C. A. Eaisin — The Serpentines of the Lleijn. 411 



its ground mass and of certain of its porphyritic felspars, although 

 others differ somewhat in their narrower longer shape.' One of the 

 Forth din lleyn examples occurs in the cliff of a small cove beyond 

 the boat-house, as if it were a broad dyke ; and a tangle of rocks on 

 the beach suggests that the porphyritic greenstone has enclosed 

 within it a compact diabase. Even in this case the intruding mass 

 need not be of much later date, but might belong to the same 

 volcanic period. This remark also applies to another rock at Forth 

 din lleyn, which is a fairly typical basalt, dark purplish or black 

 and microcrystalline ; it appears to be similar to a mass on the 

 beach at Forth wen, probably a dyke. On microscopic examination 

 the latter exhibits lath-shaped felspars, much magnetite and some 

 viridite. Further examples of this type occur to the south. 



A junction with schists is mentioned by Mr. Elsden, but I could 

 find no true metamorphic rocks. Schistose masses occur, the exact 

 nature of which, in many cases, is extremely doubtful, but I have 

 identified among them rocks of igneous origin. One slide proves 

 that a diabase was veined by calcite, and that vein sttiff and 

 igneous rock were crushed together, so that the original character 

 is masked ; but amygdaloids and microliths of the ground-mass can 

 still be recognized. 



One rock of interest is exposed at low tide on the face of the 

 promontory beyond the boat-house. It is a whiteish, sub-crystalline 

 limestone, partly a breccia, the fragments of which project on the 

 weathered fawn-coloured surface. The microscope slide seems to 

 have the structure of limestone rock, apparently dolomitised, rather 

 than of mineral vein, traversed, however, by clearly marked secondary 

 veins of calcite and of quartz. Some rather dusty or silicified en- 

 closures, which occur, seem to me to be not organic but of igneous 

 origin. So the precipitation of the calcareous material may have 

 been contemporaneous with volcanic eruptions. 



6. Car eg Fawr (South of Aberdaron). — Of this I need say little, as 

 Mr. Harker has fully described all its important characteristics.^ 

 My slides do not happen to show any indications of a perlitio 

 structure, but exhibit some of the larger cracks, which can be seen 

 in the field sometimes limiting spheroids. The crag stands squarely 

 up from the more level ground, as Mr. Harker states, like an in- 

 trusive mass; but it need not therefore belong to a later geologic 

 period. 



7. Car eg (North of Aberdaron). — At this place, thick veins or 

 masses of jasper are found, and limestone also occurs, but it is close 

 to a small outcrop of the schistose rocks of the district. The dull 

 purplish or greenish bosses and crags, which are doubtless those 

 called "serpentine,"^ are certainly igneous, apparently rather basic, 

 and some originally glassy in character. At places spheroids can be 



1 I am much indebted to Mr. Thos. Davies, F.G.S., for kindly allowing me to see 

 slides of the Lanibay porphyry, and other Irish specimens, in the collection of the 

 Miueralogical Department at the British Museum of Natural History. 

 . 2 Mr. A. Harker, loc. cit. pp. 87, 88. 



3 The Geology of North Wales. Mem. of Geol. Survey, p. 230 (2nd ed.) 



