206 GENERAL REMARKS [Ch. X. 



contains felspar, and sometimes exhibits all the constituents 

 of granite, so as to resemble foliated gneiss, and even perfect 

 granite ; and a similar quartz-rock forms a part of the granite 

 of the adjacent Grampians, as exhibited in the ridge of 

 Grianan. The limestone of Tirey, one of the Western Isles 

 of Scotland, passes in the same manner into the gneiss with 

 which it alternates: it contains concretions of hornblende, 

 which mineral abounds in the gneiss ; but of all the compo- 

 nent parts of the latter, quartz is most generally in union 

 with the limestone, and is probably intimately combined 

 therewith, on account of its great hardness : it contains lumps 

 of granite, and is traversed by granite veins, so that its 

 junction therewith is of the same character as that of Glen 

 Tilt. 



Intimate as the connection of the granitic and schistose 

 rocks thus appears to be, there is yet another point of view in 

 which their mutual relations are still more closely united. It 

 has been stated that both the granitic and the schistose groups 

 individually exhibit in each district, or in different parts of 

 the same district, characteristic minerals, the nature of which 

 seems to regulate the composition of the respective series : 

 now this is not only the case, but the characteristic minerals, 

 in adjacent masses of granite and slate, are commonly iden- 

 tical ; so that a perfect knowledge of the mineral transitions 

 in one, may assist us in comprehending the nature of those 

 jn the other. Several instances of this connection have been 

 noticed in the preceding details, of which the following are 

 the most perspicuous. In Cornwall, shorl is the characteristic 

 accessory mineral of the granite, but it is not sufficiently 

 abundant to modify the general appearance of this rock, 

 being only partially developed, so as to give rise to occasional 

 beds of granular and compact shorl-rock ; which also occur 

 in the adjoining schistose groups, but not extensively, for 

 their shorl speedily passes into hornblende, actynolite, chlorite, 

 and various obscure substances, perhaps of an intermediate 

 nature : but by far the most prevailing mineral of the Cornish 



