108 UNIVERSITY OF COLORADO STUDIES 



is almost always yellowish-green and contains magnetite varying from 

 grains just identifiable with the unaided eye to perfect octahedrons i . 5 cm. 

 in" diameter."* Under the microscope the original mineral composition 

 cannot be exactly determined; quartz, microcline, epidote, kaolin, mag- 

 netite and small crystals of zircon are the present minerals. A specimen 

 found near Sunshine contains many grains of magnetite, but looks and 

 feels like sandstone. In thin section quartz, orthoclase, and plagioclase 

 are seen in small grains, with a little hornblende, numerous zircons, and 



grains of epidote. 



Pegmatite and Aplite 



Dikes of these rocks, from a few inches to many yards in width, 

 cut the granite in all directions, and two persistent pegmatite dikes 

 cross the area trending about N. 25 W. One passes near Sunshine 

 where it is well developed; the other, the Hoosier Dike, crosses 

 Fourmile Canyon between Salina and Wall Street and is very promi- 

 nent at Summerville and east of Gold Hill. As seen on the surface, 

 quartz and potash feldspar usually make up most of the rock, the 

 quartz often in masses of large dimensions, sometimes 35 feet across. 

 No microscopic determination of the feldspar was made except in 

 the case of graphic granite which is often locally developed. In 

 this case the feldspar was microcline in pegmatitic intergrowth with 

 quartz. Other minerals occurring in amounts variable from place to 

 place are muscovite, biotite and magnetite; and in one instance in 

 Sunbeam Gulch, hornblende in aggregates two inches in diameter is 

 found with the quartz and feldspar. Muscovite and biotite are not often 

 seen in the coarsest pegmatite, possibly having been weathered out; 

 but in the finer-textured variety crystals of either may be found up to 

 two inches in diameter. 



The aplite is composed of quartz, microcline and orthoclase, with 



small amounts of biotite, muscovite and epidote, the last two probably 



secondary. Minute crystals of apatite and zircon are enclosed in the 



feldspars. 



Lamprophyric Dikes 



A few narrow basic dikes, usually about three feet wide, are found in 



the gneissoid granite, or granite-gneiss. One of these dikes crosses the 



