Dr. Mac Culloch o;/ the Geology of Glen Tilt. 301 



The various granites described as occupying the principal parts 

 of the ridge, are also mixtures of quartz, mica, and felspar, dif- 

 fering however as much in colour and texture as they do in struc- 

 ture ; to describe such varieties is not possible, were it useful. 

 The chief varieties of the syenitic granites are found on the slopes 

 which descend to the Tilt, and they are almost invariably of a grey 

 and black colour, from the predominance of the hornblende : their 

 texture is as various as their shades of colour, the crystals of horn- 

 blende being sometimes very large and decided, and imbedded as 

 it were in a paste of quartz and felspar, while in other extreme 

 cases the minuteness and mixture of the several ingredients is such, 

 that the constituents can scarcely be discerned. In these cases the 

 specimens have sometimes the fallacious appearance of the green- 

 stones of the trap family. They may, however, be distinguished 

 from these by attending to the white ingredient, which frequently 

 consists of quartz with little or no felspar, while in all cases it con- 

 tains quartz, a substance which is but rarely found in greenstone. 

 They may generally also be recognized for what they really are, by 

 tracing them for some space, when their characters will be found 

 to become more conspicuous, either by the enlargement of the 

 constituent parts, or by the admixture of mica, a substance which, 

 except incidentally, is not found as an ingredient of greenstone 

 properly so called. These rocks, like common granite and gneiss, 

 frequently contain epidote as a constituent incidental part, and in this 

 also they are distinguished as well from that syenite to which I pro- 

 pose to limit the term, as from the greenstone of the trap family, 

 «ince epidote is but very rarely found among these latter rocks. They 

 are equally distinguished from them by the frequent presence of 

 another mineral which, as' well as the former, is peculiar to the 

 older rocks, Sphene. This is found in considerable abundance. 



