126 



LYELL'S ELEMENTS OF GEOLOGY. 



Granite Veins. 



the environs of Heidelberg, where the granite on the hanks 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 

 colour, and is seen underlying gneiss. The other is a red gra- 

 nite, which penetrates the dark variety every where in veins.* 



The accompanying sketches will explain the manner in which 

 granite veins often ramify and cut each other. (Figs. 115. and 



116.) They represent the 

 manner in which the gneiss 

 at Cape Wrath, in Suther- 

 landshire, is intersected by 

 veins. Their light colour, 

 strongly contrasted with that 

 of the hornblende-schist, 

 here associated with the 

 gneiss, renders them very 



Fig. 115. 



conspicuous. 



Granite very generally 

 assumes a finer grain, and 

 undergoes a change in min- 

 eral composition, in the veins 

 which it sends into contigu- 

 ous rocks. Thus, according to Professor Sedgwick, the main 

 body of the Cornish granite is an aggregate of mica, quartz, and 



Fig. 116. 



Granite veins traversing gneiss, Cape Wrath. 

 (MacCultoch.)t 



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



felspar ; but the veins are sometimes without mica, being a gra- 



* MacCulloch, Syst. of Geol., vol. i. p. 58. 

 t Western Islands, pi. 31. 



