ON THE SCHISTOSE VOLCANIC ROCKS OF DAETMOOR. 285 



19. On the Schistose Volcanic Rocks occurring on the west of 

 Dartmoor, with some Notes on the Structure of the Brent Tor 

 Volcano. By Frank Rutlet, Esq., F.G.S., H.M. Geological 

 Survey. (Read January 21, 1880.) 



(Communicated by permission of the Director-General of the Geological Survey.) 



After examination of the schistose beds of rock on the western 

 margin of the Dartmoor granite, I experienced great difficulty in 

 coming to any satisfactory conclusion concerning the origin and real 

 nature of many of them. These rocks had already been described 

 by Sir Henry De la Beche, in his Report on the Geology of Cornwall, 

 Devon, and W. Somerset, as schistose ashes ; and his well-known 

 keenness in detecting lithological peculiarities naturally induced me 

 to accept his opinion, especially as it seemed in some instances to be 

 a very correct one. There was, however, one point which caused 

 me considerable perplexity. I could not reconcile the very fre- 

 quent occurrence of a densely amygdaloidal structure with the per- 

 sistently schistose character of these rocks. If the amygdaloidal 

 condition implied a once vesicular structure, and if the rocks 

 should on this account be regarded as lavas, how then was the 

 schistose character to be accounted for ? I was not then acquainted 

 with any instance of schistose lava. On the other hand I could 

 not understand why, if these rocks were ashes, they should be amyg- 

 daloidal. In my memoir * on the eruptive rocks of Brent Tor and its 

 neighbourhood I endeavoured to account for the schistose structure 

 in lavas, and for the amygdaloidal condition in ashes, and in many 

 places expressed my doubt about the true character of these rocks. 

 Ever since those lines were written I have been anxious to venti- 

 late this matter, and, if possible, to arrive at a more satisfactory 

 solution of the difficulty. It is to Mr. John Arthur Phillips that I 

 owe the suggestions which have at last helped to settle the 

 question. On expressing my doubts to Mr. Phillips, and after 

 showing him the sjjecimens which I had collected, he at once stated 

 his belief that these schists were precisely similar to some which 

 he had met with in Cornwall, and which he had examined and de- 

 scribed as lavas. On re-examining my microscopic sections I was 

 still unable to satisfy myself that he was right in his conclusion ; 

 but I thought my sections were perhaps not sufficiently thin, and 

 therefore made an examination of some new and better ones which 

 were prepared by Mr. CutteU. At once I realized the truth of Mr. 

 Phillips's views. The majority of the types which I had selected 

 were evidently lavas of a more or less vesicular character, while the 

 others were ashes, fine tuff's (Schalsteins), or sedimentary schists. 



In some of the latter amygdaloidal structure is present ; and it is 

 just possible that even these may represent greatly altered lavas. 

 One section cut from a specimen collected by Sir Henry De la Beche, 

 and labelled " Vesicular ash," is an unquestionable lava. Never- 



* Memoirs of the Geological Survey, 1878. 



