Ch. XXXIII.] 



CONFORMABLE PORPHYRIES. 



571 



Veins of pure quartz are often found in granite, as in many stratified 

 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 segregation, as it is 

 called, can sometimes be shown to have clearly taken place long subse- 

 quently to the original consolidation of the containing rock. Thus, for 

 example, I observed in the gneiss of Tronstad Strand, near Drarnmen, in 

 Norway, the annexed section on the beach. It appeai-s that the alternat- 

 ing strata of whitish granitiform gneiss, and black hornblende-schist, were 

 first cut through by a greenstone dike, about 2j 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 encrusted with transpa- 

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

 mon opaque white quartz. 



Fig. 695. 



Gneiss 



Gneiss. 



We have seen that the volca- 

 nic formations have been called 

 overlying, because they not only 

 penetrate others, but spread 

 over them. Mr. Necker has 

 proposed to call the granites 

 the underlying igneous rocks, 

 and the distinction here indi- 

 cated is highly characteristic. 

 It was indeed supposed by some 



a,i. Quartz vein passing through gneiss and green- 01 tne earlier ODServei'S, tuat tne 

 stone, Tronstad Strand, near Christiania. granite of Christianiaj in Nor . 



way, was intercalated in mountain masses between the primary or paleo- 

 zoic strata of that country, so as to overlie fossiliferous shale and lime- 

 stone. 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 Professor Keilhau, whose observations on this contro- 

 verted point I had opportunities in 1837 of verifying. There are, how- 

 ever, 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 classed as plutonic rather than trappean rocks, which may 

 truly be described as interposed conformably between fossiliferous strata, 

 as the porphyries (ac, fig. 696), which divide the bituminous shales and 



Euritic porphyry alternating with primary fossiliferous strata, 

 near Christiania. 



argillaceous limestones, ff. But some of these same porphyries are 

 partially unconformable, as b, and may lead us to suspect that the others 



