Vol. 50. j IGNEOUS ORIGIN ON DARTMOOR. 347 



No. 23 also contains sphene and apatite. The former is rather 

 abundant in small granules and crystals : it is a secondary product, 

 sometimes embedded in the serpentinous substance, and at other 

 times associated with the leucoxene. 



II. Meldon. — West Okement River. 



At Meldon, under and near the railway viaduct, there are some 

 interesting rocks, the outcrop of which is entered in De la Beche's 

 geological map as ' greenstone.' Owing to vegetation and talus, no 

 actual exposure of these rocks can be seen on the banks of the river 

 below the viaduct, and their relationship to the sedimentary series 

 cannot be made out from an examination of the bed of the stream ; 

 but, judging from the manner in which they crop out higher up, and 

 on the downs above, the ' greenstones ' appear to run with the 

 bedding of the Carboniferous strata, and to dip at the same angle 

 and in the same direction as these. 



The following specimens from the outcrop adjoining the viaduct 

 have been sliced and examined : — 



Volcanic Tuffs. 



No. 25 1196. Sp. Gr. 2-71. 



„ 26 1197. „ 272. 



„ 27 1198. „ 2-70. 



Viewed macroscopically No. 25 would, I think, be taken for a 

 lava ; for, besides Mebby crystals of felspar in a compact base, 

 there are many small, lath-shaped, idiomorphic crystals of the same 

 mineral orientated in various directions. A prolonged study of this 

 specimen under the microscope has, however, satisfied me that we 

 have here only an ash which has suffered from contact-metamorphism. 

 The rock is really compounded of fragments of trachytic, felsitic, and 

 other lavas of somewhat more basic character, cemented together 

 in what now looks like the base of a felsite. This cement, or 

 base, contains great quantities of the anthophyllite described under 

 No. 8 (p. 342). 



The slice also contains a reddish-brown mica, a little quartz, and 

 dots of magnetite or ilmenite, leucoxene, and ferrite. 



Owing to the fragments of which the rock is composed being of 

 much the same colour as the matrix, it is only here and there that 

 the unaided eye is able to distinguish between them. Many of the 

 lath-shaped prisms may therefore belong to included fragments, 

 and not to the matrix, but I am prepared to admit the possibility 

 that they may be the products of metamorphism. 



Nos. 26 and 27 are rocks of a similar character. The melting- 

 down process has gone so far in these specimens that they simulate 

 very closely the appearance of igneous rocks, and they might easily 

 be taken for rhyolites. They betray their origin, however, by the 

 numerous fragments of different kinds of lavas which they contain. 

 Alteration has proceeded very far in both specimens. Mica in 



