56S 



MINERAL STRUCTURE OP 



[Ch. XXXIII 



Fig. G90. 



It is not uncommon for one set of granite veins to intersect another ; 

 and sometimes there are three sets, as in the environs of Heidelberg, 

 where the granite on the banks of the river Necker is seen to consist of 

 three varieties, differing in colour, grain, and various peculiarities of 



mineral composition. One of these, 

 which is evidently the second in 

 age, is seen to cut through an older 

 granite; and another, still newer, 

 traverses both the second and the 

 first. 



In Shetland there are two kinds 

 of granite. One of them composed 

 of hornblende, mica, felspar, and 

 quartz, is of a dark color, and is 

 seen underlying gneiss. The other 

 is a red granite, which penetrates 

 the dark variety everywhere in 

 veins.* 

 The accompanying sketches will explain the manner in which granite 

 veins often ramify and cut each other (figs. 690 and 691). They repre- 



Fig. 691. 



Granite veins traversing gneiss, Capo Wrath. 

 (MacCulloch.)t 



Granite veins traversing gneiss, at Cape Wrath, in Scotland. (MacCulloch.) 



sent the manner in which the gneiss at Cape Wrath, in Sutherlandshire, 

 is intersected by veins. Their light colour, strongly contrasted with 

 that of the hornblende-schist, here associated with the gneiss, renders 

 them very conspicuous. 



Granite very generally assumes a finer grain, and undergoes a change 

 in mineral composition, in the veins which it sends into contiguous rocks. 

 Thus, according to Professor Sedgwick, the main body of the Cornish 

 granite is an aggregate of mica, quartz, and felspar ; but the veins are 

 sometimes without mica, being a granular aggregate of quartz and fel- 

 spar. In other varieties quartz prevails to the almost entire exclusion 

 both of felspar and mica ; in others, the mica and quartz both disappear, 

 and the vein is simply composed of white granular felspar.^ 



* MacCulloch, Syst. of Geol., vol. i., p. 58. f Western Islands, pi. 31. 



% On Geol. of Cornwall, Camb. Trans., vol. i. p. 124. 



