Apkil 14, 1911] 



SCIENCE 



555 



mean proportions were forced upward. 

 Evidence is wanting to show that any of 

 these magmas in their upward movement 

 ever penetrated to the surface; apparently 

 they came to a standstill far below a cov- 

 ering of overlying sediments whose thick- 

 ness must, for the present at least, remain 

 a matter of conjecture. Excessive erosion 

 since early Eocene time has laid bare these 

 massive batholithic forms which now 

 stand out as dominant features in the 

 landscape. 



There is no evidence to warrant the opin- 

 ion that these porphyries and crystalline 

 rocks were ever connected with vents dis- 

 charging lavas, though there is, beyond the 

 boundaries of the Gallatin Range, extrava- 

 sated material of Eocene age covering 

 large tracts of country. In the northeast 

 comer of the park such surface flows are 

 well developed in the accumulation of silts 

 and ashes. Much of this material was laid 

 down under relatively quiet conditions. 

 Apparently they are much later than the 

 crystalline rocks already referred to, but 

 their age is determined by a characteristic 

 flora corresponding with the well-known 

 Fort Union beds of Montana of Eocene 

 age. 



In these extravasated lavas the influence 

 of volcanic waters may be recognized in 

 many ways, but degradation of the mass 

 has been so great that evidence of individ- 

 ual extinct hot springs is no longer trace- 

 able; moreover, it would seem impossible 

 to distinguish them from those belonging 

 to Miocene eruptions. 



MIOCENE IGNEOUS ROCKS AND THERMAL 

 "WATERS 



The Absaroka Range 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 

 eighty miles in a north-and-south direction 

 and measures nearly fifty miles in width. 

 In strong contrast 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 intrusions, ac- 

 companied by innumerable dikes and siUs, 

 offshoots from the parent stock. All this 

 was the result of long-continued, pro- 

 tracted energy, as clearly shown both by 

 geological processes and the many succes- 

 sive fossil forests. These flourished through 

 thousands of feet of eruptive material and 

 were alternately kiUed by hot fragmen- 

 tary lavas and preserved by renewed 

 streams of muds and ashes.- The luxuriant 

 vegetation which developed throughout 

 this period is regarded by all paleobotan- 

 ists as of Miocene age. All volcanic activ- 

 ity long since ceased. 



What concerns us most at the present 

 time is the influence of thermal waters de- 

 rived from deep-seated sub-erustal sources 

 upon 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 sur- 

 face 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 sub-erustal waters is shown in many 



' Arnold Hague, " Early Tertiary Volcanoes of 

 the Absaroka Eange," presidential address, Geol. 

 Soe. of Wash., 1899, Science, March 24, 1899. 



