Again, the vast table-lands of the Colorado region illustrate on a truly magnificent scale, to which there is no parallel in the Old World, the effects of atmospheric erosion on undisturbed and nearly level strata. Here we find valleys and river gorges deeper and longer than any others in the world; great winding lines of escarpment, like ranges of sea cliffs; terraced slopes rising at various levels; huge buttresses and solitary monuments, standing like islands out of the plains; and lastly, great mountain masses carved out into the most striking and picturesque shapes, yet with their lines of "bedding" clearly marked out. On the other hand, where, as is almost always the case in mountain-ranges, the stratified rocks have been folded, crumpled, twisted, and fractured by great "faults," we find a very different result. In these cases the rocks have generally been very much altered by the action of heat. For here we find crystalline schists, gneiss, granite, and other rocks in the formation of which heat has played an important part; and very often the igneous rocks have forced their way through those of sedimentary origin and altered them into what are called metamorphic rocks (see chapter v., page 156). Thus they have lost much of their original character and structure. The repeated uplifts and subsidences of the earth's crust, by which the continents of the world have been raised up out of the sea to form dry land, have, broadly speaking, thrown the rocky strata into a series of wave-like undulations. In some extensive regions these undulations are so broad and low that the curvature is quite imperceptible, and the strata appear to lie in horizontal layers, or to slope very slightly in a certain direction. This is, in a general way, the position of the strata of which plains and plateaux are composed. But in the longer and comparatively narrow mountain regions that traverse each of the great continents, forming, as it were, backbones to them, the undulations are very much more frequent, narrower, and higher. Sometimes the rocks have been thrown into huge open waves, or the folds are closely crowded together, so that the strata stand on their ends, or are even completely overturned, and thus their proper order of succession is reversed, and the older ones actually lie on the top of the newer ones. As we approach a great mountain-chain we observe many minor ridges and smaller chains running roughly parallel with it, and, as it were, foreshadowing the great folds met with in the centre of the chain and among its highest peaks. These small folds become sharper and closer the nearer we get to the main chain, and evidently were formed by the same movements that uplifted the higher ranges beyond; but the force was not so great. Thus we find the great Alpine chain flanked to the north by the smaller ranges of the Jura Mountains; and on the south, side of the Himalayas we find similar smaller ranges of hills.