SERPENTINE OP PORTHPINIIETN. 



49 



with no certain indications of bedding. The rock exhibits spots of 

 epidote, varies in colour from a dull sap-green to a greenish-purple 

 colour, is traversed by thin veins of quartz and calcite, and 

 might be taken either for a compact chloritic rock resulting from the 

 alteration of a uniform fine sediment, or for a very compact diabase. 

 Under the microscope both varieties are seen to be rather decom- 

 posed, the slides being rendered more or less opaque with fine 

 powdery ferrite, which sometimes also occurs in granules. There 

 are indications of numerous niicroliths, some doubtless being felspar, 

 and probably a plagioclasc ; these, in places, show a tufted, sometimes 

 an almost spherulitic arrangement. In this are scattered some 

 larger augite crystals, much altered but still recognizable. The 

 green variety has some faint indications of a fragmental structure, 

 and might be a tuff in which the outlines of the fragments were 

 nearly obliterated. The structure of the redder variety is more 

 distinctly that of a true igneous rock ; and as I could see no signs of 

 a division in the field, we may venture to identify these as compact 

 diabases*, very probably old flows of a basaltic lavat. The breccia 

 exhibits under the microscope a base of filmy pale-coloured viridite, 

 in which are scattered numerous granules of ferrite, crystals of 

 epidote, with other minerals of secondary formation, altered augite, 

 and felspar, and scattered fragments, apparently of more or less 

 glassy lapilli. Of the nature of some of the last there can be no 

 doubt. The rock, then, is a " diabase tuff," of which probably a 

 good deal was once a very fine dust. 



For some distance to the north of Porthclinlleyn the dubiously 

 " conglomeratic " aspect of the rock as above described continues ; 

 and then we find, on arriving within a short distance of the life- 

 boat house, sections that, even in the field, are perfectly conclusive. 

 The best occurs in a little cliff (an old quarry-face) looking seaward. 

 On the sides of this (the section is about a dozen yards wide) we 

 find a dull-green ashy-looking matrix, full of angular and subangular 

 fragments of all sizes up to 3 or 4 inches diameter of a compact 

 paler-coloured rock ; and in the middle a mass of rounded blocks (a 

 foot or two in diameter) which seems almost solid passes irregularly, 

 but with fairly marked boundaries, into breccia. There can, then, be 

 no doubt that we have here a true volcanic agglomerate, including 

 either a thickly fallen mass of bombs or a small lava coulee. This 

 view is fully confirmed by microscopic examination, — the breccia 

 exhibiting a ground-mass, as above described, full of scattered frag- 

 ments obviously of volcanic origin, some a brown slaggy glass, like 

 a kind of tachylite, others compact basaltic scoria. The examination 

 of a spheroidal mass from the agglomerate shows it to be highly de- 

 composed compact diabase. 



* The name diabase is given, though little or no chlorite can be recognized, 

 because I know of none other that exists. 



t I may mention that I have in my collection a rock microscopically identical 

 •with the above, which I collected between Glyn Garth and Beaumaris. These 

 old basalts are, I suspect, not uncommon among the Welsh Pre-Cambrians, and 

 in the field are hard to recognize. 



Q. J. a S. No. 145. e 



