of plateaus; or, less commonly, they are built up by lava-flows. After they come into existence, plateaus are subject to the same changes as plains. Mountains have originated in various ways: by diastrophism, as in the case of mountains formed by the upfolding of the superficial strata of the earth's crust or by the up-thrust of blocks of the earth's crust; by volcanoes, as in the case of volcanic peaks; and by gradation. The origin of mountains by gradation will be referred to later Like plains and plateaus mountains are modified by wind and water and air after they are formed. Physiography essays to explain how they originated and how far their present forms are the result of original uplift or upbuilding and how far the result of subsequent gradation.

Minor Relief Features The great relief forms — plains, plateaus and mountains — are affected by numerous smaller relief features. Thus, a plain or a plateau may be affected by depressions (valleys) cut out by streams. Between the valleys remain elevations. If the elevations be long and narrow, they are ridges; if short, hills. The valleys are made; the ridges and hills are left. As a result of the excavation of valleys, plains may be far from flat. Generally speaking, the valleys are deep in proportion as the land is high. The valleys of plateaus, on the whole, are deeper than those of plains, and the ridges and hills between them are therefore higher. If the ridges or hills be very high, they may be called mountains. For example, the Catskills are simply the big hills left in the erosion of the plateau which once existed where the mountains now are. The depressions between the mountainous hills have been worn out by running water. It is thus that mountains originate as the result of gradational processes operating on plateaus. Special names are given to special forms of hills or mountains, developed by erosion. Here belong buttes, large, flat-topped, high hills developed by erosion in arid regions; mesas, which are more extensive remnants of plateaus; etc.

Along valley-bottoms whether the valleys are in plains, plateaus or mountains, narrow alluvial plains are often developed. So, too, about the borders of lakes, whether in plains, plateaus or mountains, low, flat lacustrine plains of limited extent are made, either as the result of deposition in the lake or as the result of the lowering of its level or as the result of both. Lacustrine and alluvial plains are minor, secondary features in relief forms of the first order. Small plains are also developed in other ways. Along streams and lakes or along the ocean terraces are often developed. They represent other minor topographic forms developed in plains, plateaus or mountains.

Between highlands and lowlands there are

always slopes. The slopes between mountains and plateaus are parts of the mountains and plateaus, respectively. So, too the slopes between the surface of a plateau and the bottom of a valley cut in it are parts both of the plateau and of the valley. Yet slopes constitute a somewhat distinct class of topographic forms. If slopes are very steep, and especially if they are steep and high, they are called cliffs Cliffs occur along sea and lake shores as well as along valleys. It is within the province of physiography to explain how these secondary and minor features, as alluvial plains, lacustrine plains terraces, slopes, cliffs etc. arose and how their individual peculiarities of form were developed.

Another topographic form is the basin. Basins, like most topographic types, are of various sorts. Some are large, some small. They may affect the surfaces of plains or of plateaus, or they may occur among mountains. Some have outlets, and some have not. Those which have not may give rise to lakes, ponds etc. Ponds and lakes may have outlets, but below the level of the outlet is a basin which has no outlet, else there would be no lake or pond. Other so-called basins are enclosed on three sides and open on the fourth, or at least at some one point. It is customary to speak of the great depression between the Appalachian Mountains on the east, the Rockies on the west and the Height of Land on the north as the Mississippi basin, although it is by no means completely inclosed by high land. It is the function of physiography to define the various types of basins and to explain their modes of origin and their individual characteristics.

Configuration of the Sea-Bottom The configuration of the sea-bottom also falls within the province of physiography; and, while the sea-bottom is less familiar than the land, its general features are known. It is known, for example, that there are great areas of the sea-bottom elevated above their surroundings. Such areas are comparable, in some sense, to the plateaus on land. There are great areas where the ocean-bed is depressed, relatively to the areas just referred to. Such areas are comparable to the plains of the land. There are other depressions in the ocean-bed, more limited in area, which are comparable to basins on the land, though some of them are much larger, both in area and depth, than the inclosed basins on the land. Many parts of the ocean-basin have been affected by vul-canism. Volcanic peaks are, on the whole, more notable features of the ocean-bottom than of the land. In its fundamental features or features of the first order, therefore, the ocean-bed has some likeness to the land. But gradational agencies, especially degradational agencies, are hardly operative