Ch. xxxni] 



QUARTZ VEINS. 



715 



Fie. 747. 



Gneiss. 



Gneiss. 



Tlie real or apparent isolation of large or small masses of granite 

 detached from the main body, as at a, b, fig. 746, and above, fig, 739, 

 and a, fig. 745, 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 fusible than the rest, or more fitted to combine 

 readily into some form of granite. 



Veins of pure quartz are often found in granite as in many strati- 

 fied rocks, but they are not traceable, like veins of granite or trap, to 

 large bodies of rock of similar composition. They appear to have 

 been cracks, into which siliceous matter was infiltered. Such segre- 

 gation, as it is called, can sometimes be shown to have clearly taken 

 place long subsequently to the original consolidation of the contain- 

 ing rock. Thus, for example, I observed in the gneiss of Tronstad 

 Strand, near Drammen, in 

 Norway, the annexed sec- 

 tion on the beach. It ap- 

 pears that the alternating 

 strata of whitish granitiform 

 gneiss and black hornblende- 

 schist were first cut through 

 by a greenstone dike, about 

 21 feet wide ; then the crack 

 a b passed through all these 

 rocks, and was filled up with 

 quartz. The opposite walls 

 of the vein are in some parts 

 incrusted with transparent 



crystals of quartz, the middle of the vein being filled up with com- 

 mon opaque white quartz. 



We have seen that the volcanic formations have been called over- 

 lying, because they not only penetrate others, but spread over them. 

 M. Necker has proposed to call the granites the underlying igneous 

 rocks, and the distinction here indicated is highly characteristic. It 

 was indeed supposed by some of the earlier observers, that the gran- 

 ite of Christiania, in Norway, was intercalated in mountain masses 

 between the primary or palaeozoic strata of that country, so as to 

 overlie fossiliferous shale and limestone. But although the granite 

 sends veins into these fossiliferous rocks, and is decidedly posterior in 

 origin, its actual superposition in mass has been disproved by Profes- 

 sor Keilhau, whose observations on this controverted point I had 

 opportunities in 1837 of verifying. There are, however, on a smaller 

 scale, certain beds of euritic porphyry, some a few feet, others many 

 yards in thickness, which pass into granite, and deserve perhaps to be 



a, t. Quartz vein passing through gneiss and green- 

 stone, Tronstad Strand, near Christiania. 



