SUGARLOAF DISTRICT, BOULDER COUNTY, COLO. 117 



of a common magnet gives a decided titanium reaction in hydrochloric 

 acid solution. 



While quartz is not present in all the slides examined, in many sec- 

 tions there are numerous small irregular masses. In addition to the 

 apatite noted below there are frequent liquid inclusions in the quartz, 

 each with a gas bubble. Although the quartz may be in part primary, 

 there is no direct evidence, such as intergrowth with feldspar. It is 

 without doubt partly secondary, sometimes forming within the boundary 

 of an augite crystal now mostly replaced by iron ore. Since from slides 

 carrying fairly fresh pyroxene quartz is nearly or quite absent, and is 

 most plentiful in those slides which show advanced alteration of the 

 essential minerals, it seems reasonable to infer that most of the quartz 

 is secondary. 



Apatite is a common accessory in stout to needle-like crystals included 

 in feldspar, pyroxene, quartz and serpentine. The feldspars contain 

 the greatest number of apatites. Pyrite occurs sparingly in the inter- 

 stitial feldspar, following the two cleavages. Dark brown biotite was 

 observed in one section in very small quantity and much weathered. 

 In addition to the occurrences of serpentine noted above, it appears in 

 minute patches scattered through the rock, and fills minute fractures in 

 the feldspar. 



Contact Phase of Diabase. — Near the walls of the dikes the diabase 

 is decidedly finer in texture than that a few feet distant. In the one thin 

 section prepared from this finer-grained variety both the feldspar and 

 the pyroxene are of two periods of crystallization. Minute pheno- 

 crysts of augite and feldspar occur in nearly automorphic crystals. The 

 latter have a maximum diameter of i . 5 mm. ; the former are usually less 

 than one-half as large. The augite seems to be automorphic toward the 

 feldspar, but in this slide the phenocrysts are too few to warrant the 

 conclusion that the augite is the older. The augites are frequently 

 twinned. The feldspar of the second generation occurs in microlites. 

 Augite is found in the interstices in small grains with serpentine and 

 black iron ore. 



In this specimen is a fragment of quartz roughly elliptical in outline 

 with an embayment (Plate III). The longer diameter of the quartz 



