferous occurrence on Ramsey Island. Hicks has described thirty species from the neighbourhood, including two of Niobe and four of Neseuretus. Except in the coast exposures, these Cambrian Beds have been very little worked for fossils; but, nevertheless, they are remarkable for the variety of species obtained. For instance, of twenty-six species of Agnostus described by Mr. Lake in his monograph of British Trilobites, no less than fourteen have been found in the immediate vicinity of St. David's. Further research, therefore, may be expected to yield many prizes. The Ordovicians are exposed on the north of Whitesand Bay and along the northern coast, but the most notable sections are at Abereiddy Bay and Llanvirn, five miles north-east of the city. It was the study of the fossils from this locality which led Hicks to intercalate his Llanvirn group between the Arenig and Llandeilo. The beds are known to be highly fossiliferous, and rich in both trilobites and graptolites, but, so far as I am aware, no serious attempt has been made to collect any fossils but

* I.e., Tremadoc of Hicks. These Neseuretus·beds are now known, from researches in other parts of Wales, to be of Arenig age; but I have not attempted to amend Hicks's nomenclature. 126 ]. FREDERICK N. GREEN ON graptolites from them for some thirty years. Tuffs and acid lava flows occur in them and are well shown near Abereiddy. Miss Elles published a short account of the Arenig and Llanvirn graptolite zones in the Geological Magazine for J 904. correlating them as follows: Zone of Didymograptus murchisoni.-Black slates of the south end of Abereiddy Bay. Zone of D. bljfdus.-Pale slates quarried at the south end of Abereiddy Bay; also at Porth Hayog (or Llauog), Ramsey Island. Zone of D. hirtmdo.-Upper black slates of Road Uchaf and Road haf, Ramsey Island (with D. sparsus). Zone of D. exte1lSlts.-Main black slates of Road Uchaf and Road Isaf; also Porth Lleuog, north of Whitesand Bay. ? Dichograptus-beds.-Slates near Trwyn Hwrddyn, White• sand Bay, .with Dendrograptus fauna. The members of the Association will be able* to visit all these exposures and also the great sections of Llandeilo Beds, but are not likely to have a chance of studying the Bala, though this group covers a large area to the north-east. Its extension north• wards has been described by Mr. Cowper Reed. t The Silurians, though developed not far off, do not appear actually to enter our area. Upper Palaozoic.-Millstone Grit and Coal Measures come in south of a thrust at Newgale Sands, and doubtless underlie much of St. Bride's Bay. They contain some good plant-beds. The Millstone grit is a greenish delicately-jointed rock with conglomeratic bands and plant remains.

FOLDING AND FAULTING. In chronological order the next question for study is the folding and faulting of the district, much of which is of post lower Coal-measure date, belonging, no doubt, to the Armorican system, though unconformities in the lower Palreozoic series afford evidence of earlier movements. As already stated, the fundamental structures are folds with axes about east-north-east and west-north-west, showing a tendency to overturning from the north-north-west. At certain points this tendency is increased and thrusting occurs. A beautiful example of the mechanics of faulting is afforded by a little reversed fault which seems to be connected with the resistance offered by the "Dimetian" granophyre. The struc• ture appears first near Solva, as a little S-shaped fold, and, further east, can be seen at Porthyrhaw, where it is shifted by the great Porthyrhaw fault. At the precipice of Ogofyffos, one and three-

* This refers to the Easter Excursion of I9tr. +Quart. Journ. Geol. Soc., vol. Ii (1895), p. 149. THE GEOLOGY OF THE DISTRICT AROUND ST. DAVID'S. 127 quarter miles from the city, it appears as a sharp little overfold. A little west of this point the fold breaks, so that it is seen at Caerbwdy Bay as a broken overfold, the core of which, resem• bling the hull of a stranded vessel, is a well-known local curiosity. The discontinuity then becomes a simple reversed fault, which can be seen again and again in the cliffs, gradually incliped more sharply and descending in the Cambrian series. It is shown with diagrammatic clearness in the floor and side of a little bay south-south-west of St. Non's chapel, and reappears, bringing the Dimetian over the basal conglomerate at an angle of about 40° to the horizontal at Porthclais, beyond which it has not been certainly traced. In the gorge of the Upper Solva there is a fine example of

