Vol. 49-] VA:RIOLIT:E of the Lloyd. 145 10. Vallioli~R of The

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Vol. 49-] VA:RIOLIT:E OF THE LLOYd. 145

10. VAllIoLI~r of the LrrYN, and AssoeIA~xl) VowcA~c Rocxs. By ~liss CATn~.~n~, A. RxIsI~, B.Sc. (Communicated by Prof. T. G. Bol~l~Y, D.Sc., LL.D., F.R.S., V.P.G.S. Read January llth, 1893.) [PLAT'~ I.]

CONTENTS. Page I. Introduction ...... 145 II. Lithological Characters of the Igneous Rocks ...... 140 III. Spheroidal Structure ...... 148 IV. Fluidal Structure and F]ow-brecciation ...... 151 V. Variolite ...... 152 (a) General Microscopic Structure. (b) Distribution. (c) Development. VI. Pseudocrystal]ites ...... 158 VII. Secondary Minerals ...... 158 VIII. Stratigraphical Summary of the District ...... 160

I. II~TRODVCTIO~. ALTHOUGH much has now been written about the igneous rocks of the Lleyn, especially in :Mr. Alfred Harker's valuable essay,1 yet, as he expressly excludes most of the district of which I am treating, and as it receives only slight mention in other writings, some of the most interesting examples from the part coloured on the Geological Survey map as 'metamorphosed Cambrian' have remained un- noticed. I have therefore put together a few notes on the rocks which I have collected in the course of many visits during the last six years. The specimens not hitherto described include a variolite --a spherulitic, somewhat basic rock. A short summary of its occurrence will at least serve to show that we have on the mainland of our island (apart from any example in Anglesey 2) a mass of variolitic diabase with characters very similar to those of German and Alpine localities, corresponding closely in many particulars with that graphically described by Mr. J. W. Gregory from the Fichtel- gebirge, ~ and also, although perhaps less markedly, with that of the Durance, as shown by Prof. Cole and Mr. Gregory. 4 I am limiting myself mainly to the rocks which can be clearly recognized as igneous, and have given only a short notice of others in the last section of the present paper. The extensive masses of a rather indefinite and schistose character in many parts are ashy or 1 , The Bala Volcanic Series of Caernarvonshire and Associated Rocks, Cambridge, 1889. 2 See Quart. Journ. Geol. Soc. vol. xlvi. (1890) p. 331, note; also ' The Vario- lito of Coryg Gwladys, Anglesey,' G. A. J. Cole, Sci. Prec. Roy. Dub. Soc. vo~. vii. (1891) p. 112. Quart. Journ. Geol. See. vol. xlvii. (1891) p. 45. 4 1bid. vol. xlvi. (1890) p. 295. Q. J. G. S. No. 194. L Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016

146 Mxss c. A. ~sI~ oN w~oIaz~ o~ [May x893 ,

agglomeratie, but include crushed examples of igneous origin. The exact nature of some rocks, however, even when illustrated by microscope slides, is dlmcult to determine precisely. I have been obliged also to leave the geological age of the variolites an open question, since even the associated limestone, quartzose rocks, and others which are possibly sedimentary, give no trustworthy evidence bearing upon this point. As much of my work was done at University College, London, I had the advantage of submitting doubtful points to Prof. Bonney, and have to thank him for valuable advice and assistance.

II. LITHOLOGICAL CHARACTERS OF TH~ I6NROUS ROCKS. Among the holocrys~dline rocks, some at least are most probably intrusions belonging to a later period. The well-defined dolerito of Trefgraig, Hendrefor, Tyhen, and Methlan is one type 1 ; this has been already described, and very similar to it is a greenish diabase forming the headland of Trwyn Gl~s ; certain other green porphyritic rocks may be related, although they are not identical. 2 A gabbro is found east of Porth Witlin which perhaps is another pro- trusion of the Craig-y-fael mass described by Mr. Harker? It consists of plagioclase felspar, much decomposed, and of diallage, sometimes eonnec~d in an ophitic manner. The rock has been more or less affected by pressure, as even the most normal spechnen exhibits bending of the cleavage-planes and other signs of strain, while much of the mass is markedly schistose. The dykes along the shore vary somewhat in character, but many are a compact diabase with fluidal structure ; 4 microporphyritic felspars are crowded near the edge, ranging roughly parallel, as if they had been floated to the side. 5 The igneous rocks which remain to be considered appear from the microscopic characters to have a certain uniformity in composition, and to belong to the class of rather basic andesites or not very basic basalts; but it is not always easy to decide how far we are dealing with two distinct magmas, or with one lava in which subsequent

J. v. Elsden, Geol. Mag. for 1888, p. 304 ; C. A. Raisin, ibid. 1892, p. 409 ; also Alfred Harker, ' The Bala Volcanic Series, eta' p. 87. 2 A diabase at Careg contains white augite ; see Geol. Mag. for 1892, p. 412. Others occur at Porth-din-lleyn, and at the beach between Dinas-fach and Porth Oer. 3 Quart, Journ. Geol. Soc. vol. xliv. (1888) pp. 447, 448; and 'The Bala Volcanic Series, etc.' pp. 89-92. 4 See ' The Bala Volcanic Series, etc.' p. 111; also J. F. Blake, Quart. Journ. Geol. Soc. vol. xliv. (1888) p. 531. As Mr. Harker states, it cannot be proved that these dykes are of the age to which he inclines to attribute them, but I fail to understand Mr. Blake's suggestion that, if contemporaneous with the volcanic ashes, they are in some cases ' actual flows,' in others ' due to infiltration.' 5 Compare Quart. Journ. Geol. Soe. vol. xlvi. (1890) p. 310. A felstone-dyke occurs near Porth Meudwy, which is lithologically similar to the felstones of Pwllheli. This dyke, however, is to the eastward of the boundary-fault, and cuts the black shales, which are probably of Ordovician age. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016

