Vol. 65.] ORDOVICIAN AND SILURIAN ROCKS OF CONWAY. 177 



old quarry-chute. On the southern slope ahove the main mass of 

 the creamy lavas, another nodular rhyolite is seen close to the 

 summit, and may be traced all along the face of the mountain, 

 though its course is interrupted twice by faults. In this nodular 

 rhyolite the nodules measure generally only 1 or 2 inches in 

 diameter, and the rock passes down imperceptibly into a finer rock, 

 frequently pinkish in colour, which cannot be separated from it. 

 Together these form a band about 50 feet thick. This, the Main 

 Nodular Rhyolite of the district, is seen again at Bodlondeb, and 

 on the Deganwy Hills. (See fig. 3, p. 178.) 



The brecciation of the Creamy Rhyolites themselves is often 

 beautifully seen, especially when the fragments of the light- 

 coloured rock are (as it were) cemented together by a black chert- 

 like substance. This is very hard, and must not be confused with 

 the softer black substance which is of frequent occurrence in the 

 nodular bands, and has been referred to by Prof. Bonney, 1 Prof. 

 Cole, 2 and Mr. Harker. 3 Mr. Pearnsides has suggested to me 

 that it may be a product of the rock itself, separated during its 

 alteration ; it is often seen filling the joints and minutest inter- 

 stices of the rock, and is probably quite different in origin from the 

 white crystalline quartz with which the rocks are frequently 

 veined. 



(D) Upper or Coetmor-Ash Group. — A few feet only of 

 creamy rhyolites separate the Main Nodular Rhyolite from the 

 Siliceous Ashy Grits which form the basal members of the Upper 

 Ash-Group. These grits are decidedly more felspathic in the 

 western than in the eastern parts of the district. They are very 

 much thicker also in the west, where they contain two definite 

 lava-flows and two mudstone-bands. 



Of the two lavas within the group, the lower is another nodular 

 rhyolite, which is very like the Main Nodular Rhyolite ; it extends 

 only for a short distance along the southern slope of the mountain. 

 The higher is a typical creamy rhyolite, which occupies a fairly 

 wide tract a little distance south of Conway Mountain proper, and 

 forms a subsidiary hill. It is at once succeeded by the lower of 

 the two mudstone-bands, which is apparently unfossiliferous. The 

 higher band, however, contains numerous Lingulce (L. attenuata, 

 Sow.), and the associated ashy grits are much less siliceous and 

 more shaly in character than those at a lower horizon, there being 

 an obvious transition into the overlying Dicranographis-Shales. 

 These shaly grits are exposed on the southern slope of the mountain 

 along the lower path north-east of Bron-lledraeth, and again a 

 little north of .Derwendeg; while the actual transition between the 

 highest ashes and the shales is best seen in the north-western end 

 of the Railway-Cutting section. 



1 Quart. Journ. Geol. Soc. vol. xxxviii (1882) p. 294. 



2 Ibid. vol. xlii (1886) p. 186. 



3 ' Bala Volcanic Series of Caernarvonshire ' 1889, p. 29. 

 Q. J. G. S. No. 258. n 