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NEWER SERIES OF INTRUSIVE ROCKS. After the main faulting the area was invaded by basic intru• sions, belonging to several types and probably at different periods; but the relative ages have not yet been discriminated with certainty. Those north of S1. David's have been described by Dr. Vincent Elsden, whose papers should be consulted.':' He has shown the predominant intrusions to be of comple'x compo• sition, ranging from a basic biotite-norite to an acid quartz• enstatite-diorite, and finally to soda-aplite, representing simul• taneous intrusions of an imperfectly mixed magma. The proportion of magnesium is high, and the rocks are characterised by the presence of a peculiar pyroxene (sahlite or magnesium• .diopside) which is believed to be an ultra-micrographic inter• growth of rhombic and monoclinic pyroxene. They can be :studied on Carns Llidi and Lleithr and at the headlands north of Whitesand Bay. Similar rocks form large masses near Llanvirn, and to them, no doubt, Cam Ysgubor on Ramsey Island must be added. This type of rock has been found in a well south of the city, but most of the later intrusions in the immediate neighbourhood .are little dykes, often much decomposed, which fill fault-fissures and ramify plentifully, especially in the Pre-Cambrian Series. They appear to be augite-andesites and leucophyres. To the west, however, touching the coast at one point, is an extensive, irregular mass, about two miles long, formerly supposed to be built up of lavas of Pebidian age. It seems really to belong to the same group as the little dykes. In 1883 Sir described some specimens as ·olivine-diabase. So far as I have been able to examine the rocks they show great variations. They usually exhibit irregular flow• :structure and are often amygdaloidaJ. Micro-slides commonly contain much glass, and one specimen consists wholly of magnetite, augite and glass, with some hematite pseudomorphs * See the Bibliography, p. '36_ 13° ]. FREDERICK N. GREEN ON after olivine, and might be termed a limburgite. Others are augite-andesites. Glomeroporphyritic aggregates of augite and iddingsite are common. The main mass can be studied near Rhoson and Trefeithan, where the relations with the tuffs are well shown; and smaller masses are shown at Clegyr-foia and at the extreme south·west of the peninsula, notably at Carnarwig, where• large fragments of the Cambrian conglomerate have been enveloped (see photo, Quart. Journ. Geol. Soc., vol. lxiv, 1908, P·381 ). The coarse doleritic (?) masses near Solva, which may bear a laccolitic relation to the Cambrian, do not seem to have been described. Intrusions of lime-bostonite and porphyrite occur near Abercastle about seven miles north-east of St. David's. Much of Ramsey Island is composed of porphyrite. Although all these basic rocks are later than the main faulting, some movements took place after the intrusion ofat least a part of them. At Porth Sele a dyke, caught in a minor fault, has been squeezed to a schistose anthophyllite-chlorite rock. This fault is. probably a branch of the great fault which runs nearly east and west through the city.

PLIOCENE. Since the folding and, at least in part, since the injection of basic material, denudation of the mo!>t stupendous character has. taken place over the area. The Carboniferous, the whole of the lower Palreozoic, and a great depth of Pre-Cambrian (which had of course also been vastly denuded before Cambrian times), probably in all more than six miles in thickness of strata, were worn off the arch of the fold, and the next evidence of marine action is the plain, no doubt of Pliocene age, which forms the foundation of the scenery of Dewisland. The platform slopes gently westward, the general level being about 150 ft. above O.D. on Ramsey Island, 200 ft. near St. David's, and 250 ft. about Caerfarchell and Llanhowell. Rocky knolls rise above the plain in all directions, marking the outcrop of hard intrusions. This. type of landscape is impressively displayed on Ramsey Island, where the steep hills of Cam Ysgubor and Cam Llundain spring abruptly from a flat platform of sedimentary beds and porphyrite, encircled by precipitous cliffs.

