EAELY ACID BRECCIA, ABSAROKA RANGE. 273 



line grading into micropoikilitic structures, which indicate that the rock frag- 

 ments are broken portions of volcanic cones where the lavas had crystal- 

 lized as dikes and other intrusive bodies. The shape of the fragments, 

 usually angular, is such as to show that most of the breccia was formed by 

 the explosion of already solidified lavas, which had consolidated near the 

 centers of volcanic action. Few, if any, exhibit the slaggy surface of bombs. 



The phenocrysts are sharply idiomorphic, and zonal structure is well 

 developed in the feldspars, which often contain numerous glass inclusions. 

 The feldspars are almost wholly polysynthetic twins, in the lime-soda 

 feldspar series, their optical properties indicating oligoclase and oligoclase- 

 andesine. Sanidine is seldom present; in fact, is almost entirely absent. 

 It is found in some associated tuffs, but may have been derived from the 

 trachytic lava occurring in the vicinity of Junction Valley. The feldspars 

 are sometimes quite free from inclusions ; in other cases they contain minute 

 crystals of the other constituent minerals, as well as portions of the 

 groundmass. 



Biotite forms six-sided plates, often rather thick. Its color is brown to 

 red, with marked absorption parallel to the plane of cleavage, and some- 

 times with strong pleochroism between orange, red, and light yellow. 

 Greenish yellows also occur. Frequently the hornblende exhibits the same 

 colors as the biotite in the same rock, both being red, or both brown or 

 greenish brown. Apparently the same cause affected the color of both 

 minerals at the same time. Often the biotite is brown when the hornblende 

 is greenish brown, or even green. Biotite resists decomposition longer 

 than hornblende in many cases. It is often free from inclusions, but fre- 

 quently contains magnetite, apatite, or zircon, and less often the other 

 mineral constituents. In some cases it has a border of magnetite grains, 

 or may be more or less completely changed to a pseudomorph of magnetite 

 with or without pyroxene. This is usually accompanied by like changes in 

 the hornblende. 



Hornblende has its customary stout prismatic forms, usually with colors 

 like those of biotite, but green tones occur more frequently than in biotite. 

 The pleochroism is that ordinarily observed. Often free from inclusions, it 

 sometimes abounds in them, glass and magnetite being the common kinds. 

 Paramorphism to magnetite and augite occurs, as with biotite. Intergrowths 

 with pyroxene are occasionally seen. Pyroxenes are more numerous in 



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