SUGARLOAF DISTRICT, BOULDER COUNTY, COLO. 115 



the whole that of secondary magnetite. Epidote is the commonest 

 secondary mineral next to magnetite. It occurs in small amount with 

 the ferromagnesian minerals that show alteration, and sometimes fills 

 minute fissures which traverse all the minerals of the rock. Scales of 

 chlorite are often present on the pyroxene. 



HYPE RSTHENE-DI ABASE 



Dikes of diabase two or three to seventy feet wide cut both the granite 

 and the metamorphic rocks ; they are usually nearly vertical. The strong- 

 est dike passes within a mile of Sugarloaf Mountain and extends beyond 

 the area mapped both toward the south and toward the northwest, 

 having been traced approximately ten miles. Since the diabase is more 

 resistant than the granite and metamorphics, the dikes usually stand 

 distinctly, though not prominently, above the country rock and have 

 been a minor factor in shaping the topography. Cubical jointing is 

 universal, the blocks commonly weathering to spheroidal forms. Weath- 

 ered surfaces are gray to brown, with numerous small black spots due 

 to the fresher surfaces of pyroxene. 



The freshest diabase is dark greenish-gray in color. It is a holo- 

 crystalline, fine-textured rock the megascopic constituents of which are 

 black pyroxene and an equal amount of olive-green to gray-green feld- 

 spar. In color and luster the feldspar closely resembles serpentine 

 Under the microscope black iron oxide appears as a third important 

 constituent. 



The feldspar is almost entirely plagioclase with albite polysynthetic 

 twinning, and in many crystals, with the addition of Carlsbad twinning. 

 The feldspars are usually lath-shaped and, for the most part, without 

 regular terminal boundaries. In length they are rarely over 3 mm. 

 From this common type there are two variations: (1) larger subhedral 

 crystals of earlier formation; (2) allotriomorphic untwinned feldspar, 

 probably orthoclase, which crystallized later than the pyroxene. The 

 allotriomorphic variety, very subordinate in amount, is not observable 

 in many slides, nor are the subhedrons common. Since in the zone 

 normal to the brachypinacoid (010) the lath-shaped feldspars frequently 

 give extinction angles of 20°-25° they are probably in large part ande- 



