364 LIEUT. -GEN. C. A. M c MAHON ON ROCKS OP [Aug. 1 894, 



apparently, more or less rounded crystals of felspar. The felspars 

 have been converted into a soft white substance (pale buff-coloured 

 in transmitted light), which exhibits a feeble double refraction 

 between crossed nicols : some of it has been partially replaced by 

 quartz. Between crossed nicols the groundmass breaks up into an 

 isotropic base ; in this occur doubly-refracting fibres and irregular 

 patchy aggregations of fibres, which are, I think, imperfectly 

 developed mica. Here and there are undoubted leaves of mica. 

 The slice also contains numerous granules of ferrite. 



VII. Summary and Conclusion. 



In the preceding pages I have noted the occurrence of felsite and 

 trachyte at Sourton Tors ; of rhyolite, and a variety of aluminous 

 serpentine, believed to have been derived from a basaltic rock, at 

 Was Tor ; and of a dolerite in the exact situation indicated by 

 Mr. Rutley as the probable position of the throat of the ancient Brent 

 Tor crater. 



At Sourton Tors, and at Meldon, on the West Okement Biver, I 

 have recorded the occurrence of some interesting tuffs, the matrix 

 of which has been converted by contact-metamorphism into what 

 closely resembles the base of a rhyolite, and which, in extreme 

 cases, exhibits fluxion-structure, or a structure indistinguishable 

 from it. So complete is the resemblance which this matrix assumes 

 to the base of an igneous rock that I was for long doubtful 

 whether the rock was not a lava full of volcanic ejectamenta; but 

 the extreme abundance of the fragments — pieces of six or seven 

 different kinds of lava being sometimes visible in a single slice — 

 taken into consideration with the extended area over which these 

 deposits are to be found, convinced me that these beds are really 

 metamorphosed tuffs. 



The occurrence of other interesting beds on the flank of Cock's 

 Tor, not noticed by previous observers, is also noted. These beds 

 now consist of a mixture of nearly colourless augite set in a base 

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

 between crossed nicols, is seen to be made up of obscurely crystal- 

 line felspar. Many rocks reveal on their weathered surfaces the 

 secret of their primitive structure, and these beds do so in a striking 

 manner ; their weathered surface exhibiting a corded appearance, 

 like corduroy cloth, that betrays, with a clearness that leaves little 

 room for doubt, the existence of an original lamination. Certain 

 appearances in some of the slides under the microscope confirm this 

 supposition, and I consider that the Cock's Tor rocks were originally 

 beds of fine-grained volcanic dust, which, in consequence of the 

 intense contact-metamorphism engendered by the great mass of 

 the Dartmoor granite, was converted into a mixture of augite and 

 felspar. 



In my first paper on the Lizard schists I showed that the latter 

 contain numerous unaltered crystals of augite, and that the horn- 

 blende is a secondary mineral formed by aqueous agencies from the 

 augite; in a joint paper written by Prof. Bonney and myself, 



