SUGARLOAF DISTRICT, BOULDER COUNTY, COLO. Ill 



possesses no peculiarities; there is but very little in the groundmass. 

 Sericite is a common alteration product. The quartz frequently shows 

 undulatory extinction. Liquid inclusions are numerous, often in rows 

 and sometimes as "negative crystals." Minute zircons occur sparingly. 

 In one thin section examined the quartz encloses great numbers of 

 incipient forms of crystallization which do not react on polarized light 

 with the highest powers. They are needle-like and usually under 

 . 06 mm. in length. Some individuals seem to be oriented with no 

 relation to others. Again, in the same field there may appear a number 

 of lines intersecting at all angles, and closely resembling a series of dashes 

 drawn with a fine ruling-pen, where each dash represents a crystallite. 

 Or, to make another comparison, they behave similarly to a repeatedly 

 broken column of mercury in a capillary tube. Where disposed in this 

 manner the needles are usually in an absolutely straight line. 



QUARTZ-MONZONITE 



North of Lefthand Canyon about half a mile from Rowena this 

 rock is exposed in stocks of which one of the largest is shown on the 

 map. There is another fairly large exposure on the slope of Nugget 

 Hill about a mile northwest of Glendale. The occurrence of several 

 small outcrops between the two leads to the inference that there is a 

 considerable body at no very great depth. 



The monzonite is bluish- gray in color and is of fine texture for a 

 phanerocrystalline rock. With the aid of a lens bluish-gray feldspar, 

 a little quartz, numerous biotite flakes usually less than 1 mm. in diam- 

 eter and small prismatic crystals of pyroxene and hornblende, 2 mm. 

 to 5 mm. long, are seen to constitute nearly the entire rock mass. Rarely, 

 small yellow-brown crystals of titanite can be detected. The total 

 amount of ferromagnesian minerals is quite constant, but locally biotite 

 may decrease and almost disappear as the other dark minerals increase. 

 In the hand specimen it is impossible to distinguish between hornblende 

 and pyroxene. Probably as the result of flow the ferromagnesian 

 minerals tend strongly toward parallelism in planes along which the 

 rock splits readily. The same minerals occasionally segregate in masses 

 commonly less than 1 cm. in diameter. 



