70 



Dr. M*"CrLL0CH's Additional Remarks on Glen Tilt. 



but may remark that the same circumstance of uniformity in direction attends 

 on tlie pinite formerly described. 



Mica occurs also in the cavities of micaceous schist, crystallized at liberty. 

 It forms extensive groups in these ; being- sometimes accompanied by a brown 

 ochraceous powder, resulting, probably, from the decomposition of some mine- 

 ral which cannot noAv be ascertained. The crystals are of an hexagonal 

 form, and the prisms so short as to be mere scales. They vary from a quarter 

 to one-sixth of an inch in diameter, and adhere to the stone, by their edges, so 

 slightly, that they fall o IV in abundance in consequence of the jarring of the 

 hammer. Their colour is silvery-white. 



Chlorite. — This mineral occurs here in two modes. 



It is found together with titanite, as already mentioned, in cavities in mica- 

 ceous schist, forming irregular nodules ; and also occupies similar situations 

 in a blue schistose limestone, when it is generally accompanied by crystallized 

 carbonate of lime. In one place it was found of a bright golden-yellow colour, 

 with a perfectly metallic lustre ; this effect appearing to arise from an inci- 

 pient decomposition, although the specimen had lost nothing of its original 

 firmness. 



The next variety of this mineral is more remarkable, and, like the mica 

 above mentioned, it is found in cavities in micaceous schist, sometimes accom- 

 panied by crystallized felspar. The crystals are crowded together, so as to 

 confuse each other's forms in some cases ; in others they are so independent 

 as to be easily examined. When most perfect, the form is a very short hex- 

 agonal prism, terminated in each end by an obtuse hexagonal pyramid, of 

 which the apex is truncated : the surfaces are almost always striated by fine 

 lines indicating the direction of the scales into which the crystal is separable. 

 Occasionally the prism is followed by a succession of two or more pyramidal 

 frusta, each in order more obtuse than the preceding. In other instances the 

 sides of the prism and of the corresponding pyramids become curved ; and 

 thus the crystals assume a spheroidal shape. When broken, in these cases, 

 they are often found to consist of triangular laminae radiating from a centre ; 

 so that the external figure is not formed, as would at first be supposed, by a 

 succession of plates varying in size and parallel to the original prism. The 

 colour of this variety is a dark green, and the ordinary dimension of the cry- 

 stals is half an inch. 



I must here add, that in the porphyry veins, which contain both pinite and 

 mica, there is sometimes found a mineral which I am at a loss to name ; since 

 it cannot be referred to any of the known varieties of either substance. It 



