SUGARLOAF DISTRICT, BOULDER COUNTY, COLO. 105 



entirely absent from several slides examined. It is occasionally inter- 

 grown with quartz in a manner similar to that of microcline. Frequently 

 it has two good cleavages. Sometimes it shows cloudy extinction 

 indicating the strain to which the rock has been subjected. Twinning 

 is after the Carlsbad law, and comparatively rare. Alteration products 

 are identical with those of microcline. 



Plagioclase is very generally distributed in small amount, but in 

 specimens in which hornblende is plentiful it is in excess of orthoclase. 

 The maximum extinction angle of 20 in sections normal to the albite 

 lamellae would indicate that it is in large part albite or albite-oligoclase, 

 though more calcic varieties may be present in the granodiorite phase. 

 Pericline twinning frequently accompanies albite twinning, and Carls- 

 bad twins are occasionally present. Although the plagioclase is fre- 

 quently very fresh, calcite, kaolin, sericite and less often, epidote occur 

 in small quantity as alteration products. 



Quartz occurs in all the slides examined, though sometimes in small 

 quantity. The hornblendic granite carries very little quartz. Undula- 

 tory extinction is very common, especially in those sections having much 

 microcline, and the quartz is commonly fractured. Fluid inclusions 

 are abundant, usually in bands intersecting at high angles. Often one 

 strong band will be crossed by several parallel bands. 



Biotite is almost a universal constituent, and usually in plentiful 

 quantities. It is generally green to brown and strongly pleochroic. 

 Sometimes the lamellae are flexed as the result of strain. Weathering 

 commonly produces muscovite with the separation of magnetite and 

 limonite. Often green chlorite and magnetite result from the alteration 

 of biotite, and epidote is a frequent secondary mineral. The green 

 hornblende, which ranges from an accessory to the dominant ferromag- 

 nesian mineral over small areas, is strongly pleochroic. Although often 

 fresh, it sometimes shows alteration to epidote or to magnetite and 

 quartz. That the hornblende and biotite crystallized simultaneously is 

 indicated by idiomorphism of the hornblende toward biotite, but in the 

 same field, small inclusions of the latter mineral in the former. 



Muscovite may be primary where it appears to be intergrown with 

 biotite. More often it has resulted from the leaching of biotite, and 