GLACIAL. The glacial deposits have not been very closely studied.. Dr. Jehu divides them as follows: I. Lower Boulder-clay. Tough grey-blue clay with shells and chalk·flints (inte,. alia). Pits at Trefeithan, Dowrog, Tretio and Caer• farchell. SE NW Riv r M,dcll Mill C yfClYC~ II BTld.&

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I~e F~r a to J="' Pellidian. It= :l ,r:ai ned. )- Oh'3 S=. hi ill. 641S= si intrusion ult. 13 2 ]. FREDERICK N. GREEN ON

2. Sands and gravels-yellow, often with shells. Not well shown near St. David's. 3. Upper Boulder-clay-yellow-brown and very stony. No shells Whitesand Bay, Porthlisky, Caerbwdy, Abereiddy, Pont Clegyr, and near the city. 4. Rubble drift. The most abundant erratics are derived from the quartz-norite series of Carn Llidi, etc. They are found everywhere and enter largely into the composition of walls. The direction of striations at Whitesand Bay and Rhodiad show that the ice came from the north-north-west. It must have reduced Llidi, Penbery and the other northern hills considerably. Other boulders are more interesting. They are from rocks in the south-west of Scotland and occur in great quantities, notably the Ailsa Crag paisanite and the Galloway granites and porphyrites. Picrite and jasper, probably from Anglesey, are also found. I do not think the existence of Irish erratics is certainly established. A mass of hornblende-picrite near Porthlisky has been described by Professor Bonney. Some peculiar facts may here be noticed, which seem to point to the existence of interesting pre-glacial and po~'t-glacial features. The writer, however, knows too little of glacial geology to be sure whether his few scattered observations, made while working at another problem, really justify his conclusions. A band of drift runs roughly east-west not far north of St. David's. It is sectioned at Whitesand Bay, where it clearly fills an old hollow descending below sea-level. The following is the succes• sion on the southern flank, beginning from the top: 5. Blown sand. 4. Head. 3. Lenticles of blown sand. 2. Boulder-clay with varied erratics. r. Layer of grettt boulders. Cleaved Cambrian rock. A minor band runs south from the main mass to Porthlisky Bay. Pwll Trefeithan lies on it. Neither of these mark existing lines of drainage, and it is plain that for some time after the retreat of the ice the drift lay intact across Whitesand Bay, as iron-pan, marking the site of an old marsh, occurs on both sides of the valley at a height ofabout 140 ft. The Caerbwdy Valley, which contains much drift, brings a little drainage from the northern boulder-clay areas. But the main drainage of Western Dewisland passes along the steep-sided gorge of the Alan. This breach contains, so far as I have seen, no boulder-clay, though there are places where, if the valley existed pre-glacially, it might be expected to remain. North of the city is an area lying a little below the true plat• form-level, and along the northern edge of this area, near the base of the hills, a silty deposit is found. THE GEOLOGY OF THE DISTRICT AROUND ST. DAVID'S. 133

I suggest that we have here an important pre-glacial river• valley, running westward with the slope of the Pliocene platform and sectioned at Whitesand Bay. Soundings seem to point to a submarine valley-system in the neighbourhood. Porthlisky and Caerbwdy Bays are sections of tributaries formerly running north. On the retreat of the ice it was no doubt banked up behind the northern ridge, perhaps sending a tongue south through Ramsey Sound. The country was covered with a fairly uniform sheet of drift, which bore a small lake north of St. David's. This lake would be fed with quantities of water from the north, which had to escape southward. Cutting through such drift as lay there, the outflow, which summed up the water from a large area of ice, found the weak spots in the granophyre and so carved out the present Alan Valley, which follows lines of fault along most of its course. With the further retreat of the ice the water supply shrank and the River Alan was reduced to its present small dimensions. The old drainage is, however, tending to reassert itself, as the little rivulets now trickling into White• sand Bay are cutting back and should eventually intercept the upper waters of the Alan. It has been thought that the steep sides of the Alan and Solva Valleys are due to marine erosion; but, as can be seen at Solva Harbour, wave action is almost suspended in such confined places. Further, looking at Solva Harbour and Porthclais and at the neighbouring soundings, it is impossible to avoid the con• clusion that there has been a recent positive movement and that these coves are simply drowned valleys. Mr. H. H. Thomas informs me that much of St. Bride's Bay must be of comparatively recent formation, the sea having swept out softer Carboniferous beds between the hard masses now tipped by the lavas of Skomer and Ramsey Islands.

