220 GENERAL REMARKS ON GNEISS. 



of granite in all cases, can not be determined. In some 

 instances, as in Perthshire where the gneiss reposes 

 on granite, it is probable that they proceed from it; 

 but it has already been seen that there are no traces 

 of that rock in the Long Island. That is however no proof 

 of its non-existence, and the circumstance of the veins 

 being always present when the gneiss reposes on granite, 

 and absent when another rock is interposed, renders 

 it probable that in these cases granite, though invisible, 

 is still present. 



The following considerations render it probable that 

 the granite veins which traverse gneiss are posterior 

 to the including rock, and formed under circumstances 

 analogous to those under which other granite veins 

 have intruded into the schistose rocks with which they 

 interfere. 



They are accompanied by fractures or contortions of 

 the gneiss, of such a nature as to prove that it once 

 possessed a condition capable of yielding in different 

 ways to external force, while these appearances are 

 also proportioned to the number and intricacy of the 

 veins. In the schistose varieties which yield easily in 

 the direction of the laminae, the veins frequently hold 

 a parallel course to these, while an occasional flexure 

 occurs in those cases where the vein crosses them ; 

 the edges being incurvated from the thicker part, or 

 the root of the vein, towards its termination. Lastly, 

 in cases, of which an example was described in Tirey, 

 where a vein traverses a mass of limestone included 

 in the gneiss, it disturbs that substance as well as the 

 surrounding rock ; and in another parallel instance noticed 

 in Scalpa, where the included substance is serpentine, 

 the vein itself undergoes a change by participating in 

 the nature of that rock during its passage. 



It may now be remarked respecting these veins, that 

 although they sometimes resemble ordinary granite, 

 they more generally possess u distinct diameter, consisting 



