Vol. 50.] IGNEOUS ORIGIN ON DARTMOOR. 351 



are in discontinuous outcrops, and the rocks themselves, as has been 

 shown above under the head of Sourton Tors, differ materially from 

 each other. 



The following specimens of the rock under description were 

 examined : — 



Mica-diorite. 



No. 34 1144. Sp. Gr. 2-81. 



„ 35 1205. „ 279. 



„ 36 1207. „ 2-79. 



The microscopical examination of these specimens shows that 

 the rock is a mica-diorite. The groundmass consists of a meshwoi k 

 of small plagioclase-prisms, some of which are sufficiently large to 

 show porphyritic crystals. The next most abundant mineral is a 

 red mica ; this is profusely scattered over the thin slices in all the 

 specimens, and to its abundance the purple tone of the grey colour 

 of the rock is due. It is, I think, a secondary contact-mineral. 



Hornblende is not a prominent mineral, and it is altogether absent 

 in No. 35. This mineral occurs in shapeless aggregates of grains, 

 or in stumpy allotriomorphic prisms. In transmitted light it varies 

 from a pale brownish-dun colour to a very pale undecided green. 

 It is dichroic only here and there, and never strongly so. Only one 

 cleavage is distinctly seen ; and when a trace of a second cleavage 

 is to be observed (which is rarely the case), it approximates in its 

 angle of intersection more to hornblende than to augite. Extinction 

 varies from L2° to 28|°, measured from the single cleavage, and 

 averages 18°. 



Sphene is abundant ; so is apatite ; and there is a fair amount of 

 magnetite or ilmenite. 



In the triclinic felspars extinction is often nearly simultaneous in 

 both sets of twins, and is nearly straight. In others it varies from 

 6° to 14° from the plane of twinning on P, and averages 10^°. 



IV. On the Flank of Cock's Tor. 



High up on the southern flank of Cock's Tor, above the Tavistock 

 and Moreton Hampstead road, there is an interesting outcrop of 

 rocks, which have not, so far as I am aware, been described, or indeed 

 noticed, by previous observers. These rocks stand out from the 

 hillside in a low cliff about 12 feet high, and for a length of 40 or 50 

 feet. They weather out into lines suggestive of lamination, which 

 impart a ribbed appearance, somewhat like that of corduroy cloth, 

 to the beds. They have a low dip to the N.N.W. 10° W. 



The ribbed appearance above alluded to is well seen on the 

 weathered surface of one of my hand-specimens (38), and it is 

 represented in the illustration (fig. 1, p. 352). 



On the occasion of my first visit to this locality, in 1892, I col- 

 lected the specimens numbered 45 to 47 ; but at my last visit in 

 1893 I made a more complete collection, in ascending order, which 

 I enumerate on the following page : — 



2b2 



