366 E0CXS OF IGNEOUS ORIGIN ON DAETMOOB. [Aug. 1 894, 



in the present case. Prof. Judd has shown, in his well-known 

 papers on the Western Isles of Scotland and on the ancient volcano 

 of Schemnitz, that the igneous rock that was poured out at the 

 surface as a lava when " undergoing consolidation at some depth 

 from, the surface assumed a most perfectly granitic character." l 

 And he states in another paper that " the distinction between 

 plutonic and volcanic rocks — however convenient and necessary it 

 may be in practice — is a purely arbitrary one, some lavas being more 

 highly crystalline than certain portions of intrusive masses." 2 



In the present case, t* confine myself to the area embraced in 

 this paper, I should say that the epidiorites are a long way short of 

 being hypogene rocks (I have not found a particle of hypersthene or 

 diallage in any of them), and that there is nothing in their micro- 

 scopical structure to prevent them from being connected with the 

 volcanic activity of the Carboniferous age. 



Discussion. 



Mr. Rutley considered that this paper would be of great value 

 not only to those geologists who had already worked in the Brent 

 Tor area, but also to those who might do so in the future. The 

 augitic rocks described by the Author as occurring on the western 

 side of Cock's Tor, rocks which had hitherto been completely over- 

 looked, were of especial interest, and would have to be taken into 

 consideration in the future mapping of the district. The discovery 

 by General M c Mahon of a dolerite at the spot which had been 

 indicated many years ago as the probable site of the original vent of 

 the Brent Tor volcano was, he hardly needed to say, most gratifying to 

 him. He briefly explained the old diagrams used in the illustration 

 of his former paper, stating that one of the two faults, there indi- 

 cated as bounding the downthrown portion of the cone, was first 

 noted by the late Dr. Harvey B. Holl. A few remarks were also 

 made upon the schistose lavas and tuffs of the neighbourhood, and 

 on the great difficulty often experienced in assigning such rocks to 

 their respective groups. 



Prof. Bonnet said that there were two points of great interest — 

 (1) the alteration of the trachytic ashes, which seemed to be some- 

 what abnormal, and (2) the conversion of a basic ash into something 

 like a hornblende-schist. The latter was a most important contri- 

 bution to a very difficult subject. He was quite prepared to believe 

 that hornblende-schist might have this origin, though his work in 

 Sark had made him a little more doubtful than formerly about the 

 origin of the hornblende- schists of the Lizard. 



Mr. W. W. Watts also spoke. 



The Authoe, in reply, indicated the exact position of the Cock's 

 Tor beds, and thanked the Fellows present for the kind and sym- 

 pathetic way in which they had received his paper. 



1 Quart. Joum. Geol. Soc. vol. xxxii. (1876) p. 323. 



2 Op. cit. vol. xlv. (1889) p. 191. 



