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SCIENGK 



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



eruptions. How long the basalt period 

 lasted cannot now be told. In determining 

 the sequence of lavas these early basalt 

 fields play an important part, as they over- 

 lie the early series of acid and basic brec- 

 cias and underlie a somewhat similar series 

 of eruptive material designated late acid 

 breccia and late basic breccia and flows. 



Following the basalts come the late acid 

 breccias. They occur less widely distrib- 

 uted than the early acid breccias, and for 

 the most part lie within the Yellowstone 

 Park. Unlike the earlier breccias, they are 

 less deeply buried beneath later eruptive 

 material, but are piled up in successive 

 layers one upon another, forming the sum- 

 mits of several prominent peaks and broad, 

 plateau-like ridges. Over considerable areas 

 they lie spread out in thin sheets over the 

 basalt flows. Their centers of eruption 

 occupy a restricted area and seem to be in 

 every way quite independent of the earlier 

 breccias and basalts. In mineral composi- 

 tion they closely resemble the early acid 

 breccias, consisting of hornblende-andesite 

 and hornblende-mica-andesite, in places 

 mingled with a good deal of pyroxene- 

 andesite, both augite and hj'persthene be- 

 ing recognized, sometimes one and some- 

 times the other predominating. Much of 

 the brecciated material is similar in mineral 

 composition to the Ishawooa intrusive 

 bodies, which will be discussed later. 

 Nearly all of this material is fragmental, 

 and the greater part of it is made up of 

 coarse and fine tuffs. Frequently the con- 

 tact between the light- colored acid breccia 

 and the still later basic breccia is sharply 

 drawn, the latter filling up depressions and 

 levelling the accidented sui-faces of the 

 former, which occur at varying altitudes. 

 In most instances the line of demarcation 

 is not so sharply drawn, and not infre- 

 quently there is a mingling of material, as 

 if there had been a pouring-out of the 

 later rock before the complete cessation and 



closing-up of the more acid centers of 

 eruption. Occasionally these light-colored 

 rocks, from what appear to be local centers, 

 lie directly upon basic breccia made up of 

 basaltic bowlders and cementing tuffs of 

 the earlier series, without the intervening 

 basalts. Overlying these acid breccias 

 there poured forth from numerous vents a 

 second great volume of basic rocks and 

 agglomerates, 2,000 to 3,000 feet in thick- 

 ness, bearing a close resemblance to the 

 earlier basic rocks. They are found over 

 the southern portion of the Absarokas, 

 usually resting upon the basalts, the late 

 acid breccias being, as before mentioned, 

 restricted to a limited region of country. 

 Indeed, the second series of breccias forms 

 the top of nearlj- all the high plateaus and 

 the summits of the more prominent points. 

 Cross-sections exposed in deep canyons re- 

 veal grand escarpments of both breccias, 

 with intervening monotonous sheets of ba- 

 salts. Viewed in a broad way, these two 

 series of breccias are singularly alike, and 

 apparently the conditions governing their 

 eruf)tions were much the same. If we are 

 to draw any distinctions, it may be said 

 that the early breccias are apt to be sco- 

 raceous and slaggy and more chaotic in 

 their tumultuous accumulation. The later 

 breccia is more regular and distinctly 

 bedded, and is almost wholly made up of 

 both coarse and fine fragmental material, 

 carrying large bowlders that could not have 

 been thrown a great distance from the dis- 

 charging vents. Bowlders weighing a ton 

 or more are by no means uncommon. In 

 general, it may be said of these later brec- 

 cias that the coarsest material lies near the 

 present crest of the range, and is seen to 

 grow finer and more uniform, with distinct 

 bedding, as one travels either east or west. 

 To this rule, however, there are marked 

 exceptions. 



Following the late basic breccia, basalt 

 tables are found here and there capping the 