POST - GLACIAL. "Submerged forests" exist at Whitesand Bay and at Newgale Sands. The late Henry Hicks has recorded red deer and bear from the former. That at Newgale is of historical interest, having been mentioned as early as the twelfth century. A fine storm• beach occurs here. Andalusite has been found in the sands by Mr. Thomas. Several pools, which are rapidly silting up, still remain. Dowrog and Trefeithan are the most noticeable. Alluvium and peat cover considerable areas. The Burrows afford good sections of blown sand with land shells, etc. The coast displays the effect of marine denudation on rocks of varying texture and in• clination, especially at Porthlisky, where the sea has advanced between two lines of fault; and curious examples of what appear to be marine pot-holes are seen at Whitesand Bay and Carnarwig. 134 J. FREDERICK N. GREEN ON Lastly, though not strictly geological, the antiquities of the district must be mentioned. Cromlechs are numerous, but are mostly overthrown, except the splendid specimen at Longhouse, eight miles away. " Camps" and "Cliff-castles," often excel• lently preserved, abound. Mention may be made of that near Caerbwdy, ofParc-y-castell, Castell Heinif, Clegyr-foia and Clawdd• y-milwyr on St. David's Head. Whitesand Bay is the reputed site of the Roman station of Menapia, where the" Via Julia" and" Flemish Way" converged. The Cathedral is mostly late twelfth century, but contains earlier fragments; with the college, palace, and other ruins it is surrounded by a great fortified close. Most magnificent of all are the remains of Bishop Gower's palace, built in the middle of the fourteenth century, one of the noblest ruins in Europe. TABLE r.

THE PEBIDIAN TUFFS OF THE NEIGHBOURHOOD OF ST. DAVID'S. D.-Ramsey Sound Series. Mostly schistose sericite-chlorite rocks. D4.-Pulverulent vari-coloured rocks of eastern part of Porthlisky. D3.-Porcellanitic tuffs of Porthlisky, Carnarwig, and St. David's City. Perhaps ISO feet. D2.-Slaty or schistose rocks, with pale-green, speckled. papery bands. Carnarwig, Whit~sand Bay, Porthlisky, and north of St. David',. About 300 feet. Dr.-Schistose bluish rock, very largely del'eloped about Emlych, a mile north of St. David's. Also seen at Maen Bachau, where it is about zoo feet thick. C.-Caerbwdy SerIes. Acid rocks, with a peculiar quartz-chlorite matrix. Cs.-Fine-grained felspathic and porcellanitic rocks. Caerbwdy Valley, Rhodiad, Carn Fach. Perhaps 400 feet. C4--Blue or green halleflinta. Caerbwdy Valley, St. Non's Bay, Carn 'Fach. 50-90 feet. C3.-Like CS. Caerbwdy Valley, Carn Gwil Geli. C2.-" Clegyr Conglomerate" with well-rolled pebbles of quartz-porphyry .and halleflintas up to 18 inches diametn, north of elegyr Hill and Treginnis• isaf. Some fine specimens from the County School Well can be seen at the school. Exactly similar rock is splendidly exposed in the Upper Solva Valley, four miles east of St. David's. Cr.-Passage-bed. Only shown near Treginnis-isaf. CI, Cz, and C3 may be together about 1,900 feet thick, but vary greatly. B.-Treglnnis SerIes. Somewhat basic, with scattered boulders of ·red rhyolite, gritty, B2.-Hard rock, composed of green vesicular trachyte with some -rhyolite and augite-andesite in a chloritic or ferruginous matrix. 500 feet. Covers much of the south-west of the district. Hr.-Red fragments of various kinds in a chloritic matrix. Thickness -variable, sometimes only a few yards. Penyfoel, Rhoson, Carn Silin, etc. (Schistose quartz-felspar-porphyry sill.) A.-Penrhlw SerIes. Alternations of red and green tuffs. A3.-Very gritty felspathic rock with rolled rhyolite pebbles and scattered .angular boulders of hornblende-andesite (wrongly described by myself as trachyte), in which the hornblende is now indicated by ferruginous pseudomorphs. Penrhiw, Rhoson, Treginnis-uchaf, Porth HenIlys, Penyfrel ,and the extreme south-west. 150 feet. THE GEOLOGY OF THE DISTRICT AROUND ST. DAVID'S. 135

A2.-Tuffs with felsite fragments. Penrhiw Quarries and Porthlisky. Perhaps 700 feet. The upper, coarser part affords good road meta!. AI.-Red and green halleflinta. North of the city and also in the cliffs between Porthlisky and Porth Henllys. Perhaps 100 feet shown.

