278 T. S. Hunt—History of Pre-Cambrian Rocks 
to which belong the stratified petrosilex or quartziferous por- 
phyries already noticed. ese are sometimes wanting at the 
bas 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 great quartziferous-porphyry ridges 
of Carnarvonshire, Dr. Hicks and his friends proposed the 
name of Arvonian, from Arvonia, the Roman name of the 
region. 
This important conclusion was announced 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 
various pre-Cambrian rocks in parts of Carnarvonshire and 
Anglesey with Messrs. Hicks, Torell and Tawney. He subse- 
uently, in company with Dr. Hicks, visited the region in South 
ales where these older rocks had been studied, and was 
ales. 
Of the many areas of these various pre-Cambrian rocks 
In South Wales, the similar rocks were examined by him at 
St. David’s, where three small bands of an impure coarsely 
crystalline limestone are included in the Dimetian granitoid 
rock, which is here often exceedingly quartzose. b 
remarked that the Dimetian, as etatoal J i 
first recognized locality, included a great mass of Aryonian 
