2 74 UNIVERSITY OF COLORADO STUDIES 



varieties we have a=yellowish green, b=dark green, r=yellowish gre^. Absorption 

 showing r=b > a. The feldspars are all striated, showing polysynthetic twinning accord- 

 ing to both the albite and pericline laws. As no examples of Carlsbad twinning were 

 observed, it was found impossible to determine the plagioclases according to the method of 

 Michel-Levy, but, as the extinction angles average about 15° and one as high as 40° was 

 observed, the composition is probably about oligoclase-andesine. Nearly all the feldspars 

 show undulatory extinction, showing that they have been exposed to great pressure, but 

 at the same time not so much as would be expected from the appearance of many of the 

 hand specimens. This would seem to imply that considerable recrystallization had taken 

 place. 



In the very coarse-grained varieties an iron garnet is very abundant. Inclosed in 

 the garnet have been noticed quartz, plagioclase, and hornblende. The other minerals 

 mentioned above present the usual aspects. 



Pegmatite and Aplite 



Every rock noticed in this district has been intruded by pegmatite and aplite, usually 

 acid in nature, but more rarely, as in the case of the diorite pegmatite described for Guy 

 Gulch, and also another occurrence near the Gem power plant on Clear Creek, basic in 

 composition. These pegmatites, intruded in every imaginable way into older rocks, have 

 the most fantastic and varied cross-section. They frequently carry great quantities of 

 magnetite, which afterwards supplies in great part the black sands and pebbles in the 

 placers. Locally, graphic granite is often developed, probably at its best near Roscoe on 

 Clear Creek. The pegmatite rarely shows evidences of having been subjected to great 

 dynamic forces. It has a common development near bodies of sheared diorite. 



It has been found impossible to map satisfactorily the pegmatite injections, and for 

 this area no attempt has been made in this direction. Some petrographic descriptions 

 of these rocks are, however, added. 



Pegmatite {Graphic Granite). Roscoe, by C. & S. tracks 



Macroscopic. — The rock consists of bands or pencils of quartz and feldspar showing 

 on cross-section the common graphic texture. 



Microscopic. — Under the microscope the rock is seen to consist of alternating bands 

 of quartz and microcline, the former showing undulatory extinction, giving in one place 

 the appearance of twinning. The microcline has the usual cleavage texture, but with 

 one cleavage developed parallel to the quartz, the other at right angles. This would seem 

 to indicate a continued pressure in one, or perhaps two, definite directions. 

 Aplite. N. W. Corner of Evergreen Quadrangle 



Macroscopic. — The rock is a pegmatitic rock consisting of quartz, feldspar, and some 

 pyrite. The specimen is traversed by a small vein of oxidized material. 



Microscopic. — The rock under the microscope is seen to consist of a medium fine- 

 grained holocrystalline mixture of quartz, microcline, and some orthoclase, with small 

 amounts of pyrite, magnetite, and muscovite. The quartz shows undulatory extinction, 

 and the whole rock shows signs of compression, and effect of dynamic forces, as js also 

 suggested by the microcUne twinning. The specimen is seen to be traversed by a small 

 vein consisting of oxidized material, and here the feldspars have been impregnated and 

 somewhat altered. 



