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



precisely that in which Mr. Rutley placed the throat of his volcano ; 

 and the rock itself would answer very well for a lava seething up in 

 this neck from below. 



VI. Was Tor. 



Immediately north of the Lydford Junction railway-stations, and 

 overhanging the G. W. Railway on its western side, is a little 

 hill marked Was Tor on the Ordnance map. As the rocks exposed 

 here have not been described in Part II. of Mr. Rutley's memoir, 

 or alluded to in Part I. of that work, and as Mr. R. N. Worth 

 in his ' Geological Notes on the South-Western line between Lyd- 

 ford and Devonport,' also passes them over without notice, a short 

 description may be given here. 



These rocks appear to be in the same line of outcrop as the 

 Bowdon-Longstone ' greenstone ' marked on the Geological Survey 

 map ; but the Bowdon • greenstone ' is represented as stopping 

 more than | mile short of Was Tor. 



I collected four specimens from the top of the Tor and two from 

 an old quarry on its north-eastern flank. A short description of 

 them is appended below : — 



From the top of Was Tor. 



No. 52 1108. Sp. Gr. 268. 



„ 53 1110. „ 2-66. 



„ 54 1109. „ 2-94. 



•„ 55 1111. „ 295. 



From a quarry on like flank of Was Tor. 



No. 56 1112. Sp. Gr. 265. 



„ 57 1113. „ 2-68. 



No. 52 is a highly carbonaceous shale, evidently one of the Culm 

 series. This locality is coloured ' Lower Culm Basement Beds, 

 Dolerites or Tuffs,' in Mr. Ussher's Map I. in his ' British Culm 

 Measures.' Under the microscope the rock is seen to be made up of 

 grains of sand, intermingled with carbonaceous material and minute 

 flakes of mica. The carbon is arranged in wavy lines, and network 

 of lines, and the whole structure is clearly due to deposition in 

 water. The rock contains magnetite, but most of it has been dis- 

 solved out, leaving cube-shaped cavities. After the iron had been 

 removed with hydrochloric acid the carbon was easily driven off 

 by heat, the powdered rock becoming quite colourless. 



No. 53 is a fine-grained sedimentary rock of somewhat slaty type. 



No. 54 is of altogether different character. It is a compact rock, 

 something between sage-green and grey in colour. It has not the 

 smooth look or the unctuous feel of an ordinary serpentine, possess- 

 ing a flue granular structure and an appearance of roughness on the 

 fractured surface. In the field it would probably pass for an 

 ordinary compact lava : its hardness is 3'5. 



Examined chemically the rock was found to be a hydrated silicate of 

 iron and magnesia. It also contained alumina, but this was quite 



