THE PEBIDIAN VOLCANIC SERIES OF ST. DAVIDS. 



241 



14. The Pebidian Volcanic Series of St. Davids. By Prof. 

 C. Lloyd Morgan, P.G.S., Assoc.E,.S.M. (Eead January 8, 

 1890.) 



[Plate X.] 



Contents. 



1. Introbuction. 



2. The Eelation op Pebidian to Cambrian. 



a. Greneral Considerations. 

 h. In Caerbwdy Valley. 



c. In St. Non's Bay. 



d. At Ogof Goicbfa (Wliitesand Bay). 



e. In Ramsey Sound. 



/. The Materials of the Conglomerate. 

 g. Summary of the Evidence. 



3. The Pebidian Succession. 



a. The Eesults of previous Investigations. 

 h. General Eesults of the Author's Survey. 



c. Furtlier Details. 



d. Faults and their Effects. 



e. The Cambrian Overlap. 

 /The "Felsitic" Dykes. 

 g. The Porcellanites. 



4. The Eelation of Pebidian to Dimetian. 



a. Dr. Hicks's Arvonian. 

 h. The Dimetian Question. 



c. In Porthlisky Bay. 



d. In the Allan Valley at Porth Clais. 



e. At Ogof Llesugn. 



/. General Considerations. 



g. The Evidence from Included Fragments. 



o. Conclusion. 



1. Introduction. 



Two strongly contrasted interpretations of the structure and geolo- 

 gical genesis of the St. David's promontory are before geologists. 

 The first of these is that advocated for many years by Dr. Hicks 

 and finally summed up by him in 1884 (Quart. Journ. Geol. Soc. 

 vol. xl. p. 507). On this view there are at St. Davids, besides the 

 strata of Cambrian age, three distinct Pre-Cambrian systems — the 

 Dimetian, Arvonian, and Pebidian. Each is unconformable to the 

 others. The Dimt3tian was very much in its present state before the 

 Arvonian was laid down. The Arvonian halleflintas, again, w^ere 

 brought to their present mineral condition before the Pebidian series 

 was deposited. The Pebidian volcanic beds, once more, were nearly 

 as highly metamorphosed as they are at present before the Cam- 

 brian Conglomerate was formed. Each is therefore separated from 

 the succeeding group by a systematic unconformity, and we have, 

 on this view, to quote the words of Mr. Ta\vney, " a vista opened 



