MIOCENE 513 



ness of these beds is from 3000 to 4000 feet, and they are largely 

 composed of stratified volcanic ashes and tuffs, which were show- 

 ered into the lake by vents not very far away, probably in the 

 Cascades. A second and much smaller lake of this age filled the 

 central valley of Montana. The John Day age must have followed 

 the White River after an interval which, geologically speaking, was 

 quite short, and it seems arbitrary to refer the two stages to 

 different epochs. 



After the John Day lakes had been obliterated, no deposits were 

 made in the interior region for a considerable period, and, so far 

 as is yet known, the Middle Miocene is not represented in that 

 region, but in the Upper Miocene these bodies of water were rees- 

 tablished on a greater scale than ever. The Loup Fork stage is 

 composed of two substages, one of which is quite distinctly older 

 than the other, and has a much more restricted range. This first 

 substage, the Deep River, was formed in a lake, or several of them, 

 which filled the mountain basins of central and southern Montana, 

 north of the Yellowstone Park. The second substage, or Nebraska, 

 is the most widely distributed of all the fresh-water Tertiary de- 

 posits of the interior, and is partly of lake, partly of river origin. 

 The beds extend from South Dakota far into Mexico. Another 

 area of these beds is in eastern Oregon, where they unconform- 

 ably overlie the John Day. The Loup Fork is much thinner than 

 the John Day and White River, and its rocks, sands, gritSj and 

 marls are scarcely^ at all indurated, not forming bad lands. 



Besides these deposits whose geological position is established 

 by the numerous fossils which they contain, there are several others 

 which are referred to the Miocene, though with much doubt. One 

 such lake occupied the upper Sacramento valley in California, and 

 stretched far to the northeast through the narrow strait between 

 the Sierra and the Coast Range; another was in western Nevada. 

 Others again occupied valleys in the interior of British Columbia. 



The Miocene was a time of great volcanic activity among the 

 mountain ranges of the Pacific coast, and along the main range of 

 the Rocky Mountains ; the great volcanoes of Mexico and of the 

 Cascade Mountains are believed to date from this epoch. The 



