106 a. hague thermal waters in yellowstone park 



Miocene Igneous Eocks and thermal Waters 



The Absaroka Eange shuts in the Park Plateau along its eastern 

 border. Strictly speaking, it is not a mountain range, but rather a 

 rugged, deeply dissected tableland, rising from 3,000 to 4,000 feet above 

 the general level of the park. It stretches for 80 miles in a north and 

 south direction and measures nearly 50 miles in width. In strong con- 

 trast to the Eocene igneous rocks, this elevated tableland was steadily 

 built up by tumultuous accumulations of breccias, agglomerates, silts, 

 and muds, the products of violent explosive action through numerous 

 conduits from sources now concealed beneath the overlying load. Nearly 

 all phenomena of ejected lavas seen in extinct volcanic areas elsewhere may 

 be observed here. Finally the mass was penetrated by batholithic intru- 

 sions, accompanied by innumerable dikes and sills, offshoots from the 

 parent stock. All this was the result of long-continued, protracted 

 energy, as clearly shown both by geological processes and the many suc- 

 cessive fossil forests. These flourished through thousands of feet of 

 eruptive material, and were alternately killed by hot fragmentary lavas 

 and preserved by renewed streams of muds and ashes.^ The luxuriant 

 vegetation which developed throughout this period is regarded by all 

 paleobotanists as of Miocene age. All volcanic activity long since ceased. 



What concerns us most at the present time is the influence of thermal 

 waters, derived from deep-seated subcrustal sources on both the volcanic 

 ejectamenta and crystalline intrusives. The action of these heated 

 waters may be observed equally well on what were surface flows and on 

 the deeply buried intrusive masses. Such surface action may be detected 

 at a number of localities by the presence of alteration products and 

 traces of sediments, although in most cases the latter have been removed 

 by running water. Underground action of subcrustal waters is shown in 

 many places from one end of the range to the other by deposits laid 

 down from ascending igneous emanations in the form of aqueous and 

 gaseous vapors charged with mineral matter. Such deposits consist 

 essentially of quartz, galena, and copper minerals carrying both gold 

 and silver. They lie as contact products along the apophyses of the 

 massive intrusions and never occur far away from them. They were de- 

 posited after the crystalline intrusives came to a state of rest, but prob- 

 ably long before they were chilled. It may not be necessary to add, but it 

 should be borne in mind, that at the time of deposition they were much 

 farther below the surface than they are found today. Mining companies 

 have exploited the ores by shifts and tunnels, but so far as I know such 



2 Arnold Hap^e : Early Tertiary volcanoes of the Absaroka Range. Presidential ad- 

 dress, Geological Society of Washington, 1899. Science, . 



