SUGARLOAF DISTRICT, BOULDER COUNTY, COLO. 113 



full development until after the formation of plagioclase was complete. 

 But in one specimen from the margin of the intrusion the pyroxene had 

 taken on crystal boundaries before the plagioclase crystals were fully 

 developed. 



Pyroxene and hornblende occur in nearly equal amount, now one, 

 now the other predominant. The pyroxene is probably in large part 

 diopside, though augite may be present. The maximum extinction 

 angle measured from the trace of the prismatic cleavage is 45 . The 

 pyroxene is colorless and without sensible pleochroism. When, as in a few 

 instances, crystal outline appears in cross-section, the prisms and pina- 

 coids are about equally developed. Although the pyroxene is generally 

 allotriomorphic the characteristic prismatic cleavage is often seen. 

 Occasionally this cleavage is accompanied by orthopinacoidal and 

 clinopinacoidal cleavages. Orthopinacoidal twins are not rare, some- 

 times with the two main parts separated by a lamella in twinned position. 

 Intergrowths with hornblende and biotite are frequent. When pyroxene 

 and hornblende are intergrown the hornblende usually encloses small 

 anhedrons of pyroxene. Sometimes the two minerals are in parallel 

 position, often with a fairly distinct boundary between the two, but 

 irregular in form without the slightest approximation toward crystal 

 outline. It is difficult to decide how much of the hornblende is inter- 

 grown with pyroxene and how much is derived from the pyroxene. 

 The former mineral often occurs in small patches in a section of the 

 latter or it may form the greater part of the mass with pyroxene at the 

 center, or in rare cases with pyroxene at both extremities of the anhedron. 

 A single biotite crystal may surround or enclose a pyroxene individual, 

 or the two may interpenetrate with jagged edges. Often several small 

 flakes of biotite, with one or more small patches of hornblende, are 

 attached to the border of a large pyroxene. Sometimes the biotite 

 is scattered in small flakes with parallel orientation over a thin section 

 of pyroxene. 



The hornblende is the compact green variety with strong pleo- 

 chroism and an extinction angle which frequently reaches 19 . Evi- 

 dences of both primary and secondary origin of this mineral were noted 

 above. Conclusive evidence that the hornblende is in part primary 



