428 THE GEOLOGY OF THE TINTAGEL DISTRICT. [Aug. I903. 



EXPLANATION OF PLATE XXV. 



Geological map of the Tintagel and Davidstow District, on the scale ot 

 1 inch to the mile. 



Discussion. 



Prof. Bonnet expressed his sense of the value of the paper, 

 upon which he knew that the Author had expended great pains, for 

 he had seen the work at more than one stage in its progress. He 

 quite agreed that there was evidence of pressure and in some parts 

 of contact-metamorphism, although no granite was seen above 

 ground. He had thought that the obscure white spots were 

 probably a secondary felspar, and enquired whether the mineral 

 named ' clinochlore ? might not be ottrelite. 



Mr. H. H. Thomas congratulated the Author on his discovery of 

 staurolite in the metamorphosed sediments ; although this mineral 

 was by no means rare in other similar localities, it had (to the 

 speaker's knowledge) only once before been mentioned from either 

 Devon or Cornwall. The occurrence had been noted by the late 

 R. N. Worth, in Devon. This was therefore an interesting addition 

 to the list of Cornish metamorphic minerals. 



Mr. Teall said that the subdivision of the great Killas formation 

 of Cornwall was attended with considerable difficulty, in con- 

 sequence of the general absence, over large areas, of any sharply- 

 defined lithological horizons. The re-survey which was now in 

 progress was approaching the area in question, and he had no 

 doubt that the Author's detailed mapping, coupled as it was with 

 careful descriptions of the rocks, would prove to be of consider- 

 able service. He was glad that the Author had preceded the 

 Geological Survey, and had found at least one group of most 

 interesting rocks which could be mapped with comparative ease. 



The Author, after thanking the Fellows for the kind manner in 

 which the paper had been received, said, in reply to a question from 

 the President, that the only indubitable instances of contact- 

 metamorphism were from the south-eastern part of the district ; but 

 that, if contact were not responsible for the changes wrought in the 

 Volcanic Series, he was at a loss to assign a cause. These changes 

 were at least as great on the coast as in the inland part of the 

 district. So far as he knew, the metamorphism was unlike that of 

 rocks as yet described from other parts of Cornwall or from Devon ; 

 it recalled, however, in one or two instances the schists of the Start 

 district. In reply to Prof. Bonney's question, concerning the dis- 

 tinction between the clinochlore shown on the screen and ottrelite, he 

 said that the former mineral was softer, had a lower specific gravity, 

 and also lacked the bluish element in the pleochroism. Commenting 

 on Mr. Teall's remark as to the usefulness of the Volcanic Series for 

 mapping, the Author stated that he thought it probable that these 

 rocks were continued south-westward, but that he believed that their 

 metamorphic character was not maintained. 



