THE ANCIENT ROCKS OP ST. DAVID'S. 195 



could be subdivided upon stratigraphical grounds. But although 

 there was no evidence of unconformity between the strata, he 

 thought that the fact of different groups of fossils succeeding each 

 other m the same area showed that those groups existed in neigh- 

 bouring seas, and had been driven, by upheaval of the sea-bottom 

 on which they lived, into the region in which they are found. 

 Hence he maintained that a change in the forms of life is evidence 

 of unconformity in an adjoining area. 



Mr. Maw remarked that under the Cambrian rocks at Llanberis 

 there are unconformable beds, which may be the equivalents of the 

 so-called greenstones of St. David's. 



Mr. Hicks admitted that the subdivisions at present in use may 

 need to be modified. He thought that the greatest break is between 

 the Menevian aud the Liugula Flags, few species passing from one 

 to the other. He regarded the upper and lower portions of the 

 lremadoc as really distinct. 



Q. J. G. S. No. 122. 