~ol. 49.] ZH'S T.Tmrs, a.s'n xssoolxrgu voI,cX_-~c ~ooxs. 147 differentiation has taken place. Of the two extreme types, the one is a dull-greenish, compact rock, the groundmass iucluding lath- shaped microliths of felspar, accompanied by more or less viridite, ~pacite, and minute granular augito or epido~e. Generally where the viridite is an important constituent, the iron oxide and the ~ugite are absent or small in quantity. Certain more compact rocks (consisting mainly of palagonite or chlorite) were probably originally glassy, and in some the traces of a fluidal structure are still retained. (See P1. I. fig. 3.) One slide contains rich brown, isometric crystals, often imperfect, probably an early formation of chromite or picotite. 1 The second type of rock is best illustrated in a variety, the microscopic characters of which seem to justify the designation of basalt or ferruginous basalt. (P1. I. figs, 1, 2, 5.) It is compact, of a dull reddish colour, and consists of opacite or ferrite separating lath-shaped mieroliths of felspar. These are not infrequently pseudomorphosed, so that the original composition of the mass must have undergone alteration. The two types seem to be connected, for one and the same rock may show a development of each kind. 2 Both forms also exhibit spherulitie growth, and include similar porphyritic or mieroporphyritie crystals. Some of these are felspar, others a serpentinized mineral, the form and general aspect of which suggest a ferro-magnesian silicate, probably bastite, s and there seem to be two varieties present, the one being crossed and out- lined by bars of granular opacite. ]Wieroliths of a similar nature occur, either yellowish or pale greenish, with a thick border o~ iron oxide. They are acieular or shuttle-shaped, such as have been often figured. 4 Thus petrological study suggests that the rocks represent two varieties, with only slight differences in composition and structure, ~and that they were probably formed by differentiation in a magma originally homogeneous. Owing to this, the ferrite may have crystallized, and consolidation have begun, in one mass rather earlier • in the other. This view seems to accord with the field evidence. While in certain parts the green diabase or dark ferruginous basalt can be easily recognized, elsewhere they are more or less intermingled, and even cannot be separated. Thus a mo~tled red ~and green rock (at Perth Orion) appears to have been formed by a second magma intruding into the mass of diabase before it was solidified. Also a junction of two varieties within the heart of a spheroid proves that these were distinguished before the cooling and contraction were complete. 5 As will be presently described, frag- 1 The crystals are too small to allow of the hardness being tested. 2 As in the green and purplish rock from west of Hendre-uchaf, see Geol. Mat. for 1892, p. 412. 3 Fouqu~ et Miehel-L~vy, '~in~ralogie Mierographique,' 1879, pl. xxvi. figs. 1, 2; & pl. xxvii. U.S. Geol. Explor. of the 40th Parallel, vol. vi. ' Mier. Petrogr.' 1876, ~F. Zirkel, pl, i. fig. 20; J. J. H. Teall, ' Brit. Petrogr.' p. 14, fig. 5 (after Zirkel) ; F. Rutley, ' Notes on Crystallites,' Min. ]~ag. vol. ix. (1891), p. 268, 17 (Crenulites). 'fig~ This junction was examined microscopically. L2 Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016

148 Miss c. A. mt, ISr~ o~ v~IoI.rr~ ol, [May i895, ments of purple basalt have been enclosed at places in a later magma. In other examples where the two types are not clearly separated, a tendency in this direction is indicated by a development of spherulites, and is even slightly marked in fluxion-bands. Partial analyses of two of this rather varied group of rocks have been prepared in the laboratory at University College, through the kindness of Dr. Plimpten. The first apparently belongs to the class of not very basic basalts, and this is representative of my ' ferruginous' type of rock (the other type has no~ been analysed). The second represents an abnormal variety, a kind of palagonite. I. II~ SiO2 ...... 52"43 29"78 A120s ...... ~ 23"84 17"72 Fe203 j 11"34 Fee ::::~::::::: 0~0 2"eS CaO ...... 7"55 7"88 MgO ...... 1"11 12"77 GO2 ...... 6"00 6"80 Hygroscopic water ...... 0"35 Alkalies and water ~ 9"07 1@68 by difference ... j "'" 100"00 10(00

I. Variolite from cliff north of Perth Oer.~--This rock exhibits a narrow zone of distinct spherulites, with gradations to a mass confusedly crystallized, as shown in P1. I. fig. 5. The specimen analysed came from the latter part, in which small amygdules occur at intervals, consisting largely of a carbonate. This deposit, however, would form only a small proportion of the mass, and would not very materially affect the results of the analysis. II. Green palagonitie rock limiting dull red basaltic spheroids, from the boss on the beach north of Perth Oer (see p. 151). ~

III. SPH~ROIVXn STRUCTUre. The igneous rocks are often jointed into rhomboidal columns weathering to roughened ends ; and spheroidal structure, which was noted by Prof. Bonney at Porth-din-lleyn, 3 is well shown at many 1 Separate analyses of this rock were obtained by Mr. C. J. Nix and by Mr. W. Gathorne Young, the two results being to a great extent corroborative. I am further indebted to Miss E. Aston, B.Sc., for kindly making an inde- pendent estimate of the quantities in a hydrochloric acid extract :--Fe203 +A120 ~ 3"73, cao 6"87, MgO 1.06, CO 2 6"00. 2 For the analysis of this rock I have to thank Miss J. M. Hayward, B.Sc., who also made a separate calculation of the carbonates present in the rock. These amounted to 20"88, and, on the hypothesis that they were all infiltrations, the percentages in the original rock would be :--SiO 2 38"22, Al~Ozlt-Fe203 34"67, MgO 13"30, with alkalies and water amounting to 13"70. As the quantity of water (not determined with precision) was not more than about one third of this last amount, the high percentage of alkali with a low percentage of silica suggests the enquiry whether nepheline could have been an important constituent in the original rock. 3 Quart. Journ. Geol. See. vol. xxxvii. (1881) p. 50. For a general account of such structures, see ibid. vol. xxxii. (1876) pp. 140-154. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016

Vol. 49.] z~ v.Lsrs, ~r aSSOC~XT~D V0LOaNIC a0OXS. 149 places, the spheroids being somet~imes as much as 6 feet in lengtJa, and often cushioned in appearance.1 Their exterior is distinguished generally by characteristic fracture and colour. 2 It is fissured in concentric and radial directions, the interior being traversed by an irregular rhomboidal jointing (fig. 1). Many spheroids also contain

Fig. 1.--Spheroid of basalt from the beach north of Porth Oer~ showing the general direction of the cracks. a5

About ~ natural size. a ---- outer zone, with radial fissures; 5 ---- interior, with irregular rhomboidal jointings.

~ig. 2.--Green ,Theroldal diabase, as exposed on the surface of a rock in the farmyard a t Dwyrhos.

About ~!g natural size. a -- green schistose rock ; b -- spherulitic zone near ,the exterior of the spheroid.

1 Cole and Gregory, o2. cir. vol. xtvi. (1890) pp. 311, 312. and tlg. 4. 2 Yariolitic structure is often connected with this part of th$ spheroid; see below, p. 155. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016

150 ~ss c. A. ~sn~ o~ v~ouzB oF [May .893~ a zone of vesicles radially elongated, as if gases had been preventA~l from escaping. Thus, evidently, a crust was formed at an early period, before the structures were completely solid. Smaller spheroids also are enclosed within larger (fig. 3, B), illustrating successiver

Fig. 3.--SlUrry& from the diff south of Porch Orion.

a = outer, greenish schistose zone. A b = zone with reddish spherulites.

c = interior, of purplish bamlt.

A and B are, roughly, about ~u natural size, but the spheroids ~.. 9 ez vary considerably in magnitude.

"''C

stages in contraction, as was demonstrated by Prof. Bonney; z, while in a green diabase (probably an ash) north of Forth Oer the enclosing shells are rhomboidal. ~ The outer layers of a spheroid are generally schistose and shining, consisting of palagonite or of a chloritic or serpentinous aggregation, which doubtless represents an originally glassy exterior. A similar matrix surrounds fragments in certain breccias. The foliated structure is not necessarily due to subsequent pressure, but might originate by slight rolling of the spheroids, if the crust were not completely solid, a The still plastic magma would be squeezed out along surfaces of weakness.

