MIOCENE LAKES IX THE CORDILLERAS. 117 



climatic changes, and a new sheet of water resulted ; this was situated wholly 

 within the boundaries of the former Ute Lake, and it was succeeded hy still 

 another, each being characterized by its special Fauna. Of the precise limits 

 of these bodies of water little has as yet been made out ; hut the deposits 

 laid down beneath their surfaces contain an abundance of the most interest- 

 ing vertebrate remains, all considered by Professor Marsh to belong to the 

 Eocene. At the close of this period great orographic disturbances took 

 place over the Cordilleras, which resulted in the drainage or extinction of 

 the Eocene lakes, over whose surface no Miocene deposits are found, indicat- 

 ing an important palaeontological break at this period, and great changes in 

 the relative area of land and water throughout this portion of the country. 



The orogi'aphic disturbances which marked the extinction of the Eocene 

 lakes, as indicated above, gave rise to new areas of water, into which detrital 

 materials were carried in abundance, forming deposits of great thickness and 

 extent, replete with interesting vertebrate remains The Miocene lacustrine 

 areas seem to have been two in number. One of these was of vast extent, 

 and was situated to the east of the Rocky Mountains. Here, on the de- 

 pressed and eroded surface of the underlying Cretaceous strata, over the 

 region now known as" the Plains," an immense sheet of water existed during 

 the Miocene period, including the White River lake-basin of Hayden, which 

 extended along the eastern slope of the range, from the parallel of 40^ far to 

 the northeast, beyond the limits of the United States into British Columbia. 

 The other lake was situated to the west of the llTth meridian and east of 

 the Cascade Range, in Central Oregon. 



The deposits of the Miocene period in the Eastern basin have a thickness 

 of a thousand feet or more. On White River, where they have been eroded 

 into the most curious and fantastic forms, they give rise to that peculiar 

 type of country known as the Bad Lands (Mauvaises Terres). Here, also, 

 the strata are crowded with interesting fossils, all of which are the re- 

 mains of animals or plants living on land, or in and near fresh water, 

 and which have been described and figured by Leidy and Marsh. The 

 sediments of the Oregon Lake were of great thickness, equalling at least 

 4,000 feet, and being made up in large part of volcanic materials either 

 ejected from under the siu-face of the water or washed into it from the 

 surrounding volcanic summits. The abundant fossils foiuid in this region 

 also indicate the absence of the ocean, and the Miocene character of the 

 Fauna. 



