Vol. 50.] IGNEOUS ORIGIN ON DARTMOOR. 353 



No. 37, the bottom bed, appears to be a true sedimentary rock. 

 The fractured surface, examined macroscopically, resembles a slate, 

 and it has two parallel white streaks running across it. This 

 impression is confirmed by an appeal to the microscope. A fine- 

 grained parallelism of structure pervades the whole slice : the micro- 

 grains are arranged in parallel lines ; and there are streaks of 

 lighter colour running parallel to each other and to the alignment 

 of the grains. The slice contains no porphyritic crystals ; it is made 

 up of minute colourless granules, crowded with microscopic fibres 

 and leaves of mica. It contains strings of ferrite, and a fine-grained 

 white substance, opaque in transmitted light, that may be kaolin. 

 This rock is evidently a variety of slate. 



Nos. 38, 40, 43, and 46, the specific gravity of which varies from 

 3 - 15 to 3*27, are shown by their microscopical characters to be 

 identically the same rock. They are all composed alike of masses 

 of augite-crystals set in a felsitic base. Sphene is plentiful in 

 No. 40 and is present iu No. 46. Apatite is abundant in No. 46. 



The base is quite subordinate to the augite. In ordinary trans- 

 mitted light it is of a reddish-buff colour, and, with the exception of 

 No. 40, it is without any structure ; it looks like, and plays the 

 part of a glassy base. Between crossed nicols it breaks up into 

 allotriomorphic masses of dimly-polarizing felspar. In No. 43 this 

 base, or groundmass, is paler in colour ; and though it is allotrio- 

 morphic as regards the augite, and shows no internal crystalline 

 shape, it exhibits a single cleavage and straight extinction. 



The augite is of a very pale greenish or bluish-green tint, but is 

 nearly colourless. It is massed together in stumpy prisms, which 

 rarely show its crystalline form perfectly. Sometimes, however, 

 the cross-cleavage and the shape are typically developed. The 

 extinction measured from a well-marked single cleavage usually 

 varies from 35° to 45°. When cross-cleavages are visible the biaxial 

 interference-figure, as in typical augite, can be seen. 



Here and there, this nearly colourless augite has occasionally been 

 converted in situ into a strongly pleochroic, gi'eenish-blue hornblende; 

 but the augite is, on the whole, extremely fresh, and the proportion 

 of the paramorphic hornblende to the unaltered augite is very small. 



In Nos. 39 and 47 the augite is in microscopic grains, and the 

 base, 1 when examined between crossed nicols, differs from the base of 

 the specimens previously described, inasmuch as it exhibits a micro- 

 crystalline-granular structure — like the base of some rhyolitic rocks 

 and the matrix of the tuffs at Sourton Tors and Meldon. Both 

 specimens contain sphene, and No. 39 contains apatite and a little 

 secondary hornblende. The comparative lowness of the specific 

 gravity of No. 39, namely 2-85, is apparently due to the increase in 

 the proportion of the acid base to the basic augite 



1 I prefer this term, in the case of these rocks, to ' groundmass,' because in 

 ordinary transmitted light it is without, structure, looking like and playing the 

 part of a glassy base. 