* Quart. Journ. Geol,Soc. vol. xxxii. (1876) pp. 151, 153, and fig. 13. _~ 2 Compare a structure:in shale ~altered by contact, described by Mr. Harker~ ibid. vol. xliv. (1888) p. 450. 3 j. W. Gregory,/bid. vol. EviL: (:1891) p. 60. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016

Vol. 49.] Ta~. I~T.EYN, AN]) ABBOCIs V0LOkNIC ROCKS. 151

IV. FLUI]}AT. STRUCTUR~ AND I~LOW-BRECCIATION. Structure which is probably fluidal can be traced in many, more or less palagonitie, green rocks, where streaming groups and streaky blotches of minute epidote or augite-granules are drawn up t~ wave- like points. Some of these rocks have in the field an appearance of flow-brecciation, while certain microscopic sections prove that subsequent movement has taken place, breaking and displacing spherulites, 1 so that fluxion is partly masked by the effects of mechanical deformation. I attribute to some kind of fluidal action several more obscure examples of breccias found associated with volcanic masses. Thus a dull green rock, probably a devitrified andesite rather acid in composition, encloses compact fragments which prove to be more definitely crystalline, and appear to have been carried within the surrounding magma2 Another mass, 3 in the field resembling an agglomerate, when examined microscopically seems to be a few- breccia, since almost all the fragments are alike, and are cracked and slightly divided, as if broken by the effect of the heated magma (now represented by palagonite or replaced by tufted chlorite). Possibly from the same cause a separation of iron oxide and of a clear crystalline substance has taken place along the edges. The rock is a compact porphyritic diabase, rarely exhibiting fluidal structure or a few amygdules, and including one or two pieces of a brownish glass. In another breccia, with fragments of a reddish slate, sectors of spherulites are embedded, as if these had been snapped along planes of weak cohesion by a sudden flow of lava at a high temperature2 (P1. I. fig. 4.) I have described a rock from Careg in which purple spheroids (4 to 5 inches in diameter) seem to have been separated and broken by a second influx of lava. 5 Some fragments have a roughly concentric fluidal structure, as if they had been moved while still plastic, and towards their exterior are clearer spots bounded by opacite, which possibly are incipient spherulites. (P1. I. fig. 3.) The most interesting breccia, however, is found north of Perth Oer. Elongated spheroids (9 to 15 inches long) of a dull-red ferruginous basalt, fractured as previously described, are surrounded by a green, schistose matrix. A few blocks of limestone are al~o included, possibly broken from a small mass associated with the basalt. This has been affected by calcareous infiltration and vein- ing; while the limestone has acquired a colour somewhat similar to that of the igneous rock; but it can be distinguished easily by its duller texture and fracture, its paler tint, and by being broken into more irregular blocks. 6 The spheroids of basalt appear 1 In a slide prepared from the variolite, Afon Sant, Aberdaron. 2 This rock is exposed at lowest tide, below the lime-kiln north of Perth Oer. 3 Above Pared-llech-y-menyn. 4 From the isthmus, Dinas-fach. 5 Geol. Mat. for 1892, p. 412. 6 This breccia appears to be somewhat similar to the mass at Caret Gwladys, Anglesey, described by Mr. J. F. Blake, Quart. Journ. Geol. See. eel. xliv. (1888) pp. 510-11, and fig. 17 ; but at Perth Oer the inclusion of limestone-blocks is a very subordinate feature, and no crystals resembling andalusite occur. The heroidal and variolitie structures are exhibited in more than one mass on the h. One of these shows a few inclusions of limestone on its north-eastern Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016

152 ~ISS C. A. ~sIN oN WmlOLI~ 01~ [May 1893, here, as at other places, to become green and palagoni~ic at the exterior, especially where segregation of the iron oxide marks a tendency towards the formation of spherulites. But several adjacent rocks, although like this mass affeet~t by subsequent crushing, seem to be examples of flow or flow-breeciation. A separation in the magma was probably followed by a second movement of lava. Although this did not violently displace or carry along the spheroids of basalt, it seems possible that it partially thrust itself between them. If not, we have first to account for the difference in microscopic structure and in fracture as being due to the glassy and the cryptocryst~lline characters of the outer and inner parts of the spheroid respectively, and to the different results of crushing upon each. But, secondly, the analyses given previously suggest that the two rocks might be found to have a marked difference in chemical composition. On this point further investigation would be necessary. In certain neighbouring masses, which are probably flow-breccias, the fragments are more separated, irregular, and smaller, but some are rounded, possibly from being moved while still plastic. These indicate a gradation towards a variolite, although they have not a radial structure. 1 V. VARIOLITE. (a) General MicroscoTie Structure.--Corresponding to the two types of rock are two ibrms of variolite.2 One is a compact green diabase exhibiting lighNcoloured spots, which on microscopic examination prove to be spherulitcs, generally elliptical, greyish, and duller than the surrounding matrix. They include a felspathic constituent showing radial aggregation in polarized light; also minute, brownish, brightly polarizing granules, probably augite ; and a fine dust, which is much less abundant in the central and con- sequently dearer part. Towards the periphery a zone of pseudocrystallites occurs. The brecciated spherulites at Dinas-fach are similar, but are much clearer. In the second type (P1. I. fig. 5) needles of brownish iron oxide are grouped in feathery tufts or elongated sheaves. The radiate character is found only at the centre of the spherulite, or both there and at the periphery, or it is face or edge, while the larger surface exposed to the north-west might be roughly represented by the figure given by Mr. Blake, if the 'fragments' were less angular, more uniformly fractured, more irregularly disseminated, and consisted entirely of dull red basalt. From the detmls given in the description of the Careg Gwladys variolite byProf. Cole, Sei. Prec. Roy. Dub. Soc. vol. vii. (1891) p. 112, I gather that the calcareous inclusions are mainly modified igneous rock, and that the mass is therefore probably very similar to this from Perth Oer. Owing to bad weather during my visits to the Angleseydistrict, I did not reach the particular boss figured by Mr. Blake, but a general lithological similarity was evident. 1 I may here refer to my description in a former paper of structures con- netted with pyromerides in acid rocks, QuarL Journ. Geol. Soc. vol. xlv. (1889) pp. 255, 267 (2rid group). 2 The first type is illustrated in rocks from near Aberdaron by Afon Sant and at Deunant; the second from Forth Orion, Dinas-faeh, and Forth Oer. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016

