PETEOGEAPHY— SANIDIN TB ACHYTE. 513 



to yellow and the fragment to blue. In either case the appearance is strik- 

 ing, and is not often met with in such a perfect condition. It is shown in 

 Plate II, Fig. 2. 



Magnetite grains, large and small, are in abundance all through the rock, 

 the occasional large masses having a fine luster. Long, colorless needles 

 of apatite of quite good size are often noticed in the groundmass, and also 

 exceedingly minute dark colored needles, probably biotite. The larger 

 biotite crystals are very ragged and indistinct, being so much decomposed 

 as to scarcely show any traces of dichroism Some of the feldspar crystals 

 are faintly banded, but are not all plagioclase, the lines being too irregular. 



The groundmass is crystalline and composed of sanidin masses, with 

 grains of magnetite and a little biotite. There are slight traces of a fluid - 

 like structure in parts of the groundmass. 



The sanidin-trachyte [159-162] from Bear Lodge Butte has a greenish 

 color, with very glassy and transparent sanidin crystals porphyritic in the 

 groundmass, in which can also be seen large, green hornblende crystals. 

 Some of the sanidin has become opaque white, giving the rock a rough and 

 spotted look, which is added to by the small hornblende blades thickly 

 sprinkled between the white sanidin. The appearance of the rock in the 

 thin section immediately recalls that of [123], already described. It con- 

 sists of very large sanidins in a groundmass, fall of small hornblende 

 microlites and blades and having a fluid-like structure, and also large, 

 fresh and green hornblende crystals with magnetite in large grains. See 

 Plate II, Fig. 3. The sanidin crystals are very large and quite clear, 

 but have cloudy or dusty streaks and spots through them, though with few 

 inclusions of any marked character, except occasionall}' a hornblende 

 crystal. The cloudy portions are sometimes almost entirely opaque These 

 are the result of alteration and are the rnacroscopical white spots. The 

 outlines of the crystals are quite sharp and distinct from the groundmass, 

 most of them being terminated by two planes. The hornblendes are very 

 conspicuous in the slide, their colors being from greenish-brown to a vivid 

 green and dark-green, and their outlines very clean and sharp. Most of 

 them have a hexagonal form, w 7 ith two sides longer than the others, while 

 a few, especially one large crystal, have eight sides, six being predominant. 

 33 B H 



