We may, perhaps, mark these features in all the rivers of the earth. A knowledge of them is essential to understand thoroughly the natural development of a river system in its true parts; unfortunately, they have as yet been too little observed and described. Among European rivers they are found in the Guadiana, at the Saltos de Lobo; in the Douro, at the rapids below Torre de Moncorvo; in the Ebro, at Sastago, below Saragossa; in the Rhone, the rapids below Lyons, between the granite banks of Pierre Encise; in Loire, by Iguerando, below Roanne; in the Rhine, below Strasbourg, and at the narrows at Bingen, near St. Goar and Andernach; in the Weser, at the Porta Westphalica; in the Danube, at Grein, at Kloster Newburg, and at Yachtali, Drenir Kapi, (Iron Gate,) and Orsova; and in the Dnieper, the thirteen waterfalls below Yekaterinslav. The same features are repeated in all the other streams of Europe and the remaining continents. More close investigation of them will lead to important results, concerning the structure of the earth in the regions intermediate between plateaus and lowlands. As a high grade, great cataracts, sharp and bold cliffs, and mountain lakes characterize the upper course of streams, so rapids, dry lake basins, and a meandering channel characterize their middle course. Below the lowest rapids are found the level plains or lowlands which give rivers their third characteristic. Lower Course. As soon as the rivers break through the lowest range of hills which once beset their course, they deposit the debris which they bear with them, and begin the formation of diluvial plains. We find in the soil of all level places along the middle, as well as the lower course of rivers, traces of the same kinds of rock and minerals, which characterize the mountains where they rise. The rate of fall in the lower course of rivers is so slight as to be almost imperceptible. Relatively, it is the greatest in the Volga; from Kamishin to the Caspian the descent is more than 150 feet, although the distance is less than 500 miles. The Senegal, from Podor to the sea, a distance of about 200 miles, falls only about 2½ feet; the Amazon, within about the same distance, falls only 10½ feet, or about an inch to the mile. In such rivers, therefore, the tide can flow a very long way inland.