Vol. 49.] THE LLEYN, AND ASSOCIATED VOLCANIC ROCKS. 153 traceable over the whole, but interrupted by pseudocrystaUites. The iron oxide usually is concentrated towards the centre, and sometimes the outer rim is clear. The spherulitic habit, however, is not confined to structures which can be distinguished in hand-specimens. In very many slides, radial clusters of felspars are more or less regular near their centre, but have no definite boundary, adjacent groups being confusedly intercrystallized. In the green diabase the crystals are long, narrow, and straight. In the basalt they are narrow and long, but generally bent or curved, often undulating, and bordered by ferrite- granules or bars. (P1. I. fig. 2.) Even in some of the defined spherulites the felspars are wavy, lath-shaped crystals with iron oxide along their edges. Possibly this is the cause of the peculiar bending, for, if iron oxide were separated in any great quantity at an early stage, it might interfere with the development of the felspars. Thus the spherulitic growth exhibits roughly three or four different modes 1 :-- 1st, without radial structure ; 2ndly, with sheaf-like felspar radiating from a centre and giving a black cross in polarized light---the felspathic type ; 3rdly, with radiate feathery arrangement of iron oxide, which gives the reddened colour to most examples--the ferruginous type. 4thly, with grouped arrangement of distinct crystals. Although in the spherulites indications of these may be found, they are best shown in the confused, interlocking aggregates of the groundmass. (b) Distribution.--The variolite occurs in small bosses near Aber- daron and at places along the coast; it is well seen north of the beach of Porth Oer, near the small isthmus of Dinas-faeh, and in the cliff south of Porth Orion. Towards the mouth of Afon Sant, near Aberdaron, the diabase appears to form two or more patches, suggesting intrusion amid the ordinary schistose rocks. 2 The spherulites are only ~v inch across, i See Quart. Journ. Geol. Soc. vol. xli. (1885) Proc. pp. 90-91, Prof. T. G. Bonney's Pres. Address; see also J. P. Iddings, ' Spherulitic Crystallization,' Bull. Phil. Soc. Washington, vol. xi. (1891) p. 448. 2 It occurs west of the line of fault, as a low crag overlooking the small valley or rift along the line of the streamlet, and it crops out in the fields above in some furze-covered plots. I could find only a single and very rotten junction with the schistose rocks, and at that part the diabase seemed to be compact and without spherulites. The variolitie structure appeared not to be necessarily developed at the exterior of the mass, but often to be connected with a kind of lamination, which I was inclined to attribute to a movement in the magma. As previously stated, in one part at least the rozk suggested fiow-brecciation, and the spherulites are usually elliptical with a definite orientation. ]n the black mieaceous shales quarried south of the fault, east of the stream, I found a fragment of a small trilobite. Dr. Hicks very kindly examined this, and stated that it was probably the pygidium of an JEglina. The specimen, in conjunction with others from another locality, seemed to him suggestive of the Arenig Group. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016

154 ross c. A. Rxxsn~ oN WO~IOLI~ OF [May I893, and stand out as whitish pimples on the weathered surface. A diabase is also exposed in the farmyard at Dwyrhos, where the vario- li~ic structure seems to be connected with spheroids (fig. 2, p. 149). In one of these, about 1~ foot in diameter, the interior is compact and pale green, surrounded by a spherulit:ic zone, outside which the spheroid-crust and the intervening matrix are shiny, dark green, almost black, consisting of interfelted chlorite or of palagonite. The variolite at Deunant is best seen in the quarry behind the farm, where the rock is compact, dark green, jointed, and slickensided. The pale-green oval spherulites, about ~ inch in length, are often ranged end to end, and microscopic examination supports the idea that the elongation is in the direction of a flow-structure, but this would not in itself disprove the intrusive character of the mass. North of Porth Oer beach, as previously described, a purple basalt, somewhat calcareous and veined, is brecciated, probably by a second lava-flow. Both rocks exhibit variolitic structure. I could not always connect it with the exterior of spheroids, although it is often thus developed. Some varioles, perhaps ~ inch in length, not radial in their structure, appear to consist of small fragment~ of the breccia. On other surfaces, the spherulites appear as small knobs, densely crowded, about ~ inch across, which in the field bear a resemblance to amygdules contained within the basalt. In the weathering of some of these examples, a greenish or greyish envelope flakes off from a reddened centre. ~ This inner kernel is traversed by radial bars of iron oxide, sometimes grouped like iron filings about a magnet. The deposit is wanting in the outer zone, where the felspathie sheaves are more distinct, although they extend over the whole spherulite, giving a black cross in polarized light. An outer boundary, roughly elliptical in shape, is marked by a deposit of brightly polarizing granules, and similar granules are sparsely scattered over the section. These spherulites are grouped along a narrow zone in the rock, and gradations can be traced to a mass eoufusedly intercrystallized (P1. I. fig. 5). The specimen bears much resemblance to the felsite of Arran described by Prof. Bonney ; 2 but in the Lleyn slide the radial structure can be traced over both the centre and the external zone; and the granular boundary-line is not polygonal as in the Arran specimen, although I attribute it to the same cause--contraction of the rock and depo- sition along surfaces of weakness. The conditions of formation in all probability were similar, since here the second flow of lava may have caused renewed heating of rock partially consolidated, as described by Prof. Bonney in Arran. At Dinas-fach, on a weathered surface of dark red basalt are small, rather elliptical bodies (averaging ~ inch in length) paler in colour or surrounded by a paler border. 3 The microscope shows that iron 1 The partial analysis of this specimen is given on p. 148. It was taken from the low cliff, near the isolated boss on the beach described on p. 151, note. 2 Geol. NIag. for 1877, p. 510 and fig. 3. 3 Part of the surface exhibited radiate coral-like markings, which were Obably limpet-tracks similar to those noted by the Rev. Edwin Hill; see art. Journ. Geol. Soc. vol. xlv. (1889) p. 387, note. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016 u 49.] THE ~r~, XND ASSOCIArSDVOLCAniC ROCXS. 155 oxide is concentrated in the centre; but the spherulite has very little radial structure. The variolite is found between the mass of green diabase forming the islet and the breccia containing spherulitic fragments, so that here also it appears to have relations with a complex of igneous rocks. In the promontory south of Perth Orion the varioles are clearly connected with spheroids of the rock, recalling the description give~ by Mr. J. W. Gregory of a similar mass at Berneck? The interior of the spheroids is purplish in colour or mottled: the exterior is, generally green, surrounding a reddened zone. Spherulites developed quite at the outside are small and close, but the examples which at once attract attention on the cliff and on scattered blocks belong to that part of the spheroid where the green zone is passing to the next redder band (fig. 3, A, p. 150). These spherulites average about ~ inch in diameter, standing out like peas on the weathered surface, and they are divided generally into a green rim and a red interior, but the red colour in some is confined to a central speck, in others extends over the whole.' The spherulites possess a shadowy radial structure, but no distinct mineral forms, while in the interior of the spheroid the constituents of the rock become better defined, but closely intercrystallized/ Radial groups starting from adjacent centres are confusedly interlocked, and the spherulitie habit is only discovered by microscopic examination. The same relations in more rudimentary structures are found~ in the spheroidal diabase of Porth-din-lleyn on the north, and of Pared-llech-y-menyn on the south. At the former place, the spherulitic tendency in rocks which are not variolites was noted by Prof. Bonney, and he also described the spheroidal character web exhibited at certain parta. The two structures are sometimes related. Near the outer boundary in one spheroid, 4 a greenish palagonite includes small microporphyritic felspars, around which granular aggregations are clustered. These seem to form incipient: spherulitic or axiolitic structures (P1. I. fig. 6). Within this narrow peripheral zone, the spheroid is a confused mass of similar micro- lithic growth, as if the crystallization took place more slowly and uniformly ; and about an inch from the outer surface is a band of small, irregular, sometimes radial cavities. (c) Develol~nent.--u has been defined as a "devitrified spherulitic tachylyte, typically coarse in structure." 5 I take the last phrase to mean :--with spherulites visib e macroscopically, since the microscope shows complete gradations to rocks with no spherulitic tendency. The term thus retains its original connotation given in pre-microscopic days. Distinct spherulites, whether in acid or

