ABSAROKITE-SHOSHONITE-BANAKITE SERIES. 953 
and higher in lime and magnesia. It is associated with the 
rocks of analyses 4 and 5. It has nearly the chemical com- 
position of the rock called yogoite by Pirsson. Associated 
with the absarokites in several places are highly feldspathic 
rocks, richer in alkalies than the banakite of the Yellowstone 
Park. The chemical composition of one of them is shown by 
analysis 7, of Table IV. They are undoubtedly comple- 
mentary products of differentiation. 
In the Highwood and Little Belt Mountains of Montana 
there are rocks almost the same as absarokite in chemical com- 
position, but coarsely crystalline. They have been described 
by Weed and Pirsson,* the account of the rocks of Yogo Peak hav- 
ing just been published. The chemical composition of the rocks 
of Yogo Peak and those of the two rocks of Square Butte, 
Highwood Mountains, is shown by the analyses in Table V. 
taken from the articles cited. 
Pirsson states that the three rocks from Yogo Peak grade into 
one another, and are facies of one mass, whose variations are 
the result of differentation. They are granular crystalline, and 
consist essentially of orthoclase and augite in different propor- 
tions, and with subordinate amounts of plagioclase, biotite, 
magnetite; and in the syenite, hornblende; and in shonkinite, 
olivine, besides accessory minerals. In the syenite orthoclase 
exceeds augite, in yogoite orthoclase equals augite, and in 
shonkinite augite exceeds orthoclase. Chemically the series is 
characterized by comparatively low alumina, with relatively high 
potash, which is nearly constant. The sum of the alkalies 
decreases with decrease of silica and alumina. Magnesia and 
lime are fairly high. In the two rocks of Square Butte the 
shonkinite is like that of Yogo Peak, but alumina and alkalies 
are somewhat lower, and magnesia and lime higher. The syen- 
ite of Square Butte, the complementary rock with shonkinite, is 
high in alumina and alkalies, and very low in magnesia and lime, 
™WEED,W. H. and Pirsson, L.V., Highwood Mountains of Montana. Bull. Geol. 
Soc. America. Vol. VI., pp. 389-422. Rochester, 1895. Reviewed in this JOURNAL. 
Vol. III., No, 7, p. 851; also Igneous Rocks of Yogo Peak, Montana. Am. JouR. 
Sci., Vol., L., No. 300. Dec. 1895.- pp. 467-479. 
