570 



ISOLATED MASSES OP GRANITE. 



[Ch. XXXIIL 



Professor Keilhau drew my attention to several localities in the 

 country near Christiania, where the mineral character of gneiss appears 

 to have been affected by a granite of much newer origin, for some 

 distance from the point of contact. The gneiss, without losing its 

 laminated structure, seems to have become charged with a larger 

 quantity of felspar, and that of a redder colour, than the felspar usually 

 belonging to the gneiss of Norway. 



Granite, syenite, and those porphyries which have a granitiform 

 structure, in short all plutonic rocks, are frequently observed to contain 

 metals, at or near their junction with stratified formations. On the 

 other hand, the veins which traverse stratified rocks are, as a general law, 

 more metalliferous near such junctions than in other positions. Hence 

 it has been inferred that these metals may have been spread in a gaseous 

 form through the fused mass, and that the contact of another rock, in 

 a different state of temperature, or sometimes the existence of rents in 

 other rocks in the vicinity, may have caused the sublimation of the me- 

 tals.* 



There are many instances, as at Markerud, near Christiania, in Nor- 

 way, where the strike of the beds has not been deranged throughout a 

 large area by the intrusion of granite, both in large masses and in veins. 

 This fact is considered by some geologists to militate against the theory 

 of the forcible injection of granite in a fluid state. But it may be stated 

 in repiy, tnat ramifying dikes of trap also, which almost all now admit 

 to have been once fluid, pass through the same fossiliferous strata, near 

 Christiania, without deranging their strike or dip.f 



The real or apparent isolation of large or small masses of granite de- 

 tached from the main body, as at a b, fig. 694 and above, fig. 688, and 



Fig. 694. 



General view of junction of granite, and schist of the Valorsine. 

 (L. A. Keeker.) 



a, fig. 693, has been thought by some writers to be irreconcilable with 

 the doctrine usually taught respecting veins; but many of them may, 

 in fact, be sections of root-shaped prolongations of granite ; while, in 

 other cases, they may in reality be detached portions of rock having the 

 plutonic structure. For there may have been spots in the midst of the 

 invaded strata, in which there was an assemblage of materials more fusi- 

 ble than the rest, or more fitted to combine readily into some form of 

 granite. 



* Necker, Proceedings of Geol. Soc, No. 26, p. 392. 



f See Keilhau's Geea Norvegica; Christiania, 1838. 



