GEOLOGICAL HISTORY OF THE YELLOWSTONE PARK. 137 



burst; indeed the region may be considered long since extinct. These 

 volcanic rocks present a wide range in chemical and mineral composi- 

 ticm and jdiysical structure. They may all however be classed under 

 three great groups — andesites, rhyolites, and basalts — following each 

 other in the order named. In some instances, erupti<ms of basalt oc- 

 curred before the complete extinction of rhyolite, but in general, the 

 relative ageof eaeh group is clearly and sharply defined, the distribu- 

 tion and mode of occurrence of each presenting characteristics and 

 salient features frequently marked by periods of erosion. 



Andesites are the only volcanic rocks which have i)layed an impor- 

 tant part in ])roducing the present structural features of the moun- 

 tains surrounding the Park. As already mentioned, they occur in 

 large masses in the Gallatin range, while most of the culminating 

 peaks in the .Absarokas are composed of compact andesites and ande- 

 sitic breccias. On the other hand, the andesites are not confined to the 

 mountains, but played an active role in filling up the interior basin. 

 That the duration of the andesitic eruptions was long continued, is 

 made evident by the plant-remains found in ash and lava beds thnmgh 

 2,000 feet of volcanic material. The plants have as yet been too little 

 studied to define positively their geological horizons. It is quite pos- 

 sible that they may indicate marked differences of climate between the 

 lower and upper beds. 



In early Tertiary times, a volcano Durst forth in the northeast cor- 

 ner of the depressed area encircled by the Park Mountains, not far 

 ft"om the junction of the Absaroka and tSnowy ranges. While not to 

 to be compared in size and grandeur with the volcanoes of California 

 and the Cascade Ivange, it is, for the Rocky ^lountains, one of no mean 

 proportions. It rises from a base about (),500 feet above sea-level, the 

 culminating peak attaining an elevation of 10,000 feet. This gives a 

 height to the voh-anoof 8,.500 feet from base to summit, measuring from 

 the Arcluean rocks of the Yellowstone Valley to the top of Mount Wash- 

 burne. The average height of the crater rim is about 9,000 feet above 

 sea level, t\w volcano measuring 15 miles across the base. The erup- 

 tive origin of AFount Washburne has long been reciognized, and it is 

 frequently referred to as a volcano. It is however simi)ly the highest 

 peak among several others, and re])resents a later outburst which de- 

 stroyed in a measure the original rim and Ibrm of the older crater. The 

 eruptions for the most part were basic andesites. Erosion has so worn 

 away the earlier rocks, and enormtms masses of more recent lavas 

 have so obscured the original form of lava-tlows, that it is not easy for 

 an inexperienced eye to recognize a volcano and the surrounding ])eaks 

 as the more elevated points in a grand cratei- wall. By following 

 around on the ancient andesitic rim, and studying the outline of the 

 old crater, together with the <-omi»osition of its lavas, its true origin 

 and history may readily be ma<le out. This older crater has as yet 

 received no special designation. l)ut when our maps and reports are 

 finally published, this ancient geological ruin will receive an approjiriate 



