This region of essentially level plateaus, extending as it does from the hot lands of eastern Mexico nearly to the arctic circle, presents great diversity of climate and also well-marked variations in the secondary features of its relief. Of necessity it needs to be subdivided for more detailed study. The rivers flowing eastward from the Rocky Mountains have excavated valleys in the plateau region, and may be used as a basis for its subdivision. This has been done by J. W. Powell for the portion within the borders of the United States, and the terms Pecos plateau, Arkansas plateau, Platte plateau, and Missouri plateau have been proposed; this category may be extended especially to the northward, so as to include the less well-known Saskatchewan, Athabasca, Peace, and Laird plateaus. Each of these divisions is in reality a group of plateaus, for the reason that the broad areas between the eastward-flowing rivers are trenched by lateral stream channels tributary to the main waterways, and thus subdivided into smaller units. This subdivision of the plateau region by stream channels leaving flat-topped areas between them makes one instructive geographical process prominent--that is, the great table-land has been dissected. The depths of the channels cut across it depend mainly on the elevation of the land and the distance the streams have to travel to reach the sea; but modifying conditions are furnished by the degree of resistance the rocks offer to erosion, the amount of precipitation, etc. If the elevation is great, the stream can cut deeply, and leave bold secondary plateaus between them; if the distance to the sea is short, other conditions being the same, the streams can cut more deeply than when their courses are long; if the rocks are resistant, they are left in bold escarpment bordering the valleys and the margins of the secondary plateaus are well defined, but if they are soft and crumble easily, their _debris_ is washed and blown into the rivers, and a general lowering of the surface without the formation of deep trenches is the result. These and still other conditions have influenced the manner in which the Great plateaus have been dissected, and are of necessity to be considered in a critical discussion of the history of the land as recorded in its relief.