PART I. CHAPTER IX. 



129 



Fig. 120. 



gneiss. 



gneiss. 



quartz Veins Conformable Porphyries. 



above, Fig. 113., and a, Fig. 118., has been thought by some 

 writers to be irreconcilable with the doctrine usually taught re- 

 specting 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 

 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 infil- 

 tered. Such segregation, as it is called, can sometimes be shown 

 to have clearly taken place long subsequently to the original con- 

 solidation of the containing rock. .Thus, for example, in the 

 gneiss of Tronstad Strand, near Drammen, in Norway, the an- 

 nexed section is seen on the beach. It appears that the alter- 

 nating strata of whitish 

 granitiform gneiss, and 

 black hornblende-schist, 

 were first cut through by 

 a greenstone dike, about 

 2 ^ 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 incrust- 

 ed with transparent crys- 

 tals of quartz, the middle 

 of the vein being filled up 

 with common opaque white quartz. 



We have seen that the volcanic 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 indicated is 

 highly characteristic. It was indeed supposed by Von Buch, at 

 the commencement of his geological career, that the granite of 

 Christiania, in Norway, was sometimes intercalated in mountain 

 masses between the transition strata of that country, overlying 

 fossiliferous shale and limestone. But although the granite sends 

 veins into these fossiliferous rocks, and is decidedly posterior in 

 origin, the opinion expressed of its actual superposition in mass 

 has been disproved by Professor Keilhau, some of whose ob- 



a. b. Quartz vein passing through gneiss and green- 

 stone^ Tronstad Strand, near Christiania. 



