Ch. XXXIII.] 



GRANITE VEINS. 

 Fig. 63S. 



567 





Fig. 689. 



Junction of granite and limestone in Glen Tilt. (MacCulloch.) 

 a. Granite. 6. limestone, 



c. Blue argillaceous Schist. 



The conversion of the limestone in these and many other instances 

 into a siliceous rock, effervescing slowly with acids, would he difficult 

 of explanation, were it not ascertained that such limestones are always 

 impure, containing grains of quartz, mica, or felspar disseminated 

 through them. The elements of these minerals, when the rock has 

 heen subjected to great heat, may have been 

 fused, and so spread more uniformly through 

 the whole mass. 



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 

 intersect 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 

 walls sometimes preserving an exact paral- 

 lelism 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 



present similar shapes at the most northern, 



point of Scotland, and the southernmost extremity of Africa, as the 



annexed drawings will show. 



* Capt. B. Hall, Trans. Roy. Soc. Edin., vol. viL 



Granite veins traversing clay slate, 

 Table Mountain, Cape of Good 

 Hope.* 



