^. R. Hunt — Age of the Dartmoor Granites. 99 



Channel ^ — chemically identical, they differed only in specifio gravity, 

 the heavier mineral indicating the greatest compression. From these 

 facts the author argued that the Channel and Dartmoor granites were 

 both originally of pre-Devonian and probably of Archasan age. That 

 after the deposition of the Devonian and Carboniferous rocks, the 

 Dartmoor area was fissured by the same earth-movements which 

 compressed the Channel area. The sea obtained no access to the 

 Channel granite, but penetrated the Dartmoor area. The crushing 

 of the lower rocks supplied the heat, whilst the sea supplied the 

 water, wiiich under high pressure dissolved the Archsean granite to 

 be reconsolidated with salt here and there caught up in its quartzes. 

 The reconstituted granites traversing the culm slates were truly 

 post-Carboniferous ; but the mass of Dartmoor is Archeean, occasion- 

 ally altered by super-heated brine accompanied by boracic acid. 



" The President said it was unquestionable that a very ingenious 

 theory had been propounded by Mr. Hunt, although very different 

 to that which had been hitherto held by most geologists. Without 

 going very closely into the question, or dealing otherwise than 

 superficially with the paper, he thought it was perfectly clear that 

 the Dartmoor granite and the Channel granite were of quite a 

 different age, and had an entirely different origin. To that extent 

 he was in conformity with the views of the author. But the granites 

 were quite different mineralogically. There was one very great 

 difference, namely, that the Channel gi'anites were not metalliferous 

 in the way that the Dartmoor granites were. There were no granites 

 in the British islands which were so metalliferous as the Devon and 

 Cornwall granites, and it appeared to him that they had a very 

 different origin to other granites. For that simple reason, as 

 regarded the age of the Dartmoor granite, it appeared to him that 

 the usual view that it was post-Carboniferous was the one most in 

 accordance with the facts of the case, as far as he had been able to 

 judge. But when they came to the differences of the mineral com- 

 position of these granites, he thought they had further proof that the 

 alteration of Dartmoor gi'anite had been joroduced subsequently by 

 the numerous fissures formed. These fissures had been more or less 

 injected by corroding aqueous vapours containing large quantities 

 of chlorides and fluorides of heavy metals. These crystals of salt, 

 which Mr. Hunt had found in such considerable quantity, he thought 

 (it) excessively probable had been formed from hydrochloric acid gas 

 acting on the soda in the rocks. That appeared a very much more 

 reasonable explanation of the origin than that they came from the 

 sea, or had anything to do with the sea whatever." 



The question as to the marine or plutonic origin of the brine- 

 inclusions referred to is of transcendent importance in connection 

 with the question as to whether the water ejected by volcanoes is of 

 meteoric or marine origin, or derived from the interior magma. 



Hypotheses that might meet the case of pure water might well 

 fail to explain the presence of the chlorides of sodium and potassium. 



Having carefully examined the rocks in the light of Mr. Hudleston's 

 1 Note. Withdrawn, Geol. Mag. 1892, p. 291. 



