76 E. Wynrw Hughes — Geology of 



(ii) Five hundred yards almost due north of Craig-y-Dinas, near 

 the farm of Eithinog-Wen, a small amount of solid rock is present. 

 A careful examination shows that this rock again compares closely 

 with the rhyolitic series of Cil-y-Coed. Porphyritic quartz and pink 

 felspars can be seen even in hand specimens. The groundmass is 

 fine-grained, compact, andfelsitic, differing only from the Cil-y-Coed 

 rhyolites in its pink coloration. Beyond Eithiuog - Wen a close 

 search of the slope of the ridge, that runs in a northerly direction 

 here, disclosed no further exposures. The large amount of debris, 

 however, that is present everywhere at the foot of the ridge suggests 

 the presence of the rhyolitic rock. 



(iii) At Bryn-mawr, 1,500 yards to the N.N.E. of Craig-y-Dinas 

 and 1,000 yards from Eithinog-Wen, several rock masses are exposed. 

 The distance between the first and last of these exposures is fully 

 600 yards. They all occur near the top of the ridge, aud are 

 undoubtedly portions of the same mass. The line of outcrop, if 

 continued in a southerly direction, would pass through the exposures 

 at Eithinog-Wen and Craig-y-Dinas. The largest of these exposures 

 is the quarry near the roadway close to Glyn-Llifon Park. It is at 

 present being worked for road-metal. In some cultivated land on 

 the north side of the road further exposures of this rock are also to 

 be seen. 



In hand specimens all the rocks compare closely with one 

 another, showing porphyritic crystals of quartz and felspar in a fine- 

 grained and compact groundmass. Several microscopic sections were 

 examined, and they bring out the close resemblance between these 

 rocks and the rhyolitic series at Cil-y-Coed. They show the presence 

 of porphyritic crystals of quartz and both orthoolase and plagioclase. 

 The quartz ci'ystals are much more numerous than the felspars, but 

 many of the latter still show multiple twinning. The groundmass is 

 cryptocrystalline, with patches sometimes coarser and sometimes 

 liner-grained, and it frequently shows good fluxion structure. 

 (PI. I, Eig. 1.) 



We have here, theu, a rock very similar, both macroscopically and 

 microscopically, to the rhyolitic rocks of Cil-y-Coed. 



C. Summary of the Succession. — r We have, then, in the area 

 between Pentwr and Bryn-mawr: — (i) Cambrian : (a) green slates at 

 Ysgubor-Wen, (b) green grit band at Pentwr, (c) purple slates at 

 Llech-y-dwr, (d) fine and coarse grits at Craig-y-Dinas, and also 

 (e) a much cleaved conglomerate at Craig-y-Dinas, all with a dip of 

 50° S.S.E., lying upon (ii) Pre-Cambrian, (/) rhyolites and tuffs 

 exposed at Craig-y-Dinas, Eithinog-Wen, aud Bryn-mawr. 



D. ItULATION TO THE SURROUNDING AREAS. — (i) South-east of 



Pentwr we have already noted the occurrence of Ordovician slates on 

 Y Foel. These can be traced through Tai-lon to Pen-y-garreg and 

 beyond. 



(ii) Unfortunately the Cambrian slates and grits cannot be 

 similarly traced to Cil-y-Coed. The strike of the beds in the two- 

 localities suggests the presence of a fault in the intervening country, 

 but no other evidence of it could be obtained, though this is the 

 direction of the fault marked on the Survey map (1850). 