1 Ibid. vol. xlvii. (1891) p. 48, fig. 2. s These appear to be very similar to those of Annalong, Co. Down, judging from Prof. Cole s description, Scl. Prec. Roy. Dub. See. vol. vn. (189z) p. 513. 3 Cole and Gregory, Quart. Journ. Geol_ Soc. vol. xlvi. (1890) p. 31-3 and pl. xiii. fig. 5. Obtained from the cliff near the boathouse. Cole and Gregory,o~. cir. p. 330. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016

156 ross c. A. ratRL~ os VARXOLIrS oF [May x893, basic rocks, are bounded either by mutual planes of weakness or by an intervening magma differently consolidated.~ An example of the first case is the Arran felsite, in which the polygonal boundaries, as was pointed out by Prof. Bonney, were probably due to the contraction which followed a renewed heating of the rock. 2 Where spherulites are separated by intervening palagonite, this is likely to be a modification of the original glass. If the temperature had been much raised subsequently, we might expect some marginal devitrification and the development perhaps of half-spherulites, as in the artificial example described by Prof. Bonney. 3 But the modified glass which borders cracks forms a zone outside any sph'erulitie or devitrified structure. Thus, although in a spheroid the fissures formed by contraction might allow the passage of residual heat, causing a secondary devitrification in the way suggested by Prof. Cole in Lipari obsidian,' I think that in these examples the formation of spherulites was generally a result of the original cooling. Struc- tures developed in the two ways could not always be distinguished, but, as in the experiments of dry cooling,s it would seem that both conditions occur. Considering the origin of acid spherulites, Mr. Iddings attributes the crystallization of certain portions of the magma to their being places of greater hydration ; GMr. Whitman Cross to the aggre~ga- tion of a colloid substance: In these andesites or basalts of the Lleyn we note frequently that spherulites have a concentration of iron oxide towards the centre, and since this substance is an early deposit, it is possible that, in somewhat basic madonnas, its segregation might help to induce the formation of spherulites at certain ~pots. 8 As ori~nal constituents, the spherulites seem to form in a mass where the rate of cooling is not uniform, and neither slow enough to allow of the development of well-formed crystals, nor so rapid as to give rise to a homogeneous glass. As Prof Bonney has said, the conditions are probably found in a mass kept for some ~ime at a

See J. P. Iddings, Spherulitic Crys~lli~-ation,' Bull. Phil. Soc. Wash- ingtcn, vol. xi. (1891)p. 451. 2 Geol. Mag. for 1877, p. 510. 3 Quart. Journ. Geol. Soc. vol. xli. (1885) Prec. p. 92. Min. Mag. voi. ix. (1891) p. 274. 5 Quart. Journ. Geol. Soc. vol. xli. (1885) _Prec. p. 93. Bull. Phil. Soc. Washington, vol. xi. (1891) p. 447. 7 , Constitution and Origin of Spherulites in Acid Eruptive Rocks,' /b/d. vp~ 43~, 437. The iron oxide in these rocks often is not in its original condition, but although oxidation and hydration may have occurred, it seems probable that the spherulites are marked by an excess of the ferru~nous constituent. In the well-known pyromeride-bearing felstone of Lea Rock, Shropshire, the iron oxide which causes the red-brown colour of the small spherulites may be only the result of subsequent change ; butit is possible that a smaller proportion of iron is present in the surrounding matrix, in which no large amount of viridite or of any iron-containing constituent can be recognized on microscopic examination. It would seem to be impossible here to apply the final test of chemical analysis. Compare also some of the pyromerides of Boulay Bay, Jersey. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016

Vol. 49-] z~aE LLEYN, AND ASSOCIATED VOLCANIC ROCKS. 157 low temperature) They would often occur where some freedom of motion is possible, as near the exterior of the rock-mass, at the boundary of contraction-spheroids, or along the lines of flow in a glassy magma. In spheroids, which we may consider as a typical case, the order of changes might be something like the following :---The exterior forming a thin film, the rate of cooling in the next shell is s~ checked that crystallites begin to develop. In the viscid crust, iron oxide segregates at places constituting centres of growth. Radial and concentric cracking takes place, and further cooling causes the glass to ' set'along the fissures. In the ferruginous aggregates within the crust, the constituents group themselves, and a radial arrangement dominates the growth, thus giving rise to 9 spherulites. The interior of the spheroid is still in a plastic condition,2 and as soon as the crust is sufficiently solid, the fall of temperature in the part within takes place doubtless at a fairly uniform and somewhat slower rate, so that a confused intercrystalli- zation of its elements results. At certain parts they are connected together in radial groups, but these are not definitely bounded, thus they do not weather out, and the rock where they occur is not a variolite. The same succession of conditions would be found towards the exterior of a rock-mass, so that a variolite is limited sometimes t(~ that part. A somewhat analogous case might occur in a moving lava-stream, where semi-plastic fragments are embedded in a mor~ fluid part. The movement would tend to prevent the proper formation of spherulites, but the curious aggregations along the exterior of included pieces might be an incipient development2 Some of the Lleyn variolite is thus a development of a lava ; and where the magma was not an actual flow, it seems very probable that it cooled at no great depth below the surface, but was formed ~wards the exterior of an intrusive mass. 4 Certain of the spherulites (as for example that from Deunant) are not unlike the variolite of Brian~on, a specimen of which was kindly lenb me for compaldson by Prof. ]3onney ; but in the Durance rock the crystallization is more definite, and a chloritir constituent has developed, which seems to have been cleared out of the spherulite, so that this is paler, but is surrounded by a darker rim. l Quart. Journ. Geol. Soc. vol. xlvii. (1891) p. 105. Compare also experiments of M. Claudet, referred to ibid. vol. xli. (1885) Proc. p. 90, note; Cole and Gregory, ibid. vol. xlvi. (1890) p. 326 ; g.W. Gregory, ibid. vol. xlvii. (1891) p. 61. 2 This agrees with ~[r. ft. W. Gregory's suggestion, ibid. vol. xlvii. (1891) p. 60. See also Cole and Gregory, ibid. vol. xlvi. (1890) p. 316, and Delesse, ' M~tamorphisme des Roches,' Paris, 1858, pp. 371-374. 3 Some structures at Porth Oer are probably due to similar conditions. The development is shown clearly in a specimen from Boulay Bay, which I received from Prof. Bomtey for comparison with nodules obtained near Pwllheli. See Quart. dourn. Geol. Soc. vol. xlv. (1889) p. 264. 4 See Prof. Cole's suggestion as to the Annalong dyke, Sei, Proe. Roy. Dub. ~)e. vol. vii. (1892) p. 513. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016

158 lies c. A. R~tSIN ON VARIOI~B 01~ ~May I893 ,

The variolito is also similar in many respects to pyromeride- bearing rocks. Among the differences, we find that the more basic spherulites do not tend to become so large, the usual maximum in the Lleyn examples being about ~ inch. The pyromerides also are generally distinguished by a smaller proportion of ferrite, and by clearer crystallization, due to the different chemical composition. As to form, the variole, although sometimes nearly spherical, has an elliptical outline more commonly than the acid spheralites, and this (and other characteristics) may be connected possibly with the more fluid character of a basic magma. In some examples veins and circular cracks occur, 1 similar to those seen at the exterior of pyromerides ; but here the mode of formation is clear and indis- putable, since the material of the spherulite is sharply different from that of the vein. In the more acid rock, the greater similarity of the two gives an appearance of gradation, which may have seemed to lend support to another inference.

