MIOCENE IGNEOUS ROCKS 107 



ore bodies have never proved lucrative, owing to their uncertainty and 

 lack of continuity. Similar ore bodies in the mining regions of Montana 

 and Colorado have been described by Emmons, Lindgren, Weed, Kemp, 

 and others. 



Another feature of these intrusive rocks of the Absarokas is seen in 

 the narrow rifts and shrinkage cracks filled with quartz by the ascending 

 currents from deep-seated sources. In like manner the cavities and 

 druses found in the petrified trees of the fossil forests are lined with 

 quartz crystals, due to heated siliceous waters coming up from below. 

 Today there are no hot springs or steam vents to be found in the Absa- 

 rokas, save in a feeble way on the western flanks, where the ancient 

 breccias have been penetrated by much later rhyolites. 



It has seemed necessary to present this somewhat lengthy description 

 of volcanic forces existing in Miocene time in order to bring out in 

 strong contrast the conditions prevailing during Pliocene and recent 

 times. 



Pliocene Igneous Eocks and thermal Waters 



After the pouring out of the basic breccias and lavas of the Miocene, 

 volcanic energy, for a time at least, ceased. Atmospheric agencies re- 

 moved a large body of the surface rocks and carved out drainage channels 

 in the easily disintegrated material. Following a prolonged interval of 

 comparative rest came renewed activity, with marked changes in the 

 nature of the eruptive lavas. Vast masses of rhyolite were extruded, not 

 on preexisting mountains, but over an inclosed basin, converting it into 

 a rugged tableland and submerging the flanks of the bordering ranges. 

 This sharply defined region has been designated as the Park Plateau. It 

 embraces a tract of country 50 by 40 miles, including approximately 

 2,000 square miles. Strictly speaking, it is not a plateau in the general 

 acceptation of the word, but presents a broken surface accentuated by 

 bold escarpments and abrupt slopes of lava flows. While the topography 

 of the tableland has, to some extent, been modified since Pleistocene time 

 and trenched by ice action, giving the effect of individual plateau blocks, 

 the mass can not be considered otherwise than as a geological unit. The- 

 earliest rhyolitic eruptions spread over a very uneven surface, the struc- 

 tural features of which may be fairly well inferred from exposures of 

 sedimentary rocks rising through the surrounding lavas or cropping out 

 from beneath the outer boundaries of the plateau. The rhyolite also lies 

 unconformably on the eroded surfaces of Miocene basic breccias, and not 

 infrequently occupies the older valley bottoms, clearly showing the much 

 later age of the siliceous lavas. Although sharply defined by topographic 

 relief and geological sequence, both periods of ejection are still more 



