Correspondence — Professor T. G. Bonney. 235 



(the last sometimes becoming marbles) are not 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 Sourton 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, which 

 give evidence on their weathered surface of an original laminated 

 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 com- 

 pleted, and the final stage, due to aqueous agencies, had just begun. 



The paper concludes with some remai-ks on the relationship of 

 the epidiorites to the rocks of volcanic origin. 



cos,E,Es:poIs^ID:E!3i^^o:E!. 



GEOLOGICAL DIFFICULTIES IN THE LEPONTINE ALPS. 

 Sir, — Dr. Stapff's interesting paper in your April Number seems 

 to call for a few words of explanation from myself. As regards the 

 Altkirche marble, the problem which it presents has been discussed 

 in a paper recently read before the Geological Society, to which 

 I may refer. Here, it may suffice to say, that I have not denied 

 that certain limestones near Altkii-che may contain fragments of 

 organisms, but doubt the occurrence of these in the true marble, 

 i.e. we have here not only to distinguish imitative from truly organic 

 markings (in which photographic evidence often is not sufficient), 

 but also to settle the age of the rock in which the latter occur. 



