PART I. CHAPTER IX. 



125 



Rocks altered by Granite Veins. 



The annexed diagram (Fig. 113.) represents another junction, 

 in the same district, where the granite sends forth so many veins 

 as to reticulate the limestone and schist, the veins diminishing 

 towards their termination to the thickness of a leaf of paper or a 

 thread. In some places fragments of granite appear entangled, 

 as it were, in the limestone, and are not visibly connected with 

 any larger mass ; while sometimes, on the other hand, a lump 

 of the limestone is found in the midst of the granite. The ordi- 

 nary colour of the limestone of Glen Tilt is lead blue, and its 

 texture large-grained and highly crystalline ; but where it ap- 

 proximates to the -granite, particularly where it is penetrated by 

 the smaller veins, the crystalline texture disappears, and it as- 

 sumes an appearance exactly resembling that of hornstone. The 

 associated argillaceous schist often passes into hornblende-slate, 

 where it approaches very near to the granite.* 



The conversion of the limestone in these and many other 

 instances into a siliceous rock, effervescing slowly with acids, 

 would be 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 been 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 grada- 

 tion from a tortuous vein to the most regular form of a dike, such 

 as intersect the tuffs and lavas of Ve- 

 suvius 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 Af- 

 ^ rica, as the annexed drawing will show. 



Granite veins traversing clay It is not uncommon for One set of 



Cape f granite veins to intersect another; and 

 sometimes -there are three sets, as in 



* MacCulloch, Geol. Trans., vol. iii. p. 259. 



t Capt. B. Hall, Trans. Roy. Soc. Edin., vol. vii. 