VX. PSEUDOCRYSTALLITES? Microscopic examination reveals occasionally a clear network across spherulites or other parts of the greenish diabase. The bars of the net appear under a high power to be free from minute augite and epidote, and to be formed of felspathic granules, thus presenting some similarity to spherulitic rays. In the ferruginous variolite, bars of granular iron oxide are similarly developed, bordered by a clear isotropie substance, and where they cross the rays, these (for a short distance) are free from the brown deposit. This colouring material might have been withdrawn to any system of contraction-cracks if such existed; or in the crystallization of the felspathic constituent, iron oxide would be extruded, as occurs along spherulitic rays. Ferrite is aggregated around other crystals, probably pyroxene, which in one slide vary from a square to an oblong or lath-shape, sometimes so narrow that the dark border becomes a bar ; and in other cases feathery tmfts grow out on either side of the pseudocrystallite-axis. I am thus inclined to revert to the earlier explanation of these structures, and not to follow Mr. J. W. Gregory's idea of their being caused by contraction.

VII. S•CONVARY M~NERA~. Among the secondary minerals are the usual aggregates replacing felspar. Other crystals, probably originally pyroxene, enclose serpent~nous, chloritic, or similar products, and are surrounded and crossed by deposits of iron oxide. A similar border of opaeite is seen in two slides of trachyte from Auvergne, which were kindly 1 See Mr. Iddings's description of a crescent-shaped area filled with felspar, in a spherulite from the Yellowstone Park, Bull. Phil. Soc. Washington, vol. xi. pp. 453, 454, pl. vii. fig. 4. 2 Cole and Gregory, Quart. Journ. Geol. Soc. vol. xlvi. (1890) pp. 313, 314 ; J. W. Gregory, ibid. vol. xlvii. (189]) pp. 56-58 ; Michel-L~vy,BulL Soc. G6ol. France, 3"e s6r. vol. v. (1877) p. 238. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016

Vol. 49.] Ta~. ~v.x~, AN~ ASS0OIXTSD VOLCal~ICItOCXS. 159 lent me for comparison by Prof. Bonney. 1 The minerals within amygdules and veins include epidoto, quartz, a sheaf-like zeolite, calcite and other carbonates, and iron oxides. Doubtless in many cases these substances have been transferred from the rock, as iu a dull-greenish diabase (north of Perth Oer) which has the structure of the basalt but not its colour, and contains small red amygdales, apparently filled by material drawn from the surrounding mass. ~ Some of the secondary deposits have been modified subsequently, and one veined specimen was described by Prof. Bonney, as illustrating the effects of pressure upon calcite, s A carbona~ of lime, magnesia, or iron, is one of the commonest secondary products. From microscopic study much of this appears to be dolomite or chalybite, although calci~e is also present. 4 In many diabase-tuffs, it is scattered through the slide in small, clearer, rhomboidal spaces, often including reddish hmmatite. In the microcrystalline basalts, the secondary development is discovered only on application of polarized light, when a number of bright- coloured rhomboids suddenly start into view. These consist of the carbonate replacing lath-shaped felspars, but interrupted by the iron oxide. The deposit seems at places to creep up the felspar- microliths, so as to project beyond--and cause a slight irregularity in--the boundary-line of the rhomboid (P1. I. fig. 1). Similar crystals are deposited in the Dinas-faeh breccia within the felspathie rays ; while elsewhere the wavy felspar-microliths of a small radial group have been replaced. 5 These pseudomorphosed spherulites, in polarized light, contrast with the surrounding duller mass by their bright pink and green tints. ~[uch of the change described is evidently due to infiltration, since rhombohedra of a carbonate, sparsely scattered in the mass, are thickly clustered along fissures or veins. Sometimes the rock appears to have been saturated throughout, the interchange of substance causing in both masses alterations of colour which can easily be recognized. The limestone generally assumes a pale peach tint, and fragments of basalt cemented by calcareous veins exhibit an outer zone of thin alternating laminm, then a band of deep red, while the interior retains a normal duller tint. The limestones which belong, as I believe, to the original series of rocks, would probably be a sufficient source for the calcareous deposits, without imagining the extension of Carboniferous or other masses of more uncertain age, and iron oxide must have been present in large proportion within the basalts. 1 Compare Fouqu6 & Michel-L6vy, 'Min6ralogie Mierographique,' 1879, pls. xxviii,and xxix. ; U. S. Geol. Explor. of 40th Par. re1. vi.-' Mdcr. Petr.,' 1876, F. Zirkel, pl. iv. fig. 2, & pl. v. fig. 2. 2 Radiate clusters of lath-shaped crystals project into the vesiclesalong their edge, the remainder of the space being filled with hmmatite. 3 Geol. ~lag. for 1889, p. 485. 4 It has generally a granular texture, absence of cleavage and twinning, rhomboidal form in section, and high polarization-tints. 5 This is seen in a Porth-felen basalt, and also within the spheroid described from Porth-din-lleyn. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016

