332 A. GEIKIE ON THE SUPPOSED 



contained any ; for he could not regard the green mineral as the re- 

 sult of the alteration of mica in situ, but rather as derived from the 

 interbedded basic rocks. Among 500 specimens of granite from 

 about 400 localities he could find nothing resembling the St. David's 

 rock, and he could not regard the latter as a granite at all. A rock 

 in the very heart of this supposed intrusive mass was found to be a 

 breccia with fragments (some of them waterworn) of the stratified 

 rock of the district. 



Prof, Kenabd said that he had had a collection of specimens and 

 of microscopic slides from the rocks of St. David's submitted to him 

 by the author, and had examined them in concert with Professor 

 Zirkel, of Leipzig, and Professor Wichmann, of Utrecht. The con- 

 clusions arrived at regarding them were as follows : — 



1. The so-called " Dimetian" rock of St. David's is unquestion- 

 ably a true granite. 



2. The quartz porphyries are just such rocks as might be expected 

 to occur as apophyses of the granite ; and the specimens from Bryn- 

 y-Garn, Bock House, and St. David's left no doubt on our minds 

 that such is really their origin. They cannot be confouuded with 

 rhyolitic lavas. 



3. The conglomerate from the granite-contact shows secondary 

 quartz between its pebbles. 



4. The bands of fine tuff found intercalated with, and on various 

 horizons above, the conglomerate, consist of true tuff, and cannot 

 have been derived from the mere -superficial waste of older volcanic 

 rocks. 



5. Fine foliation is well developed among the strata above the 

 conglomerate as well as in the volcanic group below. 



Mr. T. Davies did not admit that some of the rocks cited by M. 

 Eenard were granites at all. 



Mr. J. A. Phillips had found in the St. David's rock something 

 which much resembled a crystal of mica undergoing alteration into 

 epidote. 



Prof. Bonnet stated that his remarks at the last meeting were 

 founded upon a knowledge of the whole argument of the paper, 

 which he had read through as Secretary, but that he would now cri- 

 ticise a few details in the present part. He thought that there 

 were in North Wales volcanic materials in positions similar to those 

 at St. David's. He criticised some of the mineralogical details of 

 the paper, pointing out that the presence of the so-called sericite 

 proved very little either way. The quartz porphyries might, no 

 doubt, be extensions of a granitic mass ; but granitoid rock in North 

 Wales and at the Wrekin was distinctly cut by rather similar quartz 

 porphyries. For himself he thought that on the whole the conglo- 

 merate made a good base for the Cambrian ; and he felt certain that 

 whatever the so-called Dimetian might be, it was older than the 

 Cambrian conglomerate. 



The Atjthoe pointed out that certain sections of Dr. Hicks were 

 not only incorrect but quite impossible. Dr. Hicks had put in faults 

 in the most reckless manner ; but, granting them, they did not dis- 



