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Vol. 49-] VA:RIOLIT:E of the Lloyd. 145 10. Vallioli~R of The Downloaded from http://jgslegacy.lyellcollection.org/ at Orta Dogu Teknik Universitesi on January 4, 2016 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 main- land 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 evi- dence 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 porphy- ritic 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 infil- tration.' 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 pseudo- morphosed, 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 porphy- ritic 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 crys- tallized, 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.
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