PETKOGRAPIIY—SANIDIN TRACHYTE. 515 



tinctly shaped forms, many being twinned. The biotite prismatic crystals 

 are very small indeed, but are thickly scattered throng-h the rock. The 

 magnetite is generally in very small grains. The groimdmass appears to 

 be of a iine fibrous or microlitic structure, filling the interstices between 

 the sanidin crystals. It is feldspathic in nature and dusty, so that it polarizes 

 but feebly, and there is less quantity than at first sight appears. While one 

 set of the sanidin crystals are bright with crossed nicols, those in a different 

 position are dark and might be mistaken for the groundmass. Upon revolv- 

 ing the slide, however, the crystals respectively change their appearance, 

 the ger.eral aspect of the rock remaining the same, while there are some 

 crystals always bright and conspicuous. 



The specimen [167, 168] from Little Missouri Buttes is a sanidin-tra- 

 chyte, but approaching phonolite in structure and in the presence of some 

 zeolitic minerals. Its color is a quite dark, dirty-green, with an abundance 

 of white, glassy sanidin crystals conspicuously scattered through the ground- 

 mass. A few blac^'- hornblendes can be also seen The structure of the 

 groundmass is quite compact, with the sanidin from very small to large 

 crystals imbedded in it. On the surface of some of the large sanidins is a 

 yellowish-brown coating Or crust of alteration. Under the microscope, the 

 rock appears made up of large sanidin crystals, some clear and some dusty 

 or decomposed, in a yellowish-green, fibrous groundmass, with a few green 

 hornblende and brown biotite crystals. The sanidins are quite well defined 

 in form, showing generally the six-sided sections corresponding to prism, 

 base, and hemi-dome. Some are very transparent and give a fine blue or 

 yellow color in polarized light, while others seem to be altered, having 

 become quite cloudy from numerous small acicular or fibrous microlites in 

 nature like the groundmass, so that only small portions of the crystals give 

 any color. This alteration commences in streaks across the clear crystal, 

 eventually afi'ecting the whole mass ; in a few cases a radiated zeolite has 

 been formed in the fissures ctnd also in the interstices between two sanidin 

 crystals. The transparent crystals are characteristic and have cleavage 

 lines in two directions. The inclusions are portions of the groundmass and 

 small, pale-greenish columnar crystals and dark needles. Besides these are 

 numerous round and long cavities in rows and scattered through the crys- 



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