Dr. H. Hicks — Pre-Cambrian Rocks in Conglomerates. 517 



Table showing the Rocks which hate been found in the Cambrian 

 Conglomerates in different areas. 



most conclusive manner that some peculiar granitoid rocks, basic 

 and acid volcanic rocks, schistose rocks, porcellanites, and argillites, 

 similar to those which are found in the pre-Cambrian axis, occur as 

 rolled fragments in the overlying Conglomerates. As these fragments 

 are now, in all important particulars, identical in character with and 

 can be shown to have suffered all the mineralogical changes which 

 the rocks from which they were derived had undergone before they 

 were broken off, it is perfectly evident that not only must there 

 have been a considerable lapse of time, separating the Cambrian 

 from the pre-Cambrian, but also that during that interval the area 

 must have been subjected to very important physical changes. The 

 following places may be mentioned as offering important evidence 

 in support of the above views. At Ramsey Island, and Treffgarn 

 in Pembrokeshire, at Bangor, and near Llanberis and Bethesda in 

 Carnarvonshire, where the Cambrian Conglomerates rest on felsites 

 and old rhyolites, more than three-fourths of the pebbles, which are 

 frequently of very large size, have been derived from the imme- 

 diately underlying rocks. Near St. David's, and at other places 

 where the conglomerates rest on various altered volcanic tuffs, a 

 large proportion of the pebbles have been derived from those tuffs 

 after they wei'e cleaved and otherwise changed into their present 

 condition. At Porthclais, Chanter's Seat, and Port Melyn near 

 St. David's a large number of the pebbles (mostly of small size) 



