Yol. 50.] IGNEOUS ORIGIN ON DARTMOOR. 363 



subordinate to the magnesia. There was only the faintest possible 

 suspicion of lime. The acid solution reacted for titanic acid. 



Under the microscope the base in transmitted light is of extremely 

 pale tint, and seems to vary from a yellow-green to a green-yellow, 

 and with high powers it is seen to be partly of obscurely fibrous 

 structure, and in part amorphous : it is isotropic. In this base there 

 are scattered patches and granules of doubly-refracting matter, which 

 appears to be in part quartz, and in part felspar. 



The slice is profusely dotted over with filamentous threads, fibres, 

 and irregularly-shaped granules of leucoxene and magnetite or 

 ilmenite. The high specific gravity seems due to the abundance of 

 the iron and leucoxene. 



In a second and thicker slice which I have had made of No. 54, 

 and in No. 55 (a preciselj* similar rock), the original character of the 

 rock is better shown. It would seem to have been composed of a 

 network of small prisms (like the felspar-prisms in basalt), with 

 larger prisms scattered about in the groundmass. There are also 

 lacuna? which probably represent aggregations of pyroxene or 

 olivine. Some of them would do very well for the latter mineral. 

 In 55 the arrangement of the sm; 

 former presence of a glassy base. 



I think that Nos. 54 and 55 were originally basaltic lavas, and 

 that they have been altered into a sort of serpentine. The shapes 

 of the prisms remain, but the substance of the felspar and augite of 

 which they were originally composed has disappeared. The 

 resulting rock seems to be a variety of serpentine, which may be re- 

 garded as something between a normal serpentine and pseudophite. 

 In other words, I take it to be a variety of aluminous serpentine. 



There may be some finely granular or finely fibrous chlorite 

 disseminated through Nos. 54 and 55, especially in the ' lacunae,' 

 but it cannot be identified as such, and, if present, its fibres must be 

 so arranged as to produce compensation. 



The beds on the top of the Tor, and those in the quarry below 

 dip N.N.E. 



No. 56, which occurs on the top of the quarry, is a dark bluish- 

 black Carboniferous slate, weathering white, the surface of the 

 weathered portion being tinted an ochreous yellow. The dark 

 colour of the rock is due to the presence of carbon. Boiling in 

 hydrochloric acid does not remove the colouring, but on the applica- 

 tion of red heat the rock turns white. I have seen similar rocks 

 in this area weather white on the surface, and along cracks, where no 

 trap has been present, or, at any rate, where none has been visible. 



No. 57, which crops out at the bottom of the quarry, under 

 No. 56, is a rhyolite. The hand-specimen might be taken for a 

 very fine-grained amygdaloid. 



Under the microscope the groundmass is seen to be a glass 

 showing fluxion-structure here and there, and sinuous streaking 

 due to ferrite-staining. In this groundmass are numerous rounded 

 and corroded crystals of quartz, and the remains of what were, 



