146 GNEISS. 



this rock, that the appearances of these veins indicate 

 their forcible intrusion into the gneiss, and that it has 

 yielded to them under a softened state. This is 

 proved hy the connexion hetween the contortions and 

 the veins, by the intrusion of parallel veins among the 

 laminae, and by the curvature of these, in a direction 

 corresponding to the motion of the fluid, which has 

 often also insinuated itself just within the displaced 

 edge and no further : the whole marking plainly the 

 forcible fracture of the rock and a simultaneous intru- 

 sion of the substance filling the fissure. One fact 

 alone, like this, is demonstrative ; because although 

 many veins might possibly be formed in some other 

 imaginary manner, such a one as this could not be 

 the consequence of any other action. The case of 

 parallel veins is explained by the preferable facility 

 with which the schistose variety yielded in the direc- 

 tion of its laminae; while the condition of the granitic 

 kind excluded all such tendency. If it is yet neces- 

 sary to produce facts in proof of this intrusion, the 

 same granite veins* which traverse a gneiss, in Tirey, 

 traverse an -included limestone also ; both of them 

 being contorted and displaced, and the chemical com- 

 position of the latter materially altered. It is evident 

 that both rocks existed before the passage of the 

 veins ; while, although the resemblance of gneiss and 

 granite might console those who consider the veins of 

 the latter as modifications of the former, the same 

 cannot hold good respecting limestone. A similar 

 occurrence in serpentine, in Seal pa, is merely a repe- 

 tition of the same argument ; and I need not now 

 enumerate the endless examples of mechanical dispo- 

 sition which prove the same inference. 



In proceeding to enquire respecting the strata with 

 which gneiss alternates, there is a difficulty, arising 



