80 



STRATIGRAPHICAL GEOLOGY 



sometimes the ashy element appears to predominate, sometimes 

 it is a true arkose, consisting of quartz -sand and decomposed 

 felspar. The pebbles vary in size, ranging up to a foot in 

 diameter ; they consist principally of a reddish quartzite and of 

 quartz, but include some of quartz -felsite, porcellanite, schist, and 

 granitoid rock, all derived from the underlying pre-Cambrian rocks. 

 In Merioneth the equivalents of the Caerfai Series are to be 

 found in the central parts of the Harlech dome (see Fig. 19). The 



GEOLOGICAL MAP OP THE COUNTRY NEAR ST DAVID'S 



Granite 

 IPebidian 



lSolv> & 



JCaerfai 



Pen 



jMenevian 



Lmgula Flags 

 Tremadoc 

 Arenig 

 Llandeilo 



c/e /r 



fen Llechwen 



S? Davids H?, 



"j * 

 ^Ramsey I. 



Fig. 16. MAP OK PART oir iouTU WALES (after Hicks). 



great Harlech Series, believed tt be from 5000 to 6000 feet thick, 

 was not subdivided by Sir A.rRamsay and the officers of the 

 Geological Survey, but quite recently Professor Lapworth and Dr. 

 T. S. Wilson have mapped a large part of the district and have 

 determined the sequence ol the beds. Their detailed descriptions 

 have not yet been published, but they have allowed a summary of 

 the succession to be printed in a paper by Mr. A. E. Andrews, 

 and from this the following brief account is gleaned. 



The lowest i/eds occupy the coast between Egryn Abbey and 

 Llanfair and extend eastward to the Rhinog Mountains. The red 

 fe.lspathic grits of Dolwen are probably not far from the base of the 



