Items related to General geology of the Georgetown quadrangle, Colorado
Synopsis
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1910 Excerpt: ...of magnetite lie in a mosaic of elongated quartz lenses (PI. X, B). The larger magnetite individuals are much less deformed than the smaller. Futterer6 in particular has emphasized the ease with which quartz is recrystallized, and in many of the other schists and gneisses of the Georgetown quadrangle magnetite appears to be one of the more readily recrystallized minerals. The pseudomorphous gneissic structure locally present in pegmatite surrounding gneiss is described in another place (p. 90). CHARACTER OF THE PEGMATIT1C MAGMAS. The magmas in general appear to have varied from a comparatively dry form, from which the granites solidified, through molten masses rich in water from which some of the pegmatites solidified, to a body so saturated with water that the banded quartz veins deposited by it resemble water-deposited veins. The extreme fluidity of the magma which deposited the pegmatite is indicated by a number of the characteristics of that rock. The pegmatitic magma or fluid sought out the smallest cracks and crevices and deposited its material therein, and the presence of pegmat ite in small masses in nearly every outcrop in the quadrangle indicates that the older rock masses were thoroughly saturated with the pegmatitic material. That the process was one of saturation rather than of ordinary igneous injection, is shown by the presence, particularly in the biotite-sillimanite schist of the Idaho Springs formation, of isolated lenticular "eyes" of pegmatite (see p. 38) similar to the rock of the larger pegmatite masses (fig. 3, p. 39). The constituents of these "eyes" are absolutely uncrushed and can not be considered segments of a sheared pegmatite dike. The pegmatites further absorbed a considerable quantity of this schist and fr...
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