160 VARIOLITE OF THE LLEYN. [May I893,,

VIII. STRATIGI~APHICALSUMMARY Or THE DISTRICT. For study of the rocks in the field we are limited generally to exploration of the coast. Only rarely is any mass exposed inland above the surface of the drift, and most of such isolated outcrops wore marked as the so-called ' serpentines.' ~ (See Map.) :My examination of Bardsey Island was somewhat hurried, and I did not explore the cliffs along its north-eastern coast, but its rocks are evidently similar to those in the neighbouring parts of the mainland. From a boat a good view is obtained of the coarser agglomerates (including large compact blocks and streaky laterite) which form the eastern cliffs north of Yr-henllwyn. The lower part of the island to the southward, on which the lighthouse is built, consists of well-banded, ashy reeks, sometimes fine-grained, with associated quartzose and calcaxeous masses. This series apparently forms the lowest strata exposed, since the dip is fairly steady over most of the island, as marked on the Geological Survey map. The extreme southerly point exhibits agglomerate, and also a purple basaltic tuff and a limestone included within a green diabase. At three places I found a coarsely crystalline rock, more or less crushed, probably intrusive and allied to the granitoid mass near LlangwnadlJ The most northerly of these includes a green mineral which is probably an altered biotite, while in the next example the compact pinkish mass is interrupted by large plates of silvery mica, doubtless a primary constituent subsequently modified. The third rock (on the west coast) has a wavy and interrupted lamination probably caused by pressure, the results of which can be recognized also in the microscopic section. Patches of a greenish, vermiform chlorite (doubtless prochlorite) extend in thin strings among the grains of quartz, which present the appearance of having been crushed and redeposited. On the mainland, the eastern boundary-fault at Parwyd Cave brings down strata, probably of Ordovieian age, against the contorted quartzose and micaceous schists of Mynydd Cristion. These have the appearance of an ancient series, and are probably related to the schists of Anglesey. Since similar types are found on :Mynydd Ystum, a it is possible that an axis of Arehsean rock strikes roughly north-eastward near the boundary-fault. 4 Along the south coast, westward of the schists, are rocks which appear to belong to the volcanic series. Some at least seem to be tufts. The cliffs westward and northward most largely consist of ashy and agglomeratie strata, often schistose, and including grit and limestone (see :Map). Over the hills of Mynydd-y-gwyddel and Mynydd Annelog, the rock 1 T. G. Bonney, ' On the Serpentine and Associated l~ock~ of Anglesey ; with a ~ote on the so-called Serpentine of Porth-din-lleyn; Quart. gourn. Geol. See. vol. xxxvii. (1881) pp. 40-50. See Geol. Mag. for 1892, p. 408, and literature there quoted. 2 Alfred Harker, Quart. gourn. Geol. See. vol. xliv. (1888) pp. 441 446. 3 j. F. Blake, ibid. p. 533. 4 I found near Tociau a similar specimen ; it was loose, but dose to the d6bris thrown up from the shaft of a small boring made in search of coal ! Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016

The outcrops of igneous rocks beyond the important lines of fault, which extend east of Llangwnadl, are copied from Mr. Alfred Harker's map in ' The Bala Volcanic Series of Caernarvonshire,' p. 59. The areas marked ' Lower Silurian,' with their faulted boundary, arc taken from the Geological Survey ma : these strata are probably of Ordovician age. The country west of the main faa~ts which is left blank is generally covered with drift. Numerous diabase dykes cut the rocks along the coast of the Lleyn, but it is impossible to show them on a map of this scale. Q.J.G.S. No. 194. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016

162 ~iss c. A. m~Isrs o~ vA~OLITS OF [May I893, is a banded hiflleflinta, in which the layers are developed by weathering. A fine-grained argillaceous tuff can be seen near Perth Merin, and at other pla'~s north of it. These beds have a fairly continuous dip to the north-west and run out to sea in low points, while their truncated edges form a flatfish platform, sometimes at the foot of low grassy cliffs of DriP. Small volcanic centres are perhaps marked by coarse agglomerates associated with igneous masses (as at Parcd-llech-y-menyn and Maen-melyn-lleyn). ~ Else- where the diabase or basalt, sometimes standing as an isolated reef or islet, may have formed part of the pipe or reservoir of a small volcano, as at Dinas-fach, Dinas-fawr, and south of Perth Oer,while the extensive mass of variolite at Perth Orion probably is also connected with a centre of eruption. Lava-flows can be recognized in green palagonitic rocks (as at Careg, Perth Oer); aJso in vesicular basalt containing amygdules of calcite, as in a much-crushed specimen between Dinas-faeh and Perth Oer, and in a very coarsely-vesicular rock west of Perth Witlin. Volcanic rocks, massive and fragmental, have thus been traced from Bardsey Island to Porth-din-lleyn. They appear on the whole to form one series, apart from those intrusions which, judging from their lithological character, are probably of post-Ordovician age2 Green and purple layers among the schistose tufts seem to be homologous with the two types of igneous rock, and the h~lleflinta of the two southern hills is represented occasionally in interstratified bands of similar character. Grit and limestone, often in lenticular patches, varying from microscopic size to many yards in length, appear to be contemporaneous with the volcanic series, although the layers have been subsequently broken and squeezed. Gradations can be traced from limestone to ashy bands (as above Braich- y-pwll), or to a laminated basaltic mud, as if the finer volcanic d~bris was washed into calcareous waters. At Porth-din-lleyn, as I have shown, s small igneous fragments appear to be embedded. Limestone is also included in large blocks within the basalt, and in these and other examples the microscopic slide consists of very regular, uniform rhombohedra, suggestive of contacNalteration. It resembles the structure shown to me by Prof. Bonney in a piece of Trenton limestone, which had been caught up in a dyke at Corpora- tion Quarry, Montreal. The grit consists mainly of quartz and of felspar, often plagioclase, sometimes microcline. It occasionally contains flakes of chlorite or mica, fragments of mica-schist or of a compact scoriaceous~ or igneous rock ; and a grain of tourmaline occurs in one slide. The grit has evidently been derived partly from old granitoid rock and schists, and thus its accumulation mus~

1 At this place, where the streamlet falls into the sea, is St. Mary's Well, of ancient repute. The trough-shaped pool of clear water, wherein grow delicate green algm, is hollowed out at the foot of a vertical cliff of coarse agglomerate. The water is fresh and sweet, although the surface is a very few feet above the level of a calm sea at high tide. Geol. Mag. for 1892, p. 409. 8 1bid. p. 410. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016

Vol. 49.] Tag LLEYN, AND ASSOCIATED VOLCANIC ROOKS. 165

have been separated from their formation by a long interval of time, such as everywhere parts the old Arehsean from Pebidian or early Pakeozoic rooks. 1 The question of the geological age of these rocks is extremely difficult. Lithologically some of them resemble the Pebidians of Caernarvonshire or the northern district of Anglesey. At one place I: found two badly preserved fossils in an ,indurated argillito, 2 which reminded me of 0rdovician strata ; but this may be a ease of inter- folding , and thus it gives 11o absolute proof of the age. I submitted the rock to Dr. Hicks, who kindly gave me his opinion, that it seemed "to belong to the Arenig Series," 3 and that the organisms resembled phyllopod crustaceans. 4 Unless further details should be obtained, we must leave undecided the geological position of these volcanic rocks, and the question whether they belong to very late Arch~ean or to early Palmozoic time. But, the uncertainty hardly Seems ~ justify us in erecting a new system to contain these and other simi.larl~r dubious masses. The stratigraphieal interest of the area consists in it,s volcanic characters. Basaltic lavas occur, of which, as Mr. Harker points out, no: other examples have been described in Caernarvonshire; ~ and a variolite is connected with contraction-spheroids, as in German and Alpine localities. The cliffs and low crags along the coast exhibit clear dissections of volcanic structures. Some igneous rocks Seem to mark the remains of the reservoir or the vents from Which lava flowed or fragments were ejected; 6 but most of the widely extended ashy deposits, especially the finer ashes, have probably come from more distant centres. The igneous masses and coarser agglomerates in the south-west and at places along the coast suggest that important vents were probably situate in parts now covered by the sea. The volcanic formations seem to have accumu- lated near a shore-line, where limestones were formed in shallow lagoons, 7 and where the waters deposited grit and mud, derived partly from ancient granitoid rock, partly from later volcanic materials. 1 In several places, Prof. Bonney has pointed out theeontrast shown by these Archman and later series of rocks. 2 Associated with the volcanic rocks of Parcd-llech-y-menyn on the south coast. 3 Dr. Hicks reported that this specimen and two from another locality were ' probably of Llanvirn age like the Pont Seiont Beds at Caernarvon.' 4 The fossils were afterwards submitted to Prof. T. Rupert Jones, :F.R.S., who has kindly ikvoured me with the note appended to the present paper, p. 164. 5 , The Bala Volcanic Series, etc.,' 1889, p. 75. ]~r. Harker points out that the rocks at Porth-din-lleyn, previously described by Prof. Bonney, might include such examples. See papers quoted. 6 The fine-grained elastic rock near the spring, north of Mynydd Annelog, possibly represents material thrown out from the small vent to the northward, since the constituents are similar, except in size. See Geol. Mag. for 1892, p. 413. 7 The possibility of precipitation of carbonate of lime in shallow pools among volcanic detritus on land, as at the well-known Temple of Serapis, must not be overlooked.

