SUGARLOAF DISTRICT, BOULDER COUNTY, COLO. 1 25 



The feldspar phenocrysts are in large part labradorite, but more sodic 

 feldspars may be present. A few of the crystals are unstriated, but albite 

 twinning is common, sometimes accompanied by twinning after the 

 pericline, and less often, the Carlsbad law. Zonary banding is occasion- 

 ally present. Cleavage cracks are prominent, at times accompanied by 

 fractures which bear no relation to crystallographic directions. Horn- 

 blende, apatite and, more rarely, minute zircons are enclosed in the 

 feldspar. 



In addition to the secondary magnetite noted in connection with the 

 hornblende, primary magnetite, in octahedrons throughout the rock, is 

 an important accessory. Titanite in perfect wedge-shaped crystals, pris- 

 matic forms and irregular grains is fairly common. Rarely the crystals 

 are twinned. 



The groundmass is composed of lath-shaped feldspar microlites and 

 minute apatites in a glassy base. Apatite crystals are occasionally 

 included in the feldspars. The microlites are often twinned and have 

 a very low extinction angle which rarely reaches io°. Flow- structure is 

 often seen in the parallel orientation of the microlites. 



MICA-DACITE ' 



Mica-dacite is exposed in a dike west and southwest of Sunshine 

 Canyon for a distance of about three miles, only part of which is within 

 the area mapped. 2 The maximum width of the dike is not less than 

 50 feet, but the outcrop in places may be considerably more as the result 



1 Since the following description was written, Mr. Earl B. Millard, a student in the Chemistry Depart- 

 ment of the University of Colorado, has made an analysis of the freshest material that could be found in the 

 dike near Sunshine Canyon. The following is Mr. Millard's analysis: 



Sio, 6S.07 TiO. 0.26 



A1 3 3 15.67 P'O s 0.21 



Fe a 3 0.64 ZrOa 0.06 



FeO 2.98 CO, 0.80 



MgO o-85 S0 3 023 



CaO 3-32 BaO 0.00 



Na,0 3-45 SrO 0.00 



K a O 2.07 MnO °-°° 



H 3 10 ° CI 000 



100.51 

 » Mr. Hugh F. Watts, who had carefully plotted this dike on the scale of 6 in. to the mile, generousl 

 allowed me to transfer his work to the accompanying map I am also indebted to Mr. Watts for calling my 

 attention to two exposures of another rock within the area, which otherwise would probably have been over- 

 ooked. 



