128 



LYELL'S ELEMENTS OF GEOLOGY. 



Metals near Granite Isolated Masses of Granite. 



undergo a modification of mineral character. The granite, still 

 remaining unstratified, becomes charged with green particles ; 

 and the talcose gneiss assumes a granitiform structure, without 

 losing its stratification.* 



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, with- 

 out 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 graniti- 

 form structure, in short all plutonic rocks, are frequently observed 

 to contain metals, at or near their junction with stratified forma- 

 tions. On the other hand, the veins which traverse stratified 

 rocks are, as a general law, more metalliferous near such junc- 

 tions 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 metals.f 



There are many instances, as at Markerud, near Christiania, 

 in Norway, 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 

 Fig. 119. be stated in reply, that 



ramifying dikes of 

 trap, which almost all 

 now admit to have 

 been once fluid, pass 

 through the same fos- 

 siliferous strata, near 

 Christiania, without 

 deranging their strike 



ralorsine. (L. A. Necker.) or dip.} 



The real or apparent isolation of large or small masses of 

 granite detached from the main body, as at ab, Fig. 119., and 



General vieio of junction of granite and schist oftkc 

 (L. 



* Necker, sur la Val. de Valorsine, Mem. de la Soc. de Phys. de Geneve, 1828. 

 I visited, in 1832, the spot referred to in Fig. 118. 

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

 I See Keilhau's Gaea Norvegica ; Christiania, 1838. 



