CHAPTER VIII. 



THE IGNEOUS ROCKS OF THE ABSAROKA RANGE AND 

 TWO OCEAN PLATEAU AND OF OUTLYING PORTIONS 

 OF THE YELLOWSTONE NATIONAL PARK. 



By Joseph Paxson Iddings. 



INTRODUCTION. 



The volcanic lavas that were erupted from the chain of ancient vol- 

 canoes situated along the eastern border of the Yellowstone Park, and from 

 minor vents lying* outside of this range, were to a very great extent tuffs 

 and fragmental material. They were thrown over large areas of country, 

 and often traveled long distances, so that after erosion had reduced the 

 size of these vast accumulations of tuff-breccia, not only were the original 

 forms of the bodies destroyed, but deposits that may have been at one time 

 connected have become separate. Thus, it is not always possible to decide 

 whether isolated areas of volcanic breccia belong to neighboring larger 

 bodies or are the result of local eruptions. 



In the present chapter no attempt will be made to describe the mode 

 of formation or the history of the lavas of these portions of the Park, or 

 the topographic features of the Absaroka Mountains. Descriptions of 

 these will be found in chapters by Mr. Arnold Hague in Part I. The 

 petrographic treatment of the rocks in this chapter will be confined to an 

 account of their field occurrence and distribution and to a systematic 

 description of their mineralogical characteristics and composition. The 

 account of their occurrence and distribution must of necessity proceed 

 along geographical lines, which may be followed either upon the map of 

 the Yellowstone Park accompanying this monograph or on the atlas sheets 

 of folio 30 of the Geologic Atlas of the United States, issued by the Geo- 

 logical Survey. For convenience, we shall start at the northern boundary 



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