SUGARLOAF DISTRICT, BOULDER COUNTY, COLO. 109 



creek a few hundred feet east of the Crisman depot, and others are 

 exposed in Sunbeam Gulch. The more common variety is a dark 

 aphanitic rock with numerous black patches of hornblende, or hornblende 

 and biotite, in a pinkish, granular groundmass. The dark patches 

 make up nearly half the rock and are from 2 to 5 mm. in diameter. 

 The groundmass contains no mineral identifiable with the unaided eye, 

 except occasional grains of pyrite. The dikes are sharply differentiated 

 from the granite and the rock usually has a gneissoid structure. 



Thin sections of rock from a dike crossing Sunbeam Gulch contain 

 no biotite. The hornblende is almost entirely without crystal outline. 

 It appears in irregularly bounded aggregates of individuals intergrpwn 

 with jagged edges, and also as small flakes in the groundmass. It 

 resembles secondary hornblende in habit. The mineral is pale green 

 and strongly pleochroic. The small flakes constitute probably one- 

 fifth to one-fourth of the groundmass. Augite crystals are present, 

 but rare. The greater part of the groundmass is composed of unstriated 

 feldspar, though microcline occurs in small amount, and occasional 

 soda-lime feldspars are present. The feldspar grains are usually less 

 than . 5 mm. in diameter. Kaolinization is common. Among the 

 feldspars an occasional grain of quartz can be detected, but this mineral 

 is perhaps almost negligible in quantity, at least as a primary constituent. 

 Titanite in idiomorphic crystals and rounded and irregular forms is 

 quite common. Apatite and zircon crystals are numerous as inclusions 

 in the feldspar. Epidote is plentiful in grains throughout the rock, 

 and, with pyrite, fills minute fissures. 



A specimen from the dike east of Crisman contains biotite inter- 

 grown with hornblende in aggregates and scattered throughout the 

 groundmass in the same manner as that of the hornblende noted above. 

 It is yellow- brown in color and strongly pleochroic. Magnetite and 

 hematite are often associated with the biotite as alteration products. 

 Hornblende and the remaining constituents of this dike are identical 

 with those of the rock described above with the exception that the horn- 

 blende and biotite are together about equal in amount to the horn- 

 blende alone in the other specimens/* These dikes have been subjected 

 to metamorphism as the result of intense shearing stresses. 



