IOO UNIVERSITY OF COLORADO STUDIES 



inches to many feet in longest diameter, are not infrequently present. 

 Sometimes the lenses overlap, that is, a thin band of gneiss will cut 

 across a lens dividing it at an acute angle with the long axis. 



When examined with the microscope quartz proves to be the most 

 abundant constituent, sometimes almost to the exclusion of other color- 

 less minerals, when the rock approaches a quartz-schist in composition. 

 Undulatory extinction is common, and liquid inclusions, each with a 

 gas bubble, are numerous. The inclusions are commonly in rows 

 parallel to the cleavage of the rock. These rows, as well as lines of 

 fracture, can often be traced across the slide, passing from one quartz 

 grain to another without interruption or change in direction. The 

 feldspars are microcline, orthoclase, and plagioclase, the last with the 

 extinction angle of albite. The total amount of feldspars is less than 

 that of quartz. Biotite is universally present and generally with par- 

 allel orientation. Muscovite, quar.tz, magnetite and hematite are 

 common alteration products. In a few cases muscovite is present 

 in flakes 4 or 5 mm. in diameter in rock which is apparently 

 very fresh. When this is examined in thin section the muscovite appears 

 to be intergrown with the biotite, showing no evidence of having been 

 derived from it. Sillimanite occurs in less amount, usually in small 

 bundles of fibers surrounded by biotite. Epidote is a common second- 

 ary mineral, usually on the borders of mica flakes, less often associated 

 with the feldspar. The garnet is colorless in thin section, and greatly 

 fractured. In the specimens examined it shows no double refraction. 

 Zircon and apatite are enclosed in small amount in the quartz- and feld- 

 spars. The apatite crystals are often needle-like, with the long axes 

 parallel to the rock cleavage. These long crystals are usually jointed 



or broken. 



Muscovite-Schist 



The presence of muscovite in the granitic gneiss has been mentioned. 

 In the gneiss there are also bands of finely laminated muscovite-schist 

 ffom a fraction of an inch to many feet in width. One of the largest 

 exposures is in the extreme northwest corner of the area mapped. The 

 schist is really only a phase of the granitic gneiss, but because of its 

 individual character it deserves some notice. 



