GENERAL FEATURES. 217 



by Crandall Creek, extending a short distance into the Park. It is denned 

 by the accompanying map (PL XXVII). The area of volcanic rocks here 

 represented is but a small portion of the great belt of igneous material 

 that forms the mountains of the Absaroka Range, which stretches along the 

 eastern margin of the Yellowstone Park from the Bowlder Plateau on the 

 uorth to the Wind River Mountains on the south. The volcano of Crandall 

 Basin is one of a chain of volcanic centers situated along the northern and 

 eastern border of the Yellowstone Park, which are all distinguished by a 

 greater or less development of radiating dikes, and by a crystalline core, 

 eroded and exposed to a variable extent. Electric Peak and Sepulchre 

 Mountain constitute one of these centers. 



Since the volcanic ejectamenta forming the Absaroka Range have 

 been thrown from numerous centers situated at no great distance from one 

 another, it would be impossible to separate the materials which have origi- 

 nated from the different vents, since they must have intermingled; and it 

 would be incorrect to assume that any particular area of volcanic rocks 

 had been derived exclusively from one center of eruption. But since the 

 material ejected from closely associated vents may be considered to have 

 come from the same general source, or to belong to a connected series of 

 eruptions, we may regard the volcanic rocks occurring in the immediate 

 vicinity of a well-marked center of eruption as representing the various 

 results of volcanic activity which have existed at that place. The area 

 embraced within the limits of the map may not be sufficiently large to 

 include all of the material thrown out from the minor centers of eruption, 

 which, during the period of volcanic activity, must have shifted about within 

 the region of Crandall Basin, but it is large enough to furnish data from 

 which the geological history of this particular volcano may be derived. 



To understand the geology of so limited an area as that represented 

 on the map it will be necessary to explain the general features of the sur- 

 rounding region. A high and massive range of Archean granite and gneiss 

 forms the country north of Clark Fork and stretches in a northwest-southeast 

 direction. The river channel is situated within this granitic district, near- 

 its southern margin. The Paleozoic strata which overlie the Archean rocks 

 have been greatly eroded, leaving an irregular layer of limestone, which 

 dips gradually to the southwest. This limestone forms a cliff along the 

 south side of Clark Fork and extends for considerable distances up the 