TABLE II. THE LOWER PAL.tEOZoIC ROCKS OF THE NEIGHBOURHOOD OF ST. DAVID·S.

Black slates, flags and flaggy sandstones, 1,000 ft. Og)'gia ouclJii, Cal.l'mme duplicata, Chei1'1lrus sedgwicki, Trinuctezts fimbriatus, Lingula, Bellerophon, etc. Compact limestone, lying on black calcareous shales and LLANDEILO.* flags, 800 ft. Trinuc!eus lIoydii, T. .faus, Calymene cam• brensis, Asaphus t)'rannus. Fine black slates with beds of tuff, thick conglomeratic tuff at base, soo ft. Didymograptus murchtsuni, Diplograpttls Johaceus, Ogygia, Calymene, ]'rinuc!eus, A!."'glina, Lingu/a, Siphonotreta. Dark-grey flags and slates, I,SOO ft. Didymograptus nanus, D. allinis, D. indentus, D. patulus, Dtplograptus dentatus, Climacograptus, Glossograptus, kg/ina, Calymene, J/lamus, Placoparia, Phacops, Trinucleus, etc., and many mollusca. ARENIG. Black slates and sandstones, r,Soo ft. Tetragraptus, Agnostus, kgNna, Ogygia, etc. Fine black slates and shales. r,ooo ft. Didlmograptus exten• sus, D. pennatuills, D. sparsllS, Phy/logt'aptus, Ogygza scutatrix, Asaphus hom.frayt; etc., and many Uadopltora. "THEMADoc" JDark earthy flags and slates, 1,000 ft. Rich fauna, Neseuretus (Neullretus- . (several species), Niobe, numerous lamellibranchs and Beds). I brachiopods, Dendrocrinus, etc...... (Probable non-sequence.) These beds appear to have been little studied. Their total LINGULA I thickness is about 2,000 ft. The greater part consists of FLAGS I grey, flaggy sandstones, very barren, with the Lingulella (Olenus- I ,iavtsii band in the upper part. At the top fine, grey Beds). 1 shales with Parabolina. The bottom part, of. dark slates and bluish flags, with Olmus. The base ribbony and contorted. Sandstone and shales, roo ft. Or/hts hicksi. MENEVIAN 1Black flags, 3So ft. Paradoxides davidit, Anopolenus, Soleno- ~~~:~xid(S_ .!!~~I:~'.fj;o~~:t~r, ;;~~~~~;?!n;;.~,~~~!,~,~IC: . Beds). Grey flags, 300 ft. P. hicksii, COllocolyphe penh/a. SOLVA {GreY rocks, ISO ft. P. aurora, Conocoryphe bl~·o. (L . Grey sandstones with a red band, 1,500 ft. .. Conocoryphe" Po o";r "d (Ctenouphalus) So/vensts. Ba~a)oxt tS- Yellow sandstones, ISO ft. P. harknersi, Conocoryphe Ip!llii, e s . Mlcrodiscus, Agnostus, Plu/onia, Protuspongia, etc. Purple sandstone, 1,000 ft. CAERFAI Red shales, So ft. Lingulella primceva, L../errllginea, "Lepel'- (Olene/lus ditia ., (? Olenellus). Beds). [ Green, flaggy sandstone, 4So ft. Conglomerate, 60 ft. N.H.-There is a violent unconformity between this conglomerate and the underlying rocks.

* Owing to difficulty of mapping, the murchisoniand bifiduszones have not hem separated, as LLANVIRN. ]. FRED~~RICK N. GREEN ON

BIBLIOGRAPHY. A general account, with bibliography, of the ancient rocks of St. David's has been given by Prof. Bigot, "L'Archeen et Ie Cambrien," Mon. Soc. Sciences Nature/les Cherbourg, t. xxvii, 1890 .* Until the work of the Geological Survey in the district east of that to be visited is published, accounts of the stratigraphy of the Lower Pal