GEOGRAPHICAL DEVELOPMENT OF THE AMERICAS 121 



Deep River (or Smith River) stage, but occurs also in numerous 

 small, scattered and widely separated areas in Oregon, 

 Wyoming, Colorado and Texas, with local names in these 

 different states. It is most Ukely that these middle Miocene 

 formations are not strictly contemporaneous in the geological 

 sense, but rather form a closely connected and successive series. 

 The mammals of the Deep River stage leave no doubt that the 

 way of migration from the Old World was again open. 



The Lowp Fork, or upper Miocene, itself susceptible of 

 further subdivision, is by far the most extensive of the Miocene 

 formations and covers much of the Great Mains region, in 

 separate areas, from South Dakota far into Mexico. Perhaps 

 also referable to the upper Miocene is a small, but very inter- 

 esting formation, the Florissant, which is in the South Park 

 of Colorado ; it was made by very fine volcanic material 

 showered into a small and shallow lake. The finely laminated 

 papery shales of the Florissant have preserved countless 

 plants and insects and many fishes, and these throw very 

 welcome light upon the vegetation and cUmatic conditions 

 of the epoch and afford an interesting contrast to the fauna 

 and flora of the Green River shales of the lower or middle 

 Eocene. That the Florissant shales are Miocene, no one 

 questions, but' their isolated position and the fact that they 

 have yielded no mammals make it somewhat doubtful whether 

 they belong in the middle or later part of the epoch. 



In the western portion of the continent vulcanism was dis- 

 played on a grand scale during the Miocene. Mention has 

 already been made of the quantity of volcanic material in the 

 marine Miocene of California and also in the lavas and tuffs 

 of the Sierras. The magnificent cones, such as Mts. Hood 

 and Tacoma, which are the glory of the Cascades, are beheved 

 to date from this time. In Idaho and eastern Oregon and 

 Washington are the immense lava-fields of the Columbia 

 River, which are, partly at least, of Miocene date and were 

 chiefly extruded through great fissures, the lava flooding the 



