Whenever a considerable mountain chain is formed, although the actual folding of the beds is limited to the usually narrow field occupied by these disturbances, the elevation takes place over a wide belt of country on one or both sides of the range. Thus if we approach the Rocky Mountains from the Mississippi Valley, we begin to mount up an inclined plane from the time we pass westward from the Mississippi River. The beds of rock as well as the surface rises gradually until at the foot of the mountain; though the rocks are still without foldings, they are at a height of four or five thousand feet above the sea. It seems probable--indeed, we may say almost certain--that when the crust is broken, as it is in mountain-building, by extensive folds and faults, the matter which lies a few score miles below the crust creeps in toward those fractures, and so lifts up the country on which they lie. When we examine the forms of any of our continents, we find that these elevated portions of the earth's crust appear to be made up of mountains and the table-lands which fringe those elevations. There is not, as some of our writers suppose, two different kinds of elevation in our great lands--the continents and the mountains which they bear--but one process of elevation by which the foldings and the massive uplifts which constitute the table-lands are simultaneously and by one process formed. Looking upon continents as the result of mountain growth, we may say that here and there on the earth's crust these dislocations have occurred in such association and of such magnitude that great areas have been uplifted above the plain of the sea. In general, we find these groups of elevations so arranged that they produce the triangular form which is characteristic of the great lands. It will be observed, for instance, that the form of North America is in general determined by the position of the Appalachian and Cordilleran systems on its eastern and western margins, though there are a number of smaller chains, such as the Laurentians in Canada and the ice-covered mountains of Greenland, which have a measure of influence in fixing its shore lines. [Illustration: _Waterfall near Gadsden, Alabama. The upper shelf of rock is a hard sandstone, the lower beds are soft shale. The conditions are those of most waterfalls, such as Niagara._]