ROCKS (JOBRESPONDINGr TO ABSAROKITE AXD BAjSTAKITE. 353 



Near the absarokite of Cottonwood Creek is a syenitic rock which 

 corresponds in some respects to the banakite of the Stinking-water region, 

 in that it is highly feldspathic with abundant biotite and some pyroxene. 

 Its microstructure is that of trachyte-porphyry. Chemically (analysis 7) 

 it is much richer in alkalies, having 8.44 per cent of potash and 5.47 

 per cent of soda, and, as Merrill points out, is quite like the socialite- 

 syenite of Square Butte, Montana, described by Lindgren. 1 They are 

 more alkaline feldspathic modifications of magmas which have yielded 

 absarokite and the somewhat similar magma which at Square Butte has 

 crystallized into the coarse-grained rock called shonkinite. 2 



This new type of granular crystalline rock consists essentially of augite 

 and orthoclase, with biotite, olivine, magnetite, albite, and anorthoclase, 

 with accessory nepheline and sodalite, and other minerals. Its chemical 

 composition, as shown in the accompanying analysis, is similar to that of 

 the absarokites of the Yellowstone Park, except in the higher percentage 

 of lime, but the low silica and alumina and the relatively high alkalies, 

 with high potash and high magnesia and lime, show its chemical relation 

 to this group. 



In the Little Belt Mountains of Montana 3 there are rocks almost the 

 same as absarokite chemically, but coarsely crystalline, as at Square Butte. 

 And Weed and Pirsson have described the petrographical characters of the 

 rocks from Yogo Peak, whose chemical composition, together with that of 

 the rocks from Square Butte, Highwood Mountains, is shown by the analyses 

 in the table on the following page. 



'Lindgren, W., Eruptive rocks from Montana : Proc. California Acad. Nat. Sci., 2d series, Vol. 

 Ill (Part I, 1891), 1893, pp. 45-47. 



- Weed, W. H., and Pirsson, L. V., Highwood Mountains of Montana: Ball. Geol. Soc. America, 

 Vol. VI, pp. 389-422, pis. 24-26. 



3 Weed, W. H., and Pirsson, L. V., Igneous rocks of the Yogo Peak, Montana: Am. Jour. Sci., 

 3d series, Vol. L (No. 300, Dec, 1895), pp. 467-479. 



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