Vol. 50,] IGNEOUS ORIGIN ON DARTMOOR. 365 



we adhered to the idea suggested by De la Beche, and stated 

 that " the possibility of some portions of the Lizard schists having 

 resulted from the alteration of a stratified basic rock must not 

 be left out of sight." The evidence afforded by the Cock's Tor 

 beds appears to establish the correctness of this hypothesis by 

 revealing the first stage in the series of changes that converted a 

 fine-grained ash into a hornblende-schist. The Cock's Tor and the 

 Lizard rocks, studied together, also show that the main agents which 

 effected the conversion of the augite into hornblende were aqueous, 

 and that the lines of original lamination assisted the action of those 

 agents ; so that the alteration of the augite into hornblende went 

 on hand in hand with the production of schistosity. 



It only remains, in conclusion, to offer a few remarks on the 

 relations of the epidiorite and the volcanic rocks. Some former 

 writers have rather twitted De la Beche's Survey with having 

 mapped such rocks as the epidiorite and rocks of volcanic origin 

 (including ashes) under one wash of colour. But I am not sure 

 that De la Beche was so entirely wrong as his critics supposed. Their 

 criticisms were made under the impression that the rocks we now 

 call epidiorites solidified under plutonic conditions, and several 

 writers have called them gabbros. We know now that these rocks 

 are only altered dolerites ; and I do not think, from what I have seen 

 of them, that we need regard them as of very deep-seated origin. 



There is no actual evidence in the area embraced in this paper 

 that the epidiorites are intrusive rocks. They certainly, in the 

 Sourton Tors and Meldon area, appear to conform to the bedding of 

 the ash- and lava-beds. I discovered no evidence of transgression in 

 their relation to the sedimentary rocks, and I think that their in- 

 trusive habit may, in many cases, have been assumed from their 

 supposed plutonic character. But even if they should ultimately 

 turn out to be intrusive sheets, or sometimes sheets or dykes, and 

 sometimes flows, I do not think this need necessarily divorce them 

 from the volcanic eruptions of that period. Flows, sheets, and dykes 

 are associated in almost every volcano ; and in studying under the 

 microscope samples of dykes and flows collected from the old crater- 

 walls of Somma I could not discover any material difference in their 

 structure. The epidiorites of the West of Dartmoor may have been 

 comparatively deep-seated offshoots of the volcanic forces that seem 

 to have opened up numerous volcanoes in this region during the 

 Carboniferous age. I regard the numerous volcanic rocks in the 

 Tavistock-Okehampton area as the outcome of several small volca- 

 noes rather than of one large one. 



Mr. J. G. Goodchild has shown in a recent paper 1 that a holo- 

 crystalline structure may possibly be set up by contact-action in the 

 glassy magma of a lava, with the result that the rock assumes 

 " a plutonic instead of a volcanic facies." That factor must be borne 

 in mind in this region, where contact-metamorphism has been so 

 active ; but I do not think it necessary to fall back on that hypothesis 



1 Geol. Mag. 1894, p. 24. 

 Q. J. G. S. No. 199. 2 c 



