SUGARLOAF DISTRICT, BOULDER COUNTY, COLO. II9 



tity of pyrite present is very small and there is no trace of ferromag- 

 nesian minerals in the trachyte. The groundmass is composed of 

 multitudes of feldspar microlites with a very little interstitial feldspar, 

 a less amount of quartz and occasional fluorite grains or crystals. 

 Rarely the microlites reach a length of . 3 mm. Simple twins and 

 microlites with three lamellae are not uncommon. For the most part 

 the microlites have parallel extinction, but in a few instances extinction 

 angles up to 14° were noted. This comparatively high angle, together 

 with secondary calcite in the groundmass, indicates the presence of some 

 plagioclase. The fluorite is purple and generally in irregular grains, 

 but occasionally shows octahedral outline. 



LATITE 



Within the area mapped there is very little evidence as to the relative 

 age of the different dike rocks; but there is no question that the latite 

 is younger than the diabase, since dikes of the former rock are seen 

 cutting the diabase dike south of Fourmile Creek. The latite dikes, 

 as a whole, vary in width from a few feet to 100 feet or more, and present 

 a considerable range in color, texture and mineral composition. Gray 

 is the prevailing color, often of brown shade, rarely bluish. Since no 

 decidedly fresh rock is exposed the original color is unknown, but the 

 tendency in weathering is toward brown tones. In texture the rock 

 ranges from a felsitic phase with a few phenocrysts scarcely 1 mm. in 

 diameter, to a latite porphyry with abundant phenocrysts some of 

 which are 25 mm. in diameter. 



The constituent minerals are plagioclase, orthoclase sometimes 

 glassy, biotite, augite and hornblende in varying proportions, beside 

 the accessories titanite, magnetite, apatite, rutile and sodalite or haiiyn- 

 ite. The last-named mineral was found in one specimen from the 

 dike west of Long Gulch just north of Fourmile Creek. Here also was 

 found the only rutile observed, in a sagenite web enclosed in altered 

 biotite. In the phenocrystic feldspars plagioclase is commonly in 

 excess of orthoclase. Since the microlites are poorly individualized, it 

 is impossible to determine the relative importance of the feldspars in the 

 groundmass. 



