BIGHORN PASS SHEET. 71 



The high percentage of carbon dioxide, 5.80, corresponds to the 

 abundance of calcite. The comparatively low alumina and relatively high 

 alkalies are noteworthy. The potash is comparatively high for so basic a 

 rock, and accounts for the presence of abundant biotite. Magnesia is below 

 the normal percentage for a rock with so little silica. It has entered into 

 the composition of biotite, malacolite, and hornblende. No orthorhombic 

 pyroxene or olivine has been developed. A comparison of this rock with 

 several others somewhat similar in chemical composition will be made 

 later on. 



The microscopic feldspars are polysynthetic twins of lime-soda feldspar, 

 with high extinction angles, corresponding to labradorite. They are nearly 

 idiomorphic, rectangular to lath-shaped crystals, of pure substance, and 

 when not obscured by calcite they appear perfectly fresh and not at all 

 crushed. It seems as though the calcite had been derived from other 

 sources — that is, from the pyroxene, or by infiltration from the inclosing 

 limestone. The absence of strain or crushing is significant in connection 

 with the proximity of this thin sheet to the massive laccolith, and indicates 

 that this lamprophyric rock is the more recent intrusion. In places the 

 form of the feldspar is tabular. And sometimes the rectangular crystals 

 are bounded by a margin of unstriated feldspar with allotriomorphic out- 

 line, and in some cases idiomorphic outline. This feldspar has a lower 

 index of refraction than that of the inclosed feldspar, and is undoubtedly 

 orthoclase. Its mode of occurrence is precisely the same as that of the 

 orthoclase in the groundmass of the basaltic rocks, absarokite and shosho- 

 nite, described in Chapter IX. Grains of quartz constitute the last crystal- 

 lization of the groundmass. It is probable, however, that some of the 

 quartz is secondary, since it occurs in idiomorphic crystals surrounded by 

 calcite. There are many grains and crystals of magnetite and abundant 

 minute hexagonal prisms of colorless apatite. Brown biotite is in part 

 idiomorphic, in part allotriomorphic, with penetrations of plagioclase and 

 inclusions of apatite and magnetite. The monoclinic pyroxene, with large 

 angle of extinction and rather low double refraction, is almost colorless in 

 thin section, and is a diopside or malacolite. It is partly altered along 

 cracks and around the margin, with the formation of calcite and chlorite. It 

 is mostly idiomorphic, in comparatively large crystals, and does not occur 

 in microlites in the groundmass. The crystals have the ordinary form and 



