The different geological formations found in mountain districts have a very important influence in determining the direction of rivers. Mountains do not generally consist of rocks of one kind of structure, but of several. What stratification is to mountains whose geological formation is the same throughout, the superposition of different kinds of rocks is to those of composite materials. The layers may be divided into superior, inferior, and adjacent. These usually vary in respect to age, and may be traced in a regular geological seniority, as for example sandstone, gypsum, limestone, gray-wacke, and granite. These formations are either closely contiguous, or are separated merely by valleys, as for instance in the Carpathian chain, where the central granite knot is separated by valley plains from the more southern limestone chain; an example of contiguity is found on the west spur of the same Carpathian range. Wherever mountain systems of varied geological structure approach each other very closely, rivers seldom break their way through either one, but find their way along the roots of the mountains, till at last they come to a less confined place. Such river courses are often very large and deep; for the mountain streams which meet and are hemmed in by the narrow pass between the two contiguous ranges sweep all loose obstructions before them, and not only leave their path clear, but continually deepen it. We find this in the Ural, the Isère, the Rhone, Aar, Inn, in all the long and winding Alpine valleys, and in the Ebro, fed by the parallel ridges of the Pyrenees. The circle of rivers which girds the central Carpathian knot is an illustration of what was said a moment since. The Poprad, Dunayic, Arva, and Waag are found where the true Carpathian chain, which is granitic, is closely contiguous to subordinate ranges of limestone and gray-wacke. In any accurate map, the long, winding course between these two chains may be easily traced. Looking at the point where the Hartz Mountains and the Thuringian ridge touch at their roots, the groups are seen to be insulated, as it were, by the rivers which gird their bases. In the great streams of southern and southwestern Asia, too, the line of the water-courses can be traced along the narrow valleys which separate main from subordinate mountain chains; the Terek, Kooban, Koor, Aras, Euphrates, Tigris, Indus, Ganges, and probably the Chinese rivers, are all examples of this. Some streams seem to be entirely independent of all these laws in forming their channels, and to have their direction assigned to them by the freaks of nature, such, for instance, as fissures in mountain chains and clefts, which remain to indicate ancient convulsions.