﻿Yol. 64.] STEUCTTJRE OF THE ST. DAVID's AEEA. 383 



naturally took the Dimetian for the oldest rock in the district. 

 The view was strengthened by the fact, that until within the last 

 two years every known junction with the Dimetian was a fault. 

 Taking the view that it was a post-Cambrian intrusion, Sir Archibald 

 Geikie looked for ' contact-action,' and thought that he had found 

 it in the intensely-hardened aspect of some of the rocks. This 

 hardening, however, was now known to be due to silicification, and 

 was often best seen far away from the margin of the Dimetian. 

 A wholly new interpretation had been given by the Author to the 

 mode of occurrence of the fine-grained, more or less basic rocks that 

 rose like stacks out of the platform south-west of St. David's. 

 They contrasted so strongly in texture with the intrusions of 

 St. David's Head that they were first naturally considered as 

 approximately-contemporaneous sills in the Pebidian ; the Author, 

 however, had now conclusively shown that they were not only 

 post-Cambrian, but later than the faulting. 



Mr. J. V. Elsden said that, although he had seen many of the 

 sections described by the Author, he was not in a position to 

 discuss the main questions involved, as his own work had lain 

 farther north. With regard to the basic intrusions, however, 

 he had expected to find rocks of the St. David's-Head type in the 

 St. David's complex, and had searched for them somewhat 

 perfunctorily and without success. It was, therefore, with con- 

 siderable interest that he recognized this type among the Author's 

 specimens. The Author's observation that these basic intrusions 

 were not aifected by the post-Cambrian faulting was interesting, as 

 tending to support the view at which the speaker had arrived with 

 regard to the age of the St. David's Head intrusions, which might 

 prove to represent a comparatively-late episode in the history 

 of that area. He congratulated the Author upon the results of 

 his careful and detailed investigations, which seemed so equally to 

 divide the honours between previous workers. 



Prof. W. W. Watts referred to the importance, as illustrated by 

 this paper, of detailed mapping of difficult areas. All the junctions 

 of the ' Dimetian ' with the surrounding rocks in this area were 

 faults, except the one referred to by the Author, in which, as 

 in other known cases, by the departure of the fault from the older 

 rocks, a mere trace of newer rocks had been found in unconformable 

 contact with the granophyre. 



The Author desired to say that he was most grateful for the kind 

 compliments paid to his work by the Chairman and others who had 

 taken part in the discussion. Boulders of the rocks described by 

 Mr. Elsden, which diff'ered greatly from the characteristic basic 

 intrusives of the district, were very common, but they undoubtedly 

 also occurred in situ, the specimens shown being from a deep well. 



LJ.G.S. No. 255. 2 c 



