860 LIEUT.-GEN. C. A. M c MAHON ON ROCKS OF [Aug. 1894, 



That hydrothermal agents played an important part in the forma- 

 tion of hornblende in the Cock's Tor beds is clear from the fact that 

 some of my slices contain microscopic cracks stopped with this 

 mineral, and a macroscopic crack on one of my hand-specimens was 

 filled with the same material. 



It may be convenient, before passing on to another locality, to 

 compare the altered ash-beds (38-44), above described, with the 

 nearest sedimentary beds. 



No. 37, which conformably underlies these beds, would do well, 

 so far as its lithological appearance is concerned, for one of the 

 bottom Culm series. It looks like an indurated and altered Culm 

 slate. 



The next nearest bed that I was able to find cropped up on the 

 side of the Tavistock and Moreton Hampstead road, under Cock's 

 Tor, about § mile S.W. of the altered ash-beds above described. I 

 have two specimens of this rock, but they are so exactly similar to 

 each other that I need only describe one of them — viz., No. 48, 1116, 

 specific gravity 2 - 74. Macro- and microscopically considered, this 

 is much more highly metamorphosed than No. 37. Indeed, it is as 

 much metamorphosed as the highly altered Devonian beds in the 

 neighbourhood of Shaugh. 



No. 48 is a fine-grained, spotted schist, with an unctuous feel. 

 Under the microscope it is seen to be composed of plates and fibres 

 of mica and fine-grained, irregularly-shaped granules of quartz. It 

 is profusely dotted with countless grains of opaque to translucent 

 ferrite, which imparts a redness to some of the mica, the colouring 

 matter being diffused in streaks across the slice. The whole slice, 

 under the microscope, has a streaky structure, and the spots are due 

 to segregations of mica. 



In Mr. Ussher's map, No. 1 (' The British Culm Measures '), 

 the country east of Tavistock is marked as ' Culm or Devonian.' 

 As regards the above-mentioned outcrops, I think No. 37 is 

 probably Lower Culm ; but I consider No. 48, on lithological and 

 penological grounds, as undoubtedly Devonian. 



V. The Brent Tor Series. 



It is not my intention to enter into any details regarding the Brent 

 Tor volcanic rocks, so well described by Mr. Frank Butley, F.G.S., 

 in his work already referred to ; but a few supplementary remarks, 

 regarding rocks not noticed in his memoir, may not be out of place. 



Due north of St. Michael's Church, which crowns the top of 

 Brent Tor, on the side of the high road leading to the Tor, not far 

 N.E. of the Stag's Head Inn, and S.E. of the old chimney of the 

 abandoned mine at Monkstone, there is a pit from which the 

 country rock is at present taken for road-material. This pit had 

 not, I believe, been opened when Mr. Butley visited the locality 

 more than 17 years ago. 



The rock exposed in this quarry is in some respects an interesting 

 one. I collected three good specimens of it from different parts of 



