ON THE SUPPOSED PRE-CAMDRIAN ROCKS OF ST. DAVId's. 261 



19. On the Supposed Pre-Camdrian Rocks of St. Dayid's. By 

 Archibald Geikie, Esq., P.R.S., F.G.S., Director-General of 

 the Geological Survey. (Read March 21 and April 11, 1883.) 



[Plates VIII.-X.] 



Contents. page. 

 Introduction 261 



Part I. 



1. "Dimetian" 267 



2. "Arvonian" 280 



3. "Pebidian" 283 



4. Relation of " Pebidian " to Cambrian Rocks 286 



5. Conclusion 291 



Part II. 



1. Order of Succession of the Rocks 294 



2. Geological Structure of the District 308 



3. The Foliation of the District 310 



4. The Granite, Quartz-porphyry, and accompanying Metamor- 



phism 313 



5. The Diabase Dyke3 and Intrusive Sheets 323 



6. Conclusion 324 



Introduction. 



Some explanation is required of the grounds on which another 

 contribution is added to the already voluminous literature connected 

 with the rocks of St. David's. The circumstances under which I 

 have reluctantly found myself compelled to make this contribution 

 are briefly these. 



A new edition of the Rock Catalogue of the Museum of Practical 

 Geology, Jermyn Street, being required, it became necessary to 

 ascertain how far the stratigraphical order followed in previous 

 editions required to be modified by the results of more recent 

 research. In particular, the question of the existence of Pre-Cam- 

 biian rocks in Wales, which has emerged since the last issue of the 

 Catalogue, appeared to demand full recognition in any republication 

 of the work. I felt it to be due to those authors who have written 

 so largely on this subject that ample acknowledgment of the results 

 of their labours should be given. On the other hand, I was equally 

 desirous that in admitting corrections of the views expressed upon 

 the maps and sections of the Geological Survey I should do so from 

 an actual inspection of the ground, which would enable me to judge 

 how far and in what manner the required alterations should be 

 made. 



It was obvious that in maps of districts surveyed forty years ago 

 some important structures might have escaped notice, positive errors 

 might have been committed, and petrographical details could not be 

 expected to have been treated in a more satisfactory manner than 

 in other English geological works of the same date. 



The existence of Pre-Cambrian rocks in South Wales had been 

 Q.J.G.S. No. 155 x 



