GRANITE. 07 



that the rays of light are reflected from it at the same 

 angle over a wide space, leading to its detection at a 

 distance, as I have also noticed under the head of 

 gneiss. We might, in such a case, imagine the whole 

 vein to he a single crystal, deficient in external form 

 only because there was not a vacuity in which that 

 could have displayed itself. 



Though the variety of granite called graphic is not 

 limited to the veins which traverse gneiss, it is most 

 frequent in those, though seldom occupying the 

 whole of one. I need not describe what belongs to the 

 business of mineralogy ; only remarking, that it is 

 sometimes the quartz, and at others the felspar, 

 which has first crystallized, and thus determined the 

 fio;ure of the other mineral. 



o 



A third kind of granite veins has been mentioned, 

 independent of any central mass. There is no reason 

 to presuppose this ; and I have traced to the parent 

 granite all that were pointed out as such in Scotland. 

 Their great size and long persistence seem to have 

 led to the notion that they differed from the veins in 

 gneiss ; bat while it is evident that no small vein 

 could occupy a great range, they are analogous to the 

 similarly persistent veins of trap, as their connexions 

 must thus be equally difficult to ascertain. 

 granite veins traverse masses of granite, is a fact that 

 I have used to prove distinct ages in this rock; 

 while they are often marked by the same peculiarities 

 of character as when in gneiss. In those which 

 traverse this substance, the texture is uniform through- 

 out ; but in others, it very often differs in the middle 

 and at the sides. Yet they are often so amalgamated 

 with the gneiss that the boundary is (indefinable ; 

 the mica or hornblende losing their parallel directions, 



VOL. II. H 



