SUGARLOAF DISTRICT, BOULDER COUNTY, COLO. 1 27 



hand specimen (3X4 in.). Generally the dacite weathers to a light 

 greenish-gray, but on protected surfaces becomes reddish. 



The feldspar phenocrysts so far as observed are found under the 

 microscope to be plagioclase exclusively. Twinning after the albite 

 law is almost universal, sometimes accompanied by pericline, Carlsbad 

 or Baveno twinning. The twinned lamellae are usually very thick. 

 Zonally-built feldspars are very common with a sharp change in the 

 extinction angle from zone to zone. The change is invariably from the 

 more basic variety at the center to less basic toward the margin. One 

 crystal cut approximately parallel to the brachypinacoid (010) gives 

 at the center an extinction angle of 29 from the trace of the basal 

 cleavage, and near the margin an angle of 15 . A difference of 12 was 

 noted between the angle of the outer zone and that of the interior in a 

 section normal to (010). Extinction angles of 25°-35° in the zone 

 normal to (010) are not uncommon, and may be as high as 45 . The 

 largest extinction angle measured from the trace of the basal cleavage 

 on a section approximately parallel to the brachypinacoid was 32 . 

 The frequency of large extinction angles on the brachypinacoid and 

 in the zone normal to that face together with the interference figures 

 and, indirectly, the thickness of the albite lamellae, would indicate a 

 considerable percentage of the anorthite molecule. The plagioclase 

 probably ranges at least from andesine to basic labradorite. 



A few of the feldspars are fissured and faulted, the fissure being 

 filled with secondary calcite. Not infrequently the phenocrysts enclose 

 portions of the groundmass, and are sometimes corroded and penetrated 

 by deep bays. The feldspars commonly show alteration near the center 

 of the crystal, less often at the margin. The commonest alteration 

 product is calcite which often has rhombohedral cleavage. Sometimes 

 in advanced stages of weathering chlorite accompanies calcite. A less 

 frequent product is epidote accompanied by sericite. Kaolin is very 

 subordinate in quantity. 



Biotite was the first of the three essential constituents to crystallize, 

 since it can be seen, though not often, penetrating both the quartz and 

 feldspar and occasionally enclosed in the feldspar. Sometimes the 

 biotite crystals were broken and torn apart while the magma was still 



