496 EEV. J. p. BLAKE OK THE 



at the junction near Melin Ddrjdwy, and at Tyn-y-pwll in the 

 centre, near Bryn twrog. 



c. The Associates of the Granite. — These are the most diflScult to 

 understand, and I can only do my best to interpret them. The 

 descriptions hitherto given have gone on the assumption that the 

 rocks were of ordinary sedimentary origin, their crystalline cha- 

 racter being due to subsequent metamorphosis. 



But, so far as I can understand it, the whole is one great volcanic 

 complex, of which we only see the base — the rocks with fluidal 

 structure, and the scorige, if they were ever present, having disap- 

 peared. There are, in fact, besides the granite, four main types of 

 rock within the area — the halleflintas, the pelites, the gneisses, and 

 the diorites. 



1. The Halleflintas, Dr. Callaway makes his halleflinta occupy 

 a very small area, from less than half a mile on the coast to nearly 

 nothing at Gualchmai. It is a remarkable, compact rock, without 

 a sign of lamination or stratification, and consists of very minute 

 quartzose fragments. When the grey gneiss is brecciated it is 

 hard to distinguish between the two ; indeed, I think, the rock near 

 Llangwllog, noted by Dr. Callaway, is of the latter kind; this, 

 however, can only contain fragments of its own substance, whereas 

 the halleflinta contains foreign particles, of which the most easily 

 recognized are of plagioclase felspar. This rock runs in a narrow 

 band from the coast to Gualchmai, where it all along intervenes 

 between the grey gneiss and the granite. It then passes on the 

 western side of the granite tongue, and is in its most beautiful form, 

 a blue, slaty-looking, finely false-bedded rock, by the side of the 

 Holyhead road. It cannot be clearly recognized much beyond Tyn- 

 rhos, on the same line, but occurs again on the other side of the- 

 granite at Gors Mill and Gwyndy. This irregular and restricted 

 distribution is not to be wondered at in a volcanic dust, but would 

 be very puzzling in a basal rock. It so interdigitates and shades 

 off into the other rocks of the granite area, that I take them to be 

 all connected, and to be brought next the grey gneiss by a fault, of 

 which there is abundant indication in the field by the sudden change 

 of rock *. 



2. The Pelite. This is somewhat different from the rock so called 

 in the western district, inasmuch as it tends more to stratifi- 

 cation, as in the lenticular variety. It is in fact a volcanic mud, 

 which has sometimes particles large enough to justify the title 

 of an ash. There is usually no quartz in it, except in secondary 

 veins, and the whole is composed of fine polarizing flakes. "When 

 the fragments are large enough to be recognized they might be 

 derived from a granitic rock, but not from the granite itself, which 

 in places intrudes into the pelite. It cannot, however, be always 

 distinguished in the field from an endoclast of the granite and asso- 

 ciated diorite. We find this rock intervening between the halleflinta 

 and the granite on the south shore, with an intrusive vein of the 



* Drawn by Dr. Callaway in bis article in the Geol. Mag. 1880. 



