Vol. 50.] IGNEOUS ORIGIN ON DARTMOOR. 349 



the time of ray visit, was to be seen cutting across these altered ash- 

 beds, at right angles to their strike. I sliced and examined a spe- 

 cimen of it, No. 29, 1161. The slice is composed entirely of quartz 

 and a pale yellowish-red mica. Some of the mica is in radiating 

 groups : it has little dichroism and feeble double refraction. There 

 are also colonies of colourless belonites. The quartz is like the 

 quartz of the neighbouring granite : it contains a profusion of gas 

 and liquid cavities, some separate, some combined. Some of the 

 liquid cavities contain three or four rectangular crystals, as well 

 as a bubble. 



It is not improbable that this quartz-vein emanated from the 

 white granite of Meldon, which is at no great distance. 1 mention 

 the occurrence of this vein for what it is worth, as other better 

 examples may hereafter be found ; and such veins, if they can be 

 associated with the granite, will elucidate the question of the relative 

 ages of the granite and the rocks of volcanic origin in this locality. 



On the Geological Survey map, another outcrop of ' greenstone ' 

 is marked to the S.E. of the white granite between Black Down and 

 Longstone Hill. On going up from a tributary of the West Okement 

 River and mounting the flank of the Black Down, blocks of this 

 outcrop are seen which weather like a coarse-grained agglomerate. 

 The dip of the rocks here is X. 20° W. 



Volcanic Tuffs. 



' No. 30 1200. Sp. Gr. 270. 1 Q . . . , , 



o-i lorn o-fiQ i Specimens °i these rocks. 



„ 32 1202. „ 2"69. Matrix of this agglomerate. 



„ 33 1203. „ 2-65. One of the included blocks. 



No. 30 is made up of fragments of trachyte, and of altered 

 sedimentary rocks embedded in a very fine-grained microcrystal- 

 line-granular matrix, which equals in amount, if it does not 

 predominate over the fragments. The matrix must originally have 

 been a very fine dust. No. 31 is so completely composed of trachyte 

 that its clastic structure is not at first apparent. This, however, 

 •came out more clearly in a second and thicker slice, which I had 

 prepared for chemical purposes, particularly before it was subjected 

 to the action of hot acid. 



No. 32 is the matrix of a coarse-grained agglomerate, and 33 is 

 a portion of a large included fragment. Under the microscope 

 No. 32 is seen to be made up of fragments of trachytic and altered 

 sedimentary rocks, embedded in a microcrystalline-granular ground- 

 mass. No. 33 is a piece of trachyte traversed by several quartz- 

 veins, which, as is commonly the case in these rocks, exhibits a 

 micro-tessellar structure. 



All the slices contain a profusion of mica varying in colour in 

 transmitted light, from orange to greenish-brown ; the mineral is 

 completely soluble in hot, dilute hydrochloric acid. The solution 

 contains iron, alumina, magnesia, lime, and potash, and the mica 

 therefore appears to be some species allied to lepidomelane. 



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



