712 



MINERAL STRUCTURE OF 



[Ch. xxxni. 



In the plutonic, as in the volcanic rocks, there is every gradation 

 from a tortuous vein to the most regular form of a dike, such as inter- 

 sect the tuffs and lavas of Vesuvius and Etna. Dikes of granite may 

 be seen, among other places, on the southern flank of Mount Battock, 

 one of the Grampians, the opposite wall sometimes preserving an exact 

 parallelism for a considerable distance. 



As a general rule, however, granite veins in all quarters of the 

 globe are more sinuous in their course than those of trap. They pre- 

 sent similar shapes at the most northern point of Scotland, and the 

 southernmost extremity of Africa, as the foregoing drawings (figs. 741 

 and 742) will show. 



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 ISTecker is seen to consist 

 of three varieties, differing in color, 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, com- 

 posed of hornblende, mica, felspar, and quartz, is of a dark color, and 

 is seen underlying gneiss. The other is a red granite, which pene- 

 trates the dark variety everywhere in veins.* 



The accompanying sketches will explain the manner in which gran- 

 ite veins often ramify and cut each other (figs. 743 and 744). They 





Fig. 743. 



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



represent the manner in which the gneiss at Cape Wrath, in Suther- 

 landshire, is intersected by veins. The light color 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 con- 

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

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



* MacCulloch, Syst. of Geo!., vol. i. p. 58 



