Notes on Dartmoor. 149 



peculiar features, since its downthrow in some places is on the east 

 and in others on the west. 



The lowest beds present are bluish slates with numerous Bala 

 fossils. These are succeeded immediately by the Cor wen Grit 

 of Prof. Hughes. No fossils have been found in this at Corwen ; 

 but in a grit occupying a similar position at Glyn Ceiriog numerous 

 fossils have been discovered. The Corwen grit is succeeded by grey 

 slates with grit-bands ; and in Nant Cawrddu near Corwen, and 

 Nant Llechog near Pen-y-glog, these slates are followed by banded 

 black shales containing numerous graptolites of the Monograptus 

 gregarius-zone. Above these are pale bluish slates ; and nothing 

 further is exposed till we reach the Tarannons. The Corwen Grit 

 clearly forms the base of the Llandovery in this area, as suggested 

 by Prof. Hughes. 



May 24th.— W. H. Hudleston, Esq., M.A., F.R.S., 



President, in the Chair. 



The following communications were read : — 



1. " Notes on Dartmoor." By Lieut.-General C. A. McMahon, 

 F.G.S. 



The author alludes to Part II. of a memoir on the British Culm 

 Measures recently published by Mr. Ussher, P.G.S., in which the 

 view is advanced that the granite of Dartmoor resulted from the 

 metamorphism of pre-existing rocks which had in a rigid state 

 offered obstruction to a long-sustained 1ST. and S. squeeze, and that 

 their fusion and consequent consolidation were effected in situ. 



The author gives some of the results of a visit to the western 

 borders of Dartmoor. He details some examples of eruptive granite- 

 veins intruding into Culm beds in the immediate vicinity of the 

 main mass of granite. The latter, in the locality described, is 

 porphyritic down to its boundary, and the veins are also porphyritic. 

 All the circumstances lead the author to believe that these veins are 

 real apophyses from the main mass, and that the view adopted by 

 De la Beche regarding the origin of the Dartmoor granite is the true 

 one. The author alludes to some features in the Meldon granite- 

 dyke not before noted ; gives some detailed observations in the bed 

 of the River Tavey, and offers an explanation of the way he thinks 

 the fine-grained marginal variety of the granite, seen in that 

 locality, has been produced. 



The author comments on the improbability that a tremendous 

 squeeze sufficient to fuse 225 square miles of a pre-Devonian rock into 

 granite should have left the Culm Measures outside the zone of 

 marginal contact-metamorphism almost untouched. 



The author, in conclusion, alludes to the often-observed pseudo- 

 stratification of the Dartmoor granite, and urges that the cause of 

 this is not the one suggested by De la Beche, but that it is due to 

 sub-aerial agencies. 



