ROCKS RESEMBLING ABSAROKITE. 351 



amount of biotite, very little magnetite and augite, and some colorless apatite. 

 The feldspar phenocrysts are labradorite, while the feldspar of the ground- 

 mass is mainly orthoclase, with kernels of fresh feldspar that has the optical 

 characters of oligoclase. There is very little quartz, and some little chlorite 

 or serpentine. The feldspars of this rock are quite fresh, as are also the 

 biotites. A coarser-grained and more altered modification of this rock 

 occurs in another dike near the head of the main stream (1467). The 

 feldspars of the groundmass are not idiomorphic, but acicular, with the 

 microstructure characteristic of syenite-porphyries. The mineralogical 

 analogy between banakite and shoshonite is chiefly in the association of 

 labradorite phenocrysts with orthoclase microlites. Biotite and augite are 

 common to some varieties of both, while olivine is present in one variety 

 of banakite and is common to most shoshonites. These rocks are the 

 highly feldspathic modification of shoshonite magma, and are complemen- 

 tary to absarokite, which represents the least feldspathic modification of the 

 same magma. 



SIMILAR ROCKS IK MONTANA. 



Rocks almost identical with absarokite occur in the region about Boze- 

 man, Montana, and have been thoroughly described by Merrill 1 in a 

 bulletin of the United States National Museum. They are intrusive 

 bodies in part, and have been described under the head of questionable 

 basalt and lamprophyre. They are more or less decomposed in some 

 cases and quite fresh in others. Their mineral composition and habit are 

 like those of the rocks here called absarokite. The phenocrysts are olivine 

 and monoclinic pyroxene, whose chemical composition in the case of the 

 rock from near Fort Ellis is that of chrome-diopside. The rock itself is 

 unusually rich in magnesia and comparatively low in iron oxide, so that it 

 is probable that the monoclinic pyroxenes in the other absarokites are not 

 such pure diopsides, but are most likely malacolites or augites. The pyrox- 

 ene in the coarse-grained shonkinite of Square Butte, Montana, which is 

 related to absarokite chemically, has been shown by Weed and Pirsson to 

 be augite. There are no phenocrysts of feldspar, and the groundmass con- 

 tains orthoclase, or when not distinctly crystallized is found to be compara- 

 tively rich in potash and soda. The chemical compositions of the rocks 



1 Merrill, Geo. P., Notes on some eruptive rocks from Gallatin, Jefferson, and Madison counties, 

 Montana: Proc. U. S. Nat. Mus., Vol. XVII (No. 1031), 1895, pp. 637-673. 



