Il8 UNIVERSITY OF COLORADO STUDIES 



is .9 mm. Surrounding the fragment is a band of serpentine probably 

 derived from pyroxene. This band ranges in width from .06 mm. to 

 .35 mm. Bordering the serpentine is a slightly narrower band of 

 minute augite crystals. The embayment is filled with a mixture of 

 granulated quartz and serpentine, and the latter mineral fills two nar- 

 row fissures crossing the quartz. No feldspar occurs within the field 

 described. The quartz is doubtless a fragment from the wall rock 

 caught up by the magma before the latter had cooled so far as to 

 preclude corrosion of the inclusion by solution. The quartz, in turn, 

 doubtless effected a slight cooling of the magma adjacent which pos- 

 sibly, if not probably, caused the segregation of the basic mineral within 

 this area. 



TRACHYTE 



Trachyte occurs in several narrow dikes, usually less than 20 feet in 

 width. One of the most prominent is exposed on the north wall of 

 Lefthand Canyon at Rowena and has a northerly dip. Another out- 

 crop is seen one-half mile east of Rowena where the dike dips southward. 

 Exposures are commonly weathered to pink or red, and fresh material 

 can be found only in prospect holes. The unaltered rock is light gray 

 to pinkish-white, and contains numerous phenocrysts from 1 mm. to 

 5 mm. in diameter, besides occasional small grains of pyrite, in a micro- 

 crystalline groundmass. The feldspars are white when fresh, but in 

 the weathered rock become pink to black depending on the extent of 

 replacement by iron oxides. In advanced stages of alteration the 

 feldspars may be completely removed. Flow- structure is typically 

 absent. In general the texture and composition are fairly constant 

 throughout the dikes, but a dike south of Fourmile Creek grades from 

 a trachyte near the walls to a more basic rock at the center. This will 

 be mentioned again in connection with the latite of Sugarloaf Mountain. 



Under the microscope the feldspar phenocrysts of the typical rock are 

 seen to be almost exclusively orthoclase, some with Carlsbad twinning. 

 Alteration to kaolin is common. Less commonly, mixed with the kaolin, 

 are magnetite and hematite in irregular grains or powder. It is probable 

 that the iron ores are infiltrations from the country rock, since the quan- 



