918 T. G. BONNET ON THE SERPENTINE AND 



is still more conspicuous in two specimens lent to me by Mr. Allport, 

 one of which is cut transversely to the streaky structure. From 

 these, especially the latter, it is quite clear that the structure is 

 produced, not only by a general parallelism in the longer dia- 

 meters of crystals of a prismatic outline, but also by a partial sepa- 

 ration of the constituent minerals — the augitic constituent, the olivine 

 with strings of serpentine, and the magnetite being each, more 

 abundant in particular laj'ers. 



Gue Graze (no. 5). — The greater part of the slide closely resembles 

 that of no. 8, except that there is a slightly larger proportion of 

 serpentine, and the rock has not been quite so coarsely crystalline. 

 The augitic constituent, however, is less conspicuous, and there are 

 a number of small patches of a semitransparent, rather earthy- 

 looking mineral from about *01 to -03 inch in greatest length, most 

 of which seem to be coated externally by a reddish, film of iron per- 

 oxide. The augitic constituent appears, as at Coverack, to be partly 

 normal augite, with some diallage, and partly enstatite. The 

 crystals, however, are much smaller here ; so that it is more difficult 

 to determine them. The granular mineral, with polarized light, 

 very closely resembles the pseudomorphic product after the felspars 

 of the gabbros described above ; and on close examination, some 

 indications of an irregularly oblong crystal-like form may be de- 

 tected. This rock, therefore, probably differs from the others in 

 containing a small quantity of felspar ; and I may state that it is the 

 only case in which I have detected that mineral in the serpentines 

 of the Lizard*. 



Lower PradanacTc Quarry (no. 7). — The slide from this quarry, as 

 might be expected from the general resemblance of the rock, corre- 

 sponds very closely with that described above from Mullion Cove ; 

 the olivine is in much the same condition of conversion into ser- 

 pentine. It contains also little dark brown and greenish brown 

 semitransparent grains, which I feel certain are picotite. Here 

 also a banded structure is indicated by a partial separation of the 

 component minerals ; enstatite, however, in this rock, rather predo- 

 minates over the augitic constituent. 



Rill Quarry (no. 4). — This slide shows a number of subangular 

 transparent grains, generally edged with black, of clear, colourless, 

 and rather fibrous crystals of longish oblong outline, and of scattered 

 granules of magnetite, set in a very pale yellowish green serpentine, 

 now and then stained brown. A few larger, irregular, imperfectly 

 transparent granules seem to be picotite. On applying polarized 

 light the subangular grains prove to be olivine, and the fibrous 

 crystals enstatite (lengthened in the direction of the macrodiagonal). 

 There is also a crystal about 0*2 inch long, of rather rhomboidal 

 shape, which proves to be this mineral ; it exhibits the usual clca- 



* My friend Mr. Main, our Lecturer in Chemistry, kindly ascertained for me 

 the amount of A1 2 0., in this rock, and informs me that the amount is only 04 

 per cent. This would mean rather more than 1 per cent, felspar ; but then this 

 amount of Al 2 3 might be present in the pyroxenic constituents. So the deter- 

 mination is uncertain. 