~2 Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016

164 rBOF. T. R. lOlgES ON CI'PRIDII~A RAISINI~. [May I893r

)IOTa. on a Fossil CrPRrDn~A~)from the Sov~ of the LLvxN. By Prof. T. RvP~.a~ Jo~m, F.R.S., F.G.S. The two counterparts of a h~ematitie cast, one convex and one hollow, in the bluish-grey schistose mudstone, weathering brown, are too rough and imperfect in surface and in outline to give quite satisfactory evidence of the little organism they represent. It appears, however, to be a badly preserved ostracod of Cypridinad affinity, having a suboval or obovate body, with a hook and notch at the anterior end, and a strong caudal process posteriorly. It measures 9 by 5 millim. Analogous features in various Uypridina Raisini(z, sp. nov. degrees, and associated with other characteristics, exist in Uy/rr/- ['Enlarged 4 diameters.] dins Reynaudii, Milne-Edwards, C. elongata, Brady, (7. Bairdii, Brady, C. japonica, Brady, and other hooded and apiculate mem- bers of the genus ; but none of these have the hinder end tapering away to so long and strong a point. A fossil form, however, from the Carboniferous series of Sicily, ! ! described and figured by Prof. G. G. Gemmellaro (Philomedev avanfhoides, Gemm., Mem. See. Ital. Sci. ser. 3, vol. viii. 1890, p. 37, pl. v. figs. 16 and 17), is very similar, but is too attenuate, and measures 6"5 by 3 millim. Having a sufficiently distinctive form, this little fossil may be regarded as of specific value; and, in honour of Miss C. A. Raisin~ the discoverer, I call it Cy_pridina t~aisinice. As far as I remember, the oldest Cypridinad yet published is the " OyTridlna ? Internal cast of a right valve, mag. about 4 diam. :From a pebble of Palseozoio quartzite ~ in the Triassic conglomerate of Budleigh Salterten, Devon." Geol. Mag. for 1881, pp. 337 and, 347, pl. ix. fig. 7. EXPLANATION OF PLATE L Fig. 1. Ferruginous basalt, from the cliff at Maen-melyn-lleyn. Dolomite is~ seen by polarized light to replace felspar, and to form rhombohedral crystals, represented in the drawing by spotted rhomboids. The- groundmass consists of felspar, of minute crystals (left blank), which are probably augite, and of dark granules and microliths of opacite. (Magnified 30 diam.) l~ig. 2. Ferruginous basalt, from the beach of the small cove nm~h of Perth Oer. This slide shows the bent and wavy character, and somewhat spherulitie arrangement, of the felspar-microliths. The dark granular deposit is opacite, which partially separates the felspars of the groundmass. Parts of two amygdules are shown: these consist of quartz and dolomite or ankerite. (Magnified 30 diam.) I Devonian or Lower Silurian. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016

Vol. 49-] wRm~I~. OF ~E T.T.E~. 165

:Fig. 3. Breccia, from a small quarry on the hillside west of Careg (north of Aberdaron), probably due to flow. The fragments show a tendency to fluxion-structure: they are bordered by small incipient spherulitic formations, within a thick deposit of opacite. The part left clear is mainly palagonite, probably an altered basic glass. A small crystal of felspar is contained within one fragment. (Magnified 8 diam.) Fig. 4. From rock on the beach, at the eastern part of the isthmus of Dinas- lath. h breccia, including fragments of a sphcrulitie andesite aud of slate or argillite, l~he radial structure represents the spherulitie fragments, consisting mainly of felspar. The dotted parts represent pseudomorphic dolomite. The darker shaded fragments consist of ~rgillite. The part between the fragments represents mainly pa]a- gonite, and the rock is probably a flow-breccia. (Magnified 30 diam.) Fig. 5. Variolite, from the cliff north of Perth Oer. The spherulites shown on the right of the figure have a central part darkened by ferrite, and ar~ contained within rings of granular deposit, somewhat similar to the boundaries in the specimen which was described from Arran by Prof. Bonney (see p. 156). This structure passes gradually to the part without distinct spherulites, shown on the left of the drawing. The groundmass here consists of granular opacite, and of felspar- microliths. (Magnified 15 diam.) Fig. 6. Exterior zone of a spheroid from near Porth-din-lleyn, south of the lifeboat-house. This part of the spheroid shows incipient spherulitic structure in the form of non-radial aggregates, which in many cases are grouped around felspar-microliths. The slide is crossed by a vein filled with a secondary deposit or palagonite. The small group of spherules to the left of the vein, which is less shaded in the drawing, is seen uuder polarized light to be pseudomorphosed. , showing the bright pink-and-green colours of dolomite. (]Y[agmfied 20 diam.)

DiscussioN. Prof. JUDD complimented the Authoress on the evidently great amount of labour and patient research devoted to this investigation. He thought the occurrence of the spherulitie structure round the surfaces of 'pillow-like masses' similar to those described by Prof. Dana was exceedingly interesting, especially when one considered the probably very great antiquity of these Caernarvonshire rocks, tie thought, also, the suggestion that early crystallized magnetite-grains had formed the nuclei of the spherulites was a very interesting and probable o~m. Mr. HX~KER expressed satisfaction at the further discovery of variolites in Britain--rocks presenting interesting points of resemblance to those known in Savoy, Hesse, and Bavaria. He anticipated that the detailed researches of the Authoress would throw light on the nature of these spherulitie structures in basic igneous rocks. Prof. BONN~ said he believed that the variolite described by the Authoress was the third example in the United Kingdom, that discovered by Mr. Blake in Anglesey being the first, and that found by Prof. Cole in Ireland the second. He referred to the question of the origin os si)herulites , and st~ated his opinion that this paper, the fruit of much labour, would prove a valuable contribution to our knowledge of the subject. Prof. ttULT, and Prof. J. F. BLAKe. also spoke. Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016

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