Tol. 50.] IGNEOUS ORIGIN ON DARTMOOR. 361 



the pit, and have examined thin slices of them under the micro- 

 scope. 



No. 49 1101. Sp. Gr. 292. 



„ 50 1100. „ 298. 



„ 51 1099. „ 3-00. 



These are grey-coloured compact rocks, showing blebby-looking 

 felspars sparsely scattered through them. Under the microscope 

 the rock is seen to be composed of augite, pseudoinorphs of serpen- 

 tine, ilmenite, brown-red mica, sphene, apatite, a little secondary 

 quartz, and the remains of large porphyritic felspars in what was 

 probably a groundmass composed of felspathic material. 



The augite, which is of a pale brown colour, is very fresh and, for 

 the most part, in idiomorphic crystals, though some of these are to 

 some extent corroded, internally and externally, by the base or 

 groundmass. Apatite is abundant, and ilmenite is still more so. 

 Tbe latter has frequently been converted, in whole or in part, into 

 leucoxene, or into sphene. The mica is of a rich brown-red colour, 

 inclining to reddish yellow in thin leaves. The serpentine appears 

 to be in part a pseudomorph after olivine and in part after augite. 

 In some cases its derivation from the former mineral seems clear, 

 for it is in rounded masses, or in six-sided and other forms 

 characteristic of olivine. Sometimes the appearances under crossed 

 nicols indicate decidedly the original structure of that mineral. 

 The whole of the serpentine, however, cannot be referred to 

 olivine. Some of it occurs in elongated or in large irregular 

 shapes : this is more suggestive of augite, or some other variety of 

 pyroxene ; and it is material to note that some of the unaltered 

 augite occurs in similar forms. Moreover, some of the serpentine 

 exhibits a series of parallel cleavages which naturally suggest the 

 pinacoidal cleavage of that mineral. 



The large porphyritic crystals of felspar exhibit their original 

 crystallographic outlines very well, but the felspar itself has quite 

 disappeared, leaving behind opaque to translucent granular matter, 

 with inclusions of steatite and talc, which probably represent what 

 were originally endo-crystals of pyroxene enclosed in the felspar. 

 Part of the original felspars have also been converted into an iso- 

 tropic colourless substance. 



The groundmass has been changed into opaque to translucent 

 granular matter. I take it to have been originally a felspathic 

 groundmass, because the secondary granular matter into which it 

 has been changed seems to be the same as the granular matter in 

 the porphyritic crystals, and because in some cases the augites are 

 moulded upon what appear to have been lath-shaped prisms of 

 felspar radiating from the groundmass into the pyroxene. 



This rock was, in its unaltered condition, a dolerite. 



Mr. Eutley has shown in his memoir that the present hill of 

 Brent Tor is the ruin of an old volcano, and in figs. 7 and 8 of his 

 paper in the Quarterly Journal (vol. xxxvi. 1880, p. 292) he 

 places the cone in the immediate vicinity of the existing hill, and 

 on its northern side. The position of the rock above described is 



