SUGARLOAF DISTRICT, BOULDER COUNTY, COLO. 1 23 



lime is sufficient to class the rock with the andesites, and is too high 

 for a typical trachyte. Before the introduction of the term latite into 

 petrographic literature as a name for rocks intermediate between trachyte 

 and andesite the Sugarloaf rock was probably quite properly called 

 andesite, or according to some petrographers it might have been desig- 

 nated trachytic andesite. But since in the same dike there occurs a 

 a rock which is unquestionably trachyte, and since the rock under 

 discussion has mineralogical and chemical characteristics in common 

 with andesite, and others in common with trachyte, it seems well to 

 consider this dike and others similar as latite. 



Latite -Porphyry 



The short east-west dike west of Long Gulch and another about one 

 and one-fourth miles due north of this are the only exposures of latite- 

 porphyry in the area under consideration. The latter dike extends 

 westward where the rock becomes more important both in extent and 

 as a factor in the relief. 



The rock is light grey in color, and to the unaided eye appears to 

 be composed almost entirely of idiomorphic crystals of feldspar and 

 hornblende or pyroxene. The megascopic feldspars are largely ortho- 

 clase, mostly of the sanidine variety. A striking feature is the large 

 size of the best-developed phenocrysts which are about 25 mm. in 

 diameter. From one to four of these may appear in a hand specimen 

 (3X4 in.). By far the greater number of feldspars are below 10 mm. 

 in diameter, and closely packed. Carlsbad twins, best seen on cleavage 

 faces in reflected light, are not uncommon. Crystals of the ferromag- 

 nesian minerals are usually less than 3 mm. in length. 



In thin section the smallest feldspars prove to be both plagioclase 

 and orthoclase with the former the more abundant. Augite and green 

 hornblende are both present, the hornblende showing the same resorp- 

 tion phenomena and alteration products as in the latite described above. 

 The groundmass, which constitutes probably one-fourth to one-third 

 of the rock, is composed of lath-shaped feldspar microlites with parallel 

 extinction, and interstitial feldspar. Wedge-shaped titanites and stout 

 zircons occur sparingly. 



