PROWERSOSE FROM TWO BUTTES, COLORADO 167 



scopic phenocrysts, many small biotite leaves, augite grains and 

 prismoids, and a multitude of minute magnetite grains. There 

 is a gradual transition from the minute biotite and augite indi- 

 viduals to those 2 or 3"^"^ in diameter — a feature which causes the 

 porphyritic texture to be subordinate and crude. 



Feldspar is the most abundant constituent, and it seems to be 

 wholly orthoclase, occurring in ill-defined scales or tablets which 

 are characteristically arranged, and possibly intergrown, in sheaf- 

 like, imperfectly radial groups. There are no sharply defined 

 crystals, and the maximum dimension of the particles is perhaps 

 ^mm_ Because of the great number of other mineral grains which 

 are held by this orthoclase mass, and through the presence of many 

 minute dustHke interpositions, the feldspathic constituent can be 

 satisfactorily studied only in certain small areas scattered through 

 the rock, where the femic minerals are quite subordinate. 



All feldspar on the borders of the sections has a lower index of 

 refraction than the Canada balsam, no multiple twinning has been 

 observed, and the analysis of the rock shows that a soda feldspar 

 is not Ukely to be present. Probably the orthoclase contains a 

 small amount of the albite molecule. 



Biotite is quantitatively the next constituent of importance. 

 It occurs in scales which are seldom crystallographically bounded, 

 the large phenocrysts and some of the smallest flakes being the 

 prominent exceptions. The pleochroism ranges from pale yellow 

 to a reddish brown, • It is clear that the mineral is poor in iron. 

 It is quite fresh throughout the rock. 



The pyroxene occurs in prismoids, colorless or faint green as 

 a rule, but some of the stouter prisms contain a core of bright grass- 

 green color. The clino-pinacoidal extinction in such cases is about 

 36° for the green core and 45° for the outer zone. The pyroxene, 

 being fresh and very nearly free from inclusions, was isolated and 

 analyzed by Dr. W. F. HUlebrand, with the result given in Column 

 II of the table, p. 168. Its specific gravity is 3.45 at 25° C. 



This pyroxene is a strongly diopsidic augite, and by assuming 

 the Fe203 to be about one-half too high, the mineral may be calcu- 

 lated to have the composition: 6 Naa Fe^ Si^ O12 (aegirite) -|- 34 

 (Na,R) (Al Fe), Si06 + 403 Ca (Mg Fe) Si.Og (diopside). It is 



