556 



SCIENCE 



[N. S. Vol. XXXIII. No. 850 



places from one end of the range to the 

 other by deposits laid down from ascend- 

 ing igneous emanations in the form of 

 aqueous and gaseous vapors charged with 

 mineral matter. Such deposits consist es- 

 sentially 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 deposited after the crystalline in- 

 trusives 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 deposi- 

 tion they were much farther below the 

 surface than they are found to-day. Min- 

 ing companies have exploited the ores by 

 shifts and tunnels, but so far as I know, 

 such ore bodies have never proved lucra- 

 tive, owing to their uncertainty and lack of 

 continuity. Similar ore bodies in the min- 

 ing 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. To-day there are no hot 

 springs or steam vents to be found in the 

 Absarokas, save in a feeble way on the 

 western flanks, where the ancient breccias 

 have been penetrated by much later rhyo- 

 lites. 



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 ROCKS AND THERMAL 

 WATERS 



After the pouring out of the basic brec- 

 cias and lavas of the Miocene, volcanic 

 energy, for a time at least, ceased. At- 

 mospheric agencies removed a large body 

 of the surface rocks and carved out drain- 

 age channels in the easily disintegrated 

 material. Following a prolonged interval 

 of comparative rest came renewed activ- 

 ity, with marked changes in the nature of 

 the eruptive lavas. Vast masses of rhyo- 

 lite were extruded, not upon preexisting 

 mountains, but over an enclosed basin, con- 

 verting it into a rugged tableland and sub- 

 merging the flanks of the bordering ranges. 

 This sharply defined region has been desig- 

 nated as the Park Plateau. It embraces a 

 tract of country fifty by forty miles, in- 

 cluding approximately 2,000 square miles. 

 Strictly speaking, it is not a plateau in the 

 general acceptation of the word, but pre- 

 sents a broken surface accentuated by bold 

 escarpments and abrupt slopes of lava 

 flows. While the topography of the table- 

 land 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 con- 

 sidered otherwise than as a geological unit. 

 The earliest rhyolitic eruptions spread 

 over a very uneven surface, the structural 

 features of which may be fairly well in- 

 ferred from exposures of sedimentary rocks 

 rising through the surrounding lavas or 

 cropping out from beneath the outer boun- 

 daries of the plateau. The rhyolite also 

 lies unconformably upon the eroded sur- 

 faces of basic breccias, and not infre- 

 quently occupies the older valley bottoms, 

 clearly showing the much later age of the 

 siliceous lavas. Although sharply defined 

 by topographic relief and geological se- 

 quence, both periods of ejection are still 

 more strongly contrasted by marked dif- 



