﻿Vol. 66.~] TREMADOC SLATES OF SOUTH-EAST CARNARVONSHIRE. 1ST 



Plate XVI. 

 Sections across the Ynyscynhaiarn anticline, on the scale of inches to the 

 mile : — C, from Grell to Black Eock Beach ; D, from Careg-yr-Eryr to 

 Ynys-towyn. 



Plate XYII. 



Geological map of the Ynyscynhaiarn District (South-East Carnarvonshire),, 

 on the scale of 3 inches to the mile. 



Discussion. 



The President (Prof. W. J. Sollas) congratulated the Author on 

 his admirable account of a classic locality ; it was complete in every 

 respect — in mapping, in palaeontology, and in tectonics. The 

 problem of deceptive conformities was one of great importance, 

 and awaited investigation. The presence of a visible unconformity 

 at Tremadoc which could not be detected in South Wales was a case 

 in point ; another would seem to be that which in many localities 

 occurred at the base of the Upper Silurian, and in others was 

 apparently absent over wide areas. The intrusion of basic rocks 

 dong the thrust-planes afforded an additional instance to those 

 already made known by Suess. The most magnificent example was 

 afforded by the ' exotic blocks ' of the Himalaya. 



Dr. Marr said that he would have wished to refer to many 

 things, such as the position of the Dictyonema Band, and the correct 

 designation of the fossil referred to that genus ; but at that late hour 

 he would only congratulate the Author on his paper, wherein he 

 had practically completed one piece of work, and given a great 

 impetus to another. He had in this and in a previous paper shown 

 definitely what were the Arenig Beds of Arenig and the Tremadoc 

 Beds of Tremadoc, for which all students of the Lower Palaeozoic 

 rocks, at home and abroad, must thank him. He had, by his careful 

 study of the tectonics of the district, added largely to that most 

 interesting subject, the building up of North Wales, and pointed 

 the way to much future work therein. 



Prof. Watts remarked that the Author's work was bringing the 

 Tremadoc Beds of Tremadoc into line with those of Shropshire. 



Mr. J. F. N". Green observed that, near St. David's, where 

 folding and thrusting similar in many respects to the phenomena 

 described by the Author occurred in rocks of the same age, 

 quartziferous and hypersthene-bearing dolerites had been de- 

 scribed in detail by Mr. J. V. Elsden. That author and the 

 speaker came independently to the conclusion that these dolerites 

 were later than the main faulting. 



The Author, in reply, thanked the President and Fellows for 

 their very generous appreciation of his paper. 



In answer to Mr. Green, he said that, although in the district 

 under consideration he had found no fault such as could be mapped 

 later than the dolerite-intrusions, there was at Garth Hill Quarry, 

 Minffordd, a very prominent fault of such a character which had 



