. Elements of geology. Geology. PART 1. CHAPTER IX. 129 Quartz Veins Conformable Porphyries. aljove, 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
. Elements of geology. Geology. PART 1. CHAPTER IX. 129 Quartz Veins Conformable Porphyries. aljove, 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 feet wide; then the crack a h 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 i
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Keywords: ., bookauthorlyellcharlessir17, bookcentury1800, booksubjectgeology
