114 UNIVERSITY OF COLORADO STUDIES 



is presented by the characteristic crystal outline very rarely seen in cross- 

 sections. Idiomorphism is even more rare than in the pyroxene, and 

 the hornblende not infrequently involves plagioclase crystals in an 

 ophitic manner. In this relation, also, it strongly suggests its primary 

 origin. When biotite is intergrown with hornblende the association of 

 the two is almost identical with that of biotite and pyroxene. 



The biotite is brown and strongly pleochroic. Sections inclined 

 slightly to the basal pinacoid give a good uniaxial interference figure. 

 Crystal outline is almost invariably absent. In amount the biotite 

 probably exceeds either of the other ferromagnesian constituents. 



The orthoclase is ordinarily allotriomorphic, but occasionally 

 approaches idiomorphic forms, giving a porphyritic aspect to the rock 

 under the microscope. Twinning after the Carlsbad law is but occa- 

 sionally seen. The orthoclase carries inclusions of all the earlier formed 

 minerals in greater or less number. A single individual may enclose 

 titanite, magnetite, plagioclase and the three ferromagnesian minerals 

 which results in typical micropoikilitic texture. From the contact 

 specimen mentioned above, orthoclase is almost absent. Even in those 

 specimens which show considerable alteration of plagioclase, the ortho- 

 clase is nearly always fresh, but occasionally it is slightly kaolinized. 



As compared with either plagioclase or orthoclase the amount of 

 quartz is small. It is completely allotriomorphic, having taken the 

 spaces left after all the other constituents had crystallized, with the 

 possible exception that the orthoclase had not entirely crystallized before 

 the quartz began to solidify. It contains numerous liquid inclusions, 

 each with a bubble. Nearly all of these inclusions are elongated and 

 some are perfect "negative crystals." 



The monzonite contains titanite in greater amount than does any 

 other rock in the district. This mineral occurs in irregular grains and 

 in perfect crystals. The crystals are usually wedge-shaped, but are 

 occasionally prismatic. Very frequently the titanite is closely asso- 

 ciated with small masses of black iron ore. Zircon and apatite are 

 present as inclusions in all the essential minerals of the rock. Magne- 

 tite is present in considerable amount in irregular grains enclosed by, or 

 associated with, all the ferromagnesian constituents. Its habit is on 



