386 Ball — Pre- Cambrian Rocks of Georgetown, Col. 



The pegmatite dikes are sometimes rudely banded, quartz 

 being- usually segregated in the center. Such banded dikes 

 along their strike pass into coarsely granular pegmatite. The 

 various textures of the pegmatites and the granular texture of 

 the granite sometimes grade into one another in a single out- 

 crop 100 feet in diameter, and in consequence the physical condi- 

 tions which in some cases determined the production of one 

 texture could have varied little from those which produced 

 others. 



While typically massive the pegmatites are locally mashed 

 into gneisses, partly by granulation and partly by recrystalliza- 

 tion. Quartz-magnetite pegmatite was particularly subject to 

 recrystallization, producing a quartz-magnetite-gneiss in which 

 eyes and thin bands of magnetite lie in quartz. The pseudo- 

 morphous gneissic structure sometimes present in pegmatite 

 surrounding schist inclusions will be described later. 



The magma appears to have varied from a comparatively 

 dry magma 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 

 pegmatites is indicated by a number of 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 pegmatite 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 ordinary igneous 

 injection is shown by the presence in the biotite-sillimanite- 

 schist of the Idaho Spring formation of isolated lenticular 

 " eyes " of pegmatite similar to that of the larger pegmatite 

 masses. The constituents of these "eyes" are absolutely 

 uncrushed and cannot be considered segments of a sheared 

 pegmatite dike. The pegmatites further absorbed consider- 

 able masses of this schist and from the center of the larger 

 inclusions gradations occur in certain instances from pure schist 

 to schist containing thin bands of pegmatite along its folia, 

 thence to pegmatite in which only a few scroll-like figures of 

 darker pegmatite faintly suggest the crenulations of the almost 

 totally absorbed schist, and lastly to pure pegmatite. In the 

 latter, however, a fair parting is sometimes preserved, indicat- 

 ing that the structure of the schist is partially preserved by 

 metasomatic replacement, and the parting may be considered 

 a pseudomorph of that of the schist. 



A further proof of the extreme fluidity of the pegmatite 

 magma is the influence upon the mineral composition of the 



