No. 5.] HUNT — PRE-CAMBRIAN ROCKS. 269 



various pre-Cambrian rocks in parts of Carnarvonshire and 

 Anglesey with Messrs. Hicks, Torell and Tawney. He subse- 

 quently, in company with Dr. Hicks, visited the region in South 

 Wales where these older rocks had been studied, and was enabled 

 to satisfy himself of the correctness both of the observations and 

 conclusions of Dr. Hicks, and of the complete parallelism in strati- 

 graphy and in mineral composition between these pre-Cambrian 

 rocks on the two sides of the Atlantic. It may here be mentioned 

 that Dr. Torell, who, during his visit to America in 1876, had 

 an opportunity of studying, with the writer, the petrosilexes of 

 New England and Pennsylvania, which he regarded as identical 

 with the halleflinta of Sweden, at once recognized them in the 

 Arvonian series of North Wales. 



Of the many areas of these various pre-Cambrian rocks which 

 the writer was enabled to examine in company with Dr. Hicks, 

 may be mentioned the granitoid mass of Twt Hill in the town 

 of Carnarvon, and the succeeding Arvonian to Port Dinorwic, 

 followed, across the Menai strait, by the Pebidian on the island 

 of Anglesey, near the Menai bridge. Farther on, the Pebidian 

 was again met with near the railway station of Ty Croes, in the 

 southwest part of the island, succeeded by a large body of Ar- 

 vonian petrosilex, and a ridge of granitoid gneiss, fragments of 

 which make up a breccia at the base of the Arvonian series. 

 The Pebidian is again well displayed at Holyhead. 



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. It may be 

 remarked that the Dimetian, as originally defined at this, its 

 first recognized locality, included a great mass of Arvonian petro- 

 silex, the two forming a ridge which extends for some miles in a 

 northeast direction, flanked by Pebidian rocks, which are some- 

 times in contact with the one and sometimes with the other 

 series. At Clegyr bridge was seen the base of the Pebidian, 

 already mentioned as consisting of a conglomerate of Arvonian 

 fragments. Another belt of the same crystalline rocks was also 

 visited, a few miles to the eastward of the last, and not far from 

 Haverfordwest, forming, according to Hicks, a ridge several 

 miles in length and about a mile wide. Where seen, at Roch 

 Castle, it was found to consist of Arvonian petrosilex, with some 

 granitoid rock near by. The ridge is flanked on the northwest 



