﻿422 Geological Society : — 



the southern one can be traced right across the pass, hut the extent 

 of the other is not so clear. 



Examination of the marble mentioned above shows in all cases 

 that it has been considerably modified by pressure since it became a 

 crystalline rock. The author discusses the evidence of these sections, 

 and maintains that the hypothesis that the marble is an older rock 

 intercalated by thrust-faulting among Jurassic strata leads to fewer 

 difficulties than to consider it as belonging to the same system. 



In the latter part of the paper the results of a re-examination of 

 the ravine-section in the Yal Canaria, and of some studies of the 

 south side of the Val Bedretto are described, which, as the author 

 maintains, confirm the view already expressed by him, viz., that 

 the schists with black garnets, mica, kyanite, dolomite, and calcite 

 (the last sometimes becoming marbles) are rot altered Jurassic rocks 

 but are much older. 



2. ' Notes on some Trachytes, Metamorphosed Tuffs, and other 

 Eocks of Igneous Origin, on the Western Flank of Dartmoor/ By 

 Lieutenant-General C. A. McMahon, F.G.S. 



In this paper the author notices the occurrence of felsite and 

 trachyte at £ourton Tor ; of rhyolite and of aluminous serpentine 

 at Was Tor ; and of a dolerite at Brent Tor in the exact situation 

 indicated by Mr. Rutley as the probable position of the throat of 

 the Brent Tor volcano. 



The author describes extensive beds of tuffs at Sourton Tor and 

 Meldon, the matrix of which has been converted, by contact-meta- 

 morphism, into what closely resembles the base of a rhyolite, and 

 which, in extreme cases, exhibits fluxion- structure, or a structure 

 closely resembling it. The fragments included in this base are so 

 numerous that six or seven different species of lavas may be seen 

 in a single slide ; this fact, and a consideration of the extensive 

 area over which these beds extend, lead the author to believe 

 that these beds are metamorphosed tuffs and not tufaceous lavas. 



He then describes some beds on the flank of Cock's Tor, whicli 

 give evidence on their weathered surface of an original lami- 

 nated structure by exhibiting a corded appearance like corduroy 

 cloth. These beds are composed of colourless augite, set in a base 

 which in ordinary light looks like a structureless glass, but which 

 between crossed nicols is seen to be an obscurely crystalline felspar. 



The author compares these rocks with that portion of the Lizard 

 hornblende -schists for which a tufaceous origin was proposed by 

 De la Beche and other writers, including Prof. Bonney and himself. 

 He shows that the Lizard schists and the Cock's Tor rocks agree in 

 specific gravity and in some other characteristics ; and he concludes 

 that at Cock's Tor the first stage in the conversion by contact- 

 action of beds of fine volcanic ash into hornblende-schist had been 

 completed, and the final stage, due to aqueous agencies, had just 

 begun. 



The paper concludes with some remarks on the relationship of 

 the epidiorites to the rocks of volcanic origin. 



