GEOGRAPHICAL DEVELOPMENT OF THE AMERICAS 127 



thus formed and a way of migration opened, the marine fauna 

 of CaUfornia and Japan became closely similar. 



In the interior, the Pliocene continental formations and 

 faunas followed so gradually upon those of the Miocene, that 

 there is great doubt as to where the line between them should 

 be drawn. These interior formations are mostly of small 

 extent and are very widely scattered, and much remains to 

 be learned regarding the mammals of the epoch. In northern 

 Kansas are the Republican River beds, which are so doubtfully 

 Pliocene, that they may almost equally well be called upper- 

 most Miocene. Other lower Pliocene stages, representing 

 various divisions of time, are the Alachua of northern Florida, 

 the Snake Creek of western Nebraska, the Thousand Creek and 

 Virgin Valley of northwestern Nevada and the Rattlesnake 

 of Oregon. Probably middle PUocene is the Blanco of north- 

 western Texas, a valley cut in the middle and lower Miocene 

 rocks and filled in with PUocene deposits. Possibly upper 

 PUocene, or, it may be, lowest Pleistocene, are the Peace 

 Creek of southwestern Florida and the so-called "Loup River" 

 (not Loup Fork) of western Nebraska. 



The volcanic activity of the Rocky Mountain and Pacific 

 coast regions, which was so remarkable in the Miocene, con- 

 tinued into and perhaps through the PUocene. The great 

 outflow of Ught-coloured lava which built up the central 

 plateau of the YeUowstone Park is referred to the PUocene, 

 and some of the enormous fissure-eruptions which formed the 

 vast Columbia River fields of black basaltic lava were probably 

 PUocene, as some were demonstrably Miocene. Both of these 

 epochs were remarkable for volcanic activity in the western 

 part of the continent. 



The PUocene cUmate, as may be inferred from the plants 

 and marine sheUs, was colder than that of the Miocene, and 

 refrigeration was progressive, as is shown by the proportion of 

 Arctic shells in the PUocene beds of the east coast of England, 

 rising from 5 per cent in the oldest to more than 60 per cent 



