268 THE CANADIAN NATURALIST. [Vol. ix. 



Huronian, and compared the Dimetian with the Upper Laur- 

 entian of Logan. 



The similar crystalline rocks of North Wales, already noticed, 

 were now studied by Professor T. McKenny Hughes of Cam- 

 bridge, who described them in 1878. These include in Carnar- 

 Tonshire and Anglesey the greenish crystalline schists which 

 the writer in 1871 referred to the Huronian (pre-Cambrian of 

 Sedgwick, and Altered Cambrian of the geological survey), 

 certain granitoid rocks formerly described as intrusive syenite, 

 and also a reddish feldspar-porphyry which forms two great 

 ridges in Carnarvonshire. This latter was by Professor Sedg- 

 wick regarded as intrusive, and is moreover mapped as such by 

 the geological survey, though described in Ramsay's memoir on 

 the geology of North Wales as probably the result of an extreme 

 metamorphism of the lower beds of the Cambrian. The pre- 

 Cambrian age of all these rocks was clearly shown by Hughes, 

 who however considered that the whole mio-ht belons: to one 

 great stratified series ; while Hicks, from an examination of the 

 same region, regarded them as identical with the Dimetian and 

 Pebidian of South Wales. 



Dr. Hicks continued his studies in both of these reoions in 

 1878, — being at times accompanied by Dr. Torell of Sweden, 

 Professor Hughes and Mr. Tawney of Cambridge, and the 

 writer — and was led to conclude that^ beside the chloritic schists 

 and greenstones (diorites) of the Pebidian, and the older gran- 

 itoid and gneissic rocks, there exists, both in North and South 

 Wales, a third independent and intermediate series, to which 

 belong the stratified petrosilex or quartziferous porphyries already 

 noticed. These are sometimes wanting at the base of the 

 Pebidian, and at other times form masses some thousands of feet 

 in thickness. At one locality, near St. David's, a great body of 

 breccia or conglomerate, consisting of fragments of the petrosilex 

 united by a crystalline dioritic cement, forms the base of the 

 Pebidian. For this intermediate series, which constitutes the 

 quartziferous-porphyry ridges of Carnarvonshire, Dr. Hicks and 

 his friends proposed the name of Arvonia, from Arvonia the 

 Roman name of the region. 



This important conclusion was announed by Dr. Hicks at the 

 meeting of the British Association for the Advancement of 

 Science at Dublin, in August, 1878. The writer, previous to 

 attending this meeting, had the good fortune to examine these 



