haque.] THE YELLOWSTONE PARK. 339 



land. Later the pouring out of vast masses of lava converted this 

 depressed region into the Park plateau. Tertiary time in the Park was 

 characterized by great volcanic activity, enormous volumes of erupted 

 material being forced out. This activity extended through the Pliocene 

 period and probably well on into the Pleistocene. Within recent time 

 there is do evidence of any extensive outbursts j indeed, volcanic energy 

 may be considered long since extinct. 



The volcanic rocks present a wide range in chemical and mineral 

 composition. They may all, however, be classed under three great 

 groups — andesites. rhyolites. and basalts. 



Andesites have played a most important part in bringing about the 

 present configuration of the mountains surrounding the Park plateau. 

 As already mentioned, in the Gallatin range they form large masses. 

 and most of the culminating peaks in the Absarokas are composed of 

 compacl basic andesites or agglomerates accompanied by basaltic Hows. 

 Andesites, however, are not confined to the mountains, but also forced 

 their way to the surface in the interior depressed basin. 



That the duration of andesitic eruptions continued through a long- 

 period of time is made evident by plant remains embedded in volcanic 

 ashes and mud associated with layers of breccia and more or less com- 

 pact lavas, which accumulated to a depth of nearly 2,000 feet. Much 

 of this plan! material is in an excellent state of preservation and it is 

 in these beds that the well-known fossil forests occur. In the grand 

 escarpments along the Lamar valley the forest-bearing beds are 

 admirably displayed, erosion having cut numerous lateral ravines and 

 gorges in the lava beds, many trees still standing in upright position. 



In late Cretaceous or early Tertiary times, a volcano burst forth in 

 the northeast corner of the basin not far from the junction of the Ab 

 saroka and Snowy ranges. It rises from a base about 6,500 feet above 

 sea level, the culminating peak attaining an elevation of 10,000 feet. 

 This gives a height to the volcano of 3,600 feet from base to summit. 

 measuring from the Archean rocks of the Yellowstone Valley to the 

 toj) of Mount Washburne. The average height of the crater rim is 

 about 9,000 feet above sea level, the volcano measuring IT) miles across 

 the base. The eruptive origin of Mount Washburne has long been 



recognized, and it is frequently referred toasavolcano. [tis, however, 



simply the highest peak among Several others, and represents a later 

 outburst which destroyed in a measure the original rim and form of an 

 older crater. The eruptions for the most pari were basic andesites and 

 basalts. Erosion has so worn away the earlier rocks, and enormous 

 masses Of more recent lavas have so obscured the original form of lava 

 Bows, thai it is not easyfor an inexperienced eye to recognize a volcano, 

 and that the surrounding peaks are the more elevated points in a 

 grand crater wall. By following around the ancient andesitic rim, and 



