PETEOGRAPUY— SANIDINOTJC.OCLASKTRACmYTE. 507 



The plagioclase is often crystallized inside of the sanidiii and nj)on the 

 edges of the crystals. The banded appearance of the twin lamellations is so 

 broad and beautiful as to resemble greatly that of labradorite ; a few of the 

 independent crystals are very large. 



There are numerous quartz grains of irregular shape throughout the 

 groundmass and occasionally a large mass more regular in form. The bio- 

 tite is in very large, broken, and decomposed crystals, either long, prismatic 

 in shape or in hexagonal plates. Although both forms are much torn and 

 broken and but faintly dichroitic, the distinct laminations can be easily seen. 

 It is, moreover, quite dark and opaque in many places. Magnetite occurs 

 in small grains, abundantly scattered through the groundmass and also in 

 quite large masses, having the usual submetallic luster. The groundmass 

 is crystalline very distinctly indeed, being an aggregation mostly of rectan- 

 gular and ragged sanidin crystals, Avith some quartz, magnetite, and a little 

 biotite in small brown leaves. The siHca was determined to be 61. SG per 

 cent. 



The sanidin-oligoclase-trachyte [144] from between the forks of Bear 

 Butte Creek is brownish in color and of a very rough texture, with macro- 

 scopical crystals of feldsjiar, between which can be seen dark-green needles 

 and crystals of biotite. In the section, were observed large and tolerably 

 clear sanidin and plagioclase crystals in a fine-grained, light-brown ground- 

 mass with biotite and magnetite. 



The large sanidin crystals are quite clear, and give fine colors in polar- 

 ized light, but there are still a great many fissures and cracks, along which 

 they are cloudy and altered. Microlites, as beautifully distinct long needles 

 and tolerably stout prisms and rounded glass masses, are among the many 

 inclusions of these sanidin crystals. The colorless needles are probably 

 apatite, with which are associated magnetite and biotite. Some of the 

 sanidins are twinned, the two parts polarizing differently. The plagioclase 

 is in large crystals and of distinct outline, although the)" are often formed 

 in conjunction with the orthoclase. The twin striations are in some cases 

 very broad and distinct, while in other crystals they are so fine and delicate 

 as to resemble the sanidin striations arising from structure. Generally they 

 are sharp and plain. The crystals have similar inclusions to those of the 



