Ch. XXXHI.] 



STRUCTURE OF GRANITE VEINS. 



711 



the undulating outline of the granite intersecting different strata, and 

 occasionally intruding itself in tortuous veins into the beds of clay- 

 slate and limestone, from which it differs so remarkably in compo- 

 sition. The limestone is sometimes changed in character by the prox- 

 imity of the granitic mass or its veins, and acquires a more compact 

 texture, like that of horn stone or chert, with a splintery fracture, and 

 effervescing freely with acids. 



The foregoing diagram (fig. 740) represents another junction, in the 

 same district, where the granite sends forth so many veins as to reticu- 

 late 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 lime- 

 stone, 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 ordinary color of the limestone of Glen 

 Tilt is lead blue, and its texture large-grained and highly crystalline ; 

 but where it approximates to the granite, particularly where it is pene- 

 trated by the smaller veins, the crystalline texture disappears, and it 

 assumes 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. 



Fig. 741. 



Fig. 742. 



Granite veins traversing clay slate, 

 Table Mountain, Cape of Good Hope.t 



Granite veins traversing gneiss, Cape Wrath. 

 (MacCulloch.)* 



* MacCulloch, Geo!. Trans., vol. iii. p. 259. 



f Capt. B. Hall, Trans. Roy. Soc. Edinburgh, vol. vii. 



X Western Islands, pi. 31. 



