C A. Raisin — The Serpentines of the Lleyn. 409 



and elsewhere,^ but we have yet no complete account. I have, 

 therefore, attempted to give a short summary, as preliminary to 

 some discussion of the district. 



In four of the exposures the rocks, which are much alike, are 

 recognised as diabase by Mr. Harker. Although abstaining from 

 any definite opinion of their geological position, he notices certain 

 characters in describing I'ocks, which, in eastern Carnarvonshire, 

 are shown to be of Bala age. Masses from near Pwllheli and from 

 other parts of the county certainly present much resemblance ; and 

 the four examples which I am considering might very well be 

 intrusions along a N.E. to S.W. line, roughly in the direction of the 

 strike or a strike fault. Lithologically they consist mainly of an 

 ophitic dolerite undergoing a process of uralitisation, but the bosses 

 also include more compact varieties, and many parts of the rock 

 exhibit secondary changes due to crushing or development of 

 minerals (often epidote). 



1. Trefgraig."^ — At least three varieties are shown in the quarry. 

 (a) An ophitic dolerite, the felspar being somewhat decomposed, 

 colourless augite, clear and well-preserved, occurring in one slide, 

 in another a pale greenish hornblende. The latter mineral includes 

 fibrous alteration, products and added growth, which sometimes 

 surround a central core with a different crystalline orientation. 

 Ilmenite is developed in the usual form of bars like sagenite. 

 (6.) A finer-grained microporphyritic diabase. Veins which occur 

 in the slide in some cases cross a felspar crystal, cementing it with 

 felspar. A structure of this kind is attributed by Mr. Harker to 

 infiltration along a line of " maximum shearing strain." ^ In the 

 Trefgraig slide, however, the felspar of the vein, containing only 

 very minute enclosures, contrasts sharply with the more decomposed 

 substance of the original mineral. Each boundary is marked by 

 bending of the crystal planes, and we should expect that any 

 infiltration would have spread beyond this line. Again, although 

 other parts of the vein are filled with serpentinous mineral, it seems 

 at some places where it heals up broken augite to consist of felspar. 

 (c). A fine-grained devitrified basalt or andesite, which contains a 

 few slightly larger microliths of felspar and augite. It is associated 

 with the dolerite first described, into which it seems to intrude, the 

 larger ci-ystals of the coarser rock being apparently snapped and 

 sometimes crushed along the boundary line. 



Some evidence of mechanical disturbance can be traced in the 

 Trefgraig mass, and one specimen from near the edge is much 

 crushed. 



2. Hendrefor.^ — The rock appears to be a uralitic diabase, retaining 



1 Notes on the Igneous Eocks of Lleyn. By J. V. Elsden, 1888, Geol. Mag. 

 Decade III. Vol. V. p. 303. The Bala Volcanic Series of Carnarvonshire and 

 Associated Rocks. By A. Harker, 1889, Cambridge, pp. 83, 86-88, etc. 



2 The Monian system of Rocks. Rev. J. F. Blake, Q.J.G.S., 1888, vol. xliv. 

 p. 531. The Bala Volcanic Series. A. Harker, pp. 83, 87. 



^ The Bala Volcanic Series of Carnarvonshire. A. Harker, p. 26. See also 

 ;Q.J.G.S., 1889, vol. xlv. p. 253, fig. 4. 



* Mr. J, V. Elsden, loc. cit. p. 304. Mr. A. Harker, loc. cit. p. 87. . 



