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SCIENCE. 



[N. S. Vol. IX. No. 221. 



These breccias and lavas were ejected from 

 numerous fissures, vents and centers of ex- 

 plosive energy. Infinite detail as regards 

 mineral composition and texture, and great 

 complexity in mode of occurrence, may be 

 observed. Viewed in a broad way and re- 

 duced to its simplest terms, the Absaroka 

 Range consists of an uplifted volcanic 

 region, presenting from one eud to the 

 other great uniformity, and even simplic- 

 ity, in its main geological features. It 

 is essentially a dissected plateau, deeply 

 trenched by incisive gorges, ofiering ex- 

 posures varying from 2,000 to 5,000 feet of 

 nearly horizontal or only slightly inclined 

 lavas. To this there are, of course, some 

 exceptions, as is natural in any volcanic 

 region. Notwithstanding the varied and 

 complex manifestations of the eruptive 

 breccias from many sources of outflow, this 

 entire body of extrusive material has been 

 divided broadly into six epochs, based upon 

 their relative age and general sequence of 

 lavas. They represent, in the geological 

 history of the mountains, as many distinct 

 phases of volcanic eruption. Beginning 

 with the earliest in the order of eruption, 

 they have been designated as follows : 

 early acid breccia, early basic breccia, early 

 basalt sheets, late acid breccia, 1-ate basic 

 breccia, late basalt sheets. 



Briefly stated, the interpretation of this 

 history, as I understand it, is somewhat as 

 follows : 



So far as is known, the oldest volcanic 

 rocks recognized in the Absarokas consist of 

 a series of eruptives made up almost en- 

 tii'ely of fragmental material, usually light 

 in color, varying from grayish white to 

 purple. In mineral composition they range 

 from hornblende-andesite to hornblende- 

 mica-andesite. Some of the siliceous varie- 

 ties have developed phenocrysts of quartz 

 in sufficient amount to be classed as dacites. 

 These breccias appear to have been thrown 

 out with violent explosive action from nu- 



merous centers, but from none of them was 

 any large amount of material piled up ; at 

 least if it was thrown out it was subse- 

 quently worn down by atmospheric agencies. 

 In no instance do they attain great eleva- 

 tion, the exposures being due to extensive 

 erosion and deep trenching of narrow can- 

 yons. They are known only in the northern 

 end of the range, and there in limited area, 

 being buried beneath vast accumulations of 

 still later material. These centers appear 

 to be independent of later eruptions. 



Overlying these acid breccias is a vast 

 amount of volcanic ejectamenta, with here 

 and there interbedded basaltic flows, the en- 

 tire body having accumulated in many places 

 to a height of several thousand feet. They 

 occur far more widely distributed over the 

 mountains than any other group of breccias, 

 stretching both in its length and breadth 

 from one end of the range to the other. 

 They constitute nearly all the northern 

 portion of the Absarokas, as well as the 

 northeast corner of the Park. Unlike the 

 early acid breccia, they are usually dark 

 colored, owing to the amount of ferro-mag- 

 nesian minerals present. The material 

 consists largely of hornblende-pyroxene- 

 andesite, proxene-andesite and basalt. 

 Constant modifications and transitions oc- 

 cur, but over the entire area the prevailing 

 rock is pyroxene-andesite, passing into 

 slightly less basic rocks carrying horn- 

 blende on the one hand and into basaltic 

 forms on the other. By far the greatest 

 portion of this eruptive material is formed 

 of coarse agglomerates, sombre in color, 

 held together by varying amounts of ce- 

 menting ash and silts of similar composi- 

 tion. The prevailing colors are black and 

 brownish gray, while the finer silts and 

 mud flows free from large bowlders are 

 light brown, in strong contrast to the mass 

 of the breccia. 



It is difficult to describe in few words 

 such volumes of volcanic material scattered 



