SUGARLOAF DISTRICT, BOULDER COUNTY, COLO. 121 



clase, like the plagioclase, is often fractured, and like the latter, seldom, 

 if ever, has a good cleavage. Occasionally it exhibits zonal banding, 

 and sometimes Carlsbad twins are present. Kaolin, quartz and sericite 

 appear as secondary minerals in the altered phenocrysts. 



Biotite is plentiful, often in hexagonal flakes. It is dark brown in 

 basal sections and gives a uniaxial interference figure. A resorption 

 border with minute grains of magnetite is almost universally present. 

 Magnetite and limonite accompanied by quartz are frequently derived 

 from weathered biotite. 



Pyroxene is the freshest ferromagnesian constituent. It is pale 

 green in color and weakly pleochroic. Orthopinacoidal twins are com- 

 mon; these are often seen in cross- sections of the mineral when the 

 nicols are crossed. The high extinction angle, frequently reaching 45 , 

 indicates that the mineral is augite. With the augite are sometimes 

 associated quartz, magnetite and calcite or epidote as alteration products. 



The hornblende crystals are often twinned. Extinction angles range 

 from 1 4 to io°. Pleochroism is strong, the color ranging from yellow- 

 ish-green to deep green with a tinge of brown. The crystals almost 

 invariably show a resorption border with minute magnetite crystals. 

 Alteration products are magnetite and quartz, the latter forming near 

 the center of the crystal. Sometimes the groundmass bordering the 

 hornblende is stained by a yellowish substance which is perhaps limonite. 

 Many crystals are completely replaced by magnetite. 



Titanite occurs in idiomorphic crystals, sometimes wedge-shaped, 

 again prismatic. The crystals often show rough cleavage cracks, and 

 twinning is common. Apatite is enclosed in all the essential minerals 

 and occurs sparingly in the groundmass. An apparently primary 

 quartz crystal 1 . 35 mm. in diameter was observed in one section. 



The felty groundmass is composed of feldspar microlites with inter- 

 stitial quartz and possibly feldspar, besides numerous grains of magne- 

 tite and much secondary calcite. Although the quartz may be in part 

 primary, much is doubtless secondary. The feldspar microlites are 

 never more than . 10 mm. long and are often simply twinned. Since 

 they almost always, if not invariably, give parallel extinction they are 

 doubtless high in alkali. However, the exact extinction angle is deter- 



