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 uppermost 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 Pliocene is the _Blanco_ of northwestern Texas, a valley cut in the middle and lower Miocene rocks and filled in with Pliocene deposits. Possibly upper Pliocene, 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, continued into and perhaps through the Pliocene. The great outflow of light-coloured lava which built up the central plateau of the Yellowstone Park is referred to the Pliocene, and some of the enormous fissure-eruptions which formed the vast Columbia River fields of black basaltic lava were probably Pliocene, as some were demonstrably Miocene. Both of these epochs were remarkable for volcanic activity in the western part of the continent. The Pliocene climate, as may be inferred from the plants and marine shells, was colder than that of the Miocene, and refrigeration was progressive, as is shown by the proportion of Arctic shells in the Pliocene beds of the east coast of England, rising from 5 per cent in the oldest to more than 60 per cent in the latest beds. In the Arctic regions the cold must have been severe, at least during the latter half of the epoch, for in the succeeding Pleistocene we find an Arctic fauna already fully adapted to the extreme severity of present day polar conditions and time was necessary for such an adaptation. In the western interior the climate was not only colder, but also drier than it had been in the Miocene, the desiccation which had begun in the latter epoch becoming progressively more and more marked.