140 GEOLOGICAL HISTORY OF THE YELLOWSTONE PARK. 



The rock from Madison Plateau was collected on the north side of 

 Madison Canyon and was selected as a typical rock coYerin<>' large areas 

 of the Park. It is purplish-gray in color, rough in texture, porphy- 

 ritic in structure, and characterized by well-developed sanidin and 

 quartz. The obsidian, from Obsidian Cliif, is an excellent example of 

 pure volcanic glass, wholly devoid of porphyritic crystals. In general 

 the investigations of the laboratory confirm the observations of the 

 field geologist, that the differences exhibited by the volcanic i)roduct are 

 not of cliemical or mineral composition, but rather of physical con- 

 ditions under which the magma has cooled. 



I have dwelt suinewhat in detail upon tlie nature of these rocks for 

 two reasons: First, because of the dififtculty met with by the scientific 

 traveller in recognizing the uniformity and simplicity of chemical com- 

 position of the rhyolite magma over the entire i)lateau, owing to its 

 great diversity in superficial habit; second, on account of their geolog- 

 ical importance in connection with the unrivalled display of the gey- 

 sers and hot springs. Tliat the energy of tlie steam and thermal 

 waters dates well back into the period of volcanic action, there is in 

 my opinion very little reason to doubt. As the energy of this under- 

 ground heat is to day one of the most impressive features of the 

 country, I will defer commenting upon the geysers and hot springs 

 until speaking of the present condition of the Park. 



Although the rhyolite ernptions were probably of long duration and 

 died out slowly, there is, I think, evidence to show that they occupied 

 a clearly and sharply defined period between the andesites and basalt 

 eruptions. Sin(;e the outpouring of this enormous body of rhyolite 

 and building up of tlie plateau the region has undergone profound 

 faulting and displacement, lifting up bodily immense blocks of lava 

 and modifying the surface features of the country. Following the rhy- 

 olite came the period of basalt eruptions, which, in comparison with 

 the andesite and rhyolite eras, w^as, so far as the Park was con- 

 cerned, insignificant, both as regards the area covered by the basalt 

 and its influence in modifying the physical aspect of the region. The 

 basalt occurs as thin sheets overlying the rhyolite and in some 

 instances as dikes cutting the more acidic rocks. It has broken out 

 near the outer edge of the rhyolite body and occurs most frequently 

 along the Yellowstone Valley, along the western foothills of the Gal- 

 latin Eange and Madison Plateau, and again to the southward of the 

 Falls River basin. 



After the greater part of the basalt had been poured out came the 

 ghicial ice, which widened and deepened the pre-existing drainage 

 channels, cut profound gorges through the rhyolite lavas and 'modelled 

 tlie two volcanos into their present form. Over the greater part of the 

 Cordillera of the central and northern Rocky Mountains wherever the 

 peaks attain a suCQcieutly high altitude to attract the moisture-laden 

 clouds evidences of the former existence of local glaciers are to be 



