PETROGEAPHY— SANIDIN-TEACHYTE. 509 



The groundmass is mostly made up of the small sanidin crystals men- 

 tioned before, but between them appears to be a very fine partially micro- 

 litic mass, which remains dark with crossed nicols. 



The sani din-trachyte [149] from the central peak of Inyan Kara is 

 dark in color, being gray, inclining to greenish. Macroscopical crystals of 

 sanidin are visible, as well as some black prismatic hornblendes. It is very 

 different from the preceding rhyolites, having much less luster and being 

 darker. 



In the section, it is immediately seen that sanidin is in great preponder- 

 ance, the groundmass being composed of it and there being also a great 

 many large, fine crystals. Hornblende and biotite, with magnetite are 

 scattered through the groundmass in large and small crystals and grains. 

 There is also some plagioclase. The large sanidin crystals are very char- 

 acteristic, polarizing in fine colors and showing the different colored zones 

 of the formation of the crystals. Sometimes a well formed section with 

 perfect outline has a very different color from the more roughly shaped 

 mass forming the exterior coating, the interior for instance being light 

 brown and faintly zoned, while the other part is a brilliant purple. This 

 zonal structure is quite usual in this rock and is shown in Plate II, Fig. 1. 

 The inclusions are long and very delicate prismatic microlites, which may 

 be apatite. 



The hornblende is present in large crystals, somewhat broken and full 

 of fissures, but still polarizing quite strongly from yellowish-green to dark- 

 green, although they are but faintly dichroitic Many of them have crys- 

 tals of sanidin and magnetite imbedded in the interior, the form of the 

 sanidin being quite perfect. Besides these large crystals, there are innum- 

 erable quite small and irregular ones, many of which in quite a striking 

 manner are tipped with magnetite on both ends, and nearly every one has 

 a large grain of magnetite in some part of it. Not to be confounded with 

 the hornblende is the biotite, which occurs in large crystals, very much 

 decomposed and broken to such a degree that their former outlines are 

 shown only by the rows of magnetite grains till remaining in their stead. 

 The biotite is light-brown and strongly dichroitic, changing to very dark- 

 brown upon turning one nicol; it is also deeply striated and full of magnet- 



