Ball — Pre-Cambrian Rooks of Georgetown, Col. 387 



pegmatite exerted by the rock in which the pegmatite was 

 injected. Biotite is the characteristic femic mineral of peg- 

 matites which inject the granites already described, the 

 quartz-monzonite-gneiss, the quartz-bearing diorite, and the 

 hornblende gneiss, although hornblende is often the typical dark 

 mineral of pegmatites in the last two formations. Muscovite 

 is the typical femic mineral of pegmatite dikes in the Idaho 

 Springs formation, and 98 per cent of the occurrences are in 

 this formation. The pegmatite dikes in quartz-monzonite areas 

 are characteristically without femic minerals and often contain, 

 instead of an alkali feldspar, a soda-lime feldspar. Magnetite 

 alone of the abundant femic constituents of the pegmatites is 

 not influenced by the rock intruded and was evidently an 

 original constituent of the pegmatite magma. In areas of 

 unusually complex injection where the Idaho Springs forma- 

 tion and the granular igneous rocks are in approximately equal 

 development, both biotite and muscovite are often associated 

 with one another. Where the pegmatite passes from a fairly 

 large area of granite to an area of the Idaho Springs formation, 

 the substitution of muscovite in place of biotite is usually 

 coextensive with the boundaries of the two formations. Where, 

 on the other hand, the pegmatite has traversed a single forma- 

 tion for a long distance and then passes to a second formation, 

 there is a distinct "lag" in the introduction of the mineral 

 characteristic of the pegmatite in the second formation. This 

 is well seen on the north side of the quartz-monzonite batholith 

 where the quartz-feldspar pegmatite extends several hundred 

 yards into the biotite-sillimanite-schist of the Idaho Springs 

 formation, before muscovite, the characteristic mineral of peg- 

 matites in that formation, is present. 



A chemical discussion of these observations is impossible, 

 since the composition of the pegmatite is unknown. It is 

 indeed probable that the pegmatitic substances left the cool- 

 ing mass of granite at widely varying times and at their birth 

 probably varied in chemical composition. Field evidence, 

 however, indicates that the pegmatitic substances prior to 

 solidification abstracted sufficient material from the enclosing 

 rocks to materially modify their chemical composition. The 

 lag already mentioned indicates that the change in composition 

 did not occur immediately upon entering a formation, but only 

 after the pegmatitic magma had traversed it for some distance. 



The Granites and Granite-Porphyry. — The granites asso- 

 ciated with the pegmatites are massive granular rocks char- 

 acterized by a fine or medium-grained allotriomorphic texture. 

 While the granites intergrade, not only with pegmatite but 

 with one another, certain types are rather well defined. 



Am. Jour. Sci.— Fourth Series, Vol. XXI, No. 125.— May, 1906. 



27 



