on the black and rugged surface of a vast current of lava, which seems unbroken, and which we can trace nearly up to the distant summit of that majestic cone which Pindar called the pillar of heaven.'"' This ravine, therefore, has been eroded by the stream in the course of about two centuries; but, as Lyell observes, it has not yet cut down to the ancient bed of which it was dispossessed, and the probable position of which he indicates in a diagram. 2. Valleys of Auvergne.-What has happened once in the case of the Simeto has happened several times in many of the valleys of Central France. These valleys have been excavated to a great depth through strata of fresh-water origin, and have been flooded from time to time by streams a b Fig. 36. Diagram Section across Valley of Chanonat, Auvergne. a, the Gergovia platform. b, the La Serre platform. d, the Chanonat gorge. of liquid lava from the neighbouring heights. Each successive lava-flow must necessarily have occupied the lowest levels to which it had access; but they now form terraces or platforms at various heights, from 1,500 feet downwards to the level of the present watercourses. These valleys were originally described by Mr. Poulett Scrope, who says:2 "Take, for instance, the two basaltic platforms of Gergovia and La Serre, and the bed of basalt which occupies the bottom of the intervening valley of Chanonat. Here are three long sheets of lava, each of which, by its gradual inclination in the same direction, and by the remains of the volcanic vent from which it was erupted, is proved to 1 "Principles of Geology," tenth edition, vol. i. p. 358. 2 "The Volcanoes of Auvergne," 1825, and "Geol. Mag." vol. iii p. 195. L have flowed down the valley." The platform of La Serre is everywhere from 300 to 500 feet below that of Gergovia, and the valley must have been deepened by that depth during the interval between the two outflows. Again, by the same reasoning, the valley of Chanonat must have been entirely excavated between the epoch of the La Serre lava and the production of the more recent bed of basalt, 500 feet below. Finally, the Chanonat rivulet has worn a new channel, from 20 to 50 feet below the level of the pebble bed on which the most recent of the three lava currents reposes (see fig. 36). From this and numerous other instances, he justly concludes that "the erosive force of the streams which now border or intersect these lava currents, together with the action of rain, frost, and other atmospheric forces, have alone hollowed out the extensive systems of valleys" which are found in the province of Auvergne. Floods doubtless occurred from time to time, and materially assisted in the erosion; but it is very clear that these valleys do not owe their origin to any one unusual or cataclysmal flood, or to any sudden disruption of the rocks. Sir A. Geikie has so excellently described the impression which the valleys of Auvergne make upon the mind of a geologist, that I venture to quote several passages in full: : "No one, whose observations have been confined to a country which has been above the sea only since the glacial period, or the contours of which have been smoothed over by the ice-sheets of that time, can readily form an adequate idea of the denuding effect of water flowing over the surface of the land. . . . The first impression received from a landscape, like that round Le Puy, is rather one of utter bewilderment. The upsetting of all one's previous estimates of the power of rain and rivers is sudden and complete. It is not without an effort, and after having analyzed the scene, feature by feature, that the geologist can take it all in. But when he has done so, his views of the effects of subaerial disintegration become permanently altered, and he quits the district with a rooted conviction that there is almost no amount of waste and erosion of the solid framework of the land, which may not be brought about in time by the combined influence of springs, frost, rain, and rivers.' 3. Valleys of the Eifel.-The country of the Lower Eifel, where it is traversed by the rivers Rhine and Moselle, affords another excellent instance of the excavation of valleys by river erosion. The country is composed of beds of hard rock, bent and inclined in various directions, and the general surface forms a gently undulating table-land, about 1,000 or 1,200 feet above the level of the sea. Through this country the rivers flow at the bottom of deep and winding valleys, forming the magnificent curving slopes which are the chief feature in the scenery of the district. This is particularly striking in the case of the Moselle, which works its way in so tortuous a course that some of its bends form loops, and curve back, so as to leave but a narrow ridge between the two reaches of the river. This singularly winding valley is both narrow and deep, being closely environed by steep, precipitous banks, 800 to 1,000 feet in height; and between their opposing bases, there is frequently no more room than is just sufficient for the river itself. The country on either side is furrowed by numerous naarrow ravines, with steep and often precipitous sides, which cut down, as they near the Moselle, to depths of 600 or 800 feet below the surface of the plain, and curve from side to side in equally sinuous courses. The only explanation which can be given of such a wonderful system of valleys is that the streams originally ran over the surface of the plain, and have constantly maintained the winding courses of their early channels, gradually engraving them deeper and deeper into the land, as the work of erosion and excavation proceeded. The action of rain has been co-extensive with that of the streams, wearing down the valley sides, and widening them into the shape they now possess. 4. The most wonderful instances, however, of the erosive power of rivers are to be found in the deep and trench-like valleys of the Western States of North America. The Colorado is formed by the union of the Green and 1 "Geological Sketches," by A. Geikie, 1882, pp. 93, 121. Grand Rivers, but the Green River may be regarded as the continuation of the main stream. This rises in Wyoming, and is joined by the Grand in Utah, whence the Colorado runs through Utah and Arizona to the Gulf of California. The Uinta Mountains are traversed by the Green River in a double series of gorges (or cañons), the deepest parts of which have walls from 2,000 to 2,500 feet high. Then, after running through a more open valley, the river traverses three great inclined plateaux, in a series of cañons which have received the names of Desolation, Gray, and Labyrinth Cañons. It then enters Stillwater Cañon, and in this, more than 1,200 feet below the general surface of the country, the Green and Grand Rivers unite their waters. The Colorado, below this junction, at once enters Cataract Cañon, and proceeds through a still more wonderful and gigantic series of gorges, which have received the names of Glen Cañon, Marble Cañon, and Grand Cañon. Glen Cañon is 149 miles long, but is not very deep; Marble Cañon is only 200 feet deep at its head, but steadily increases in depth, till near the lower end its walls are 3,500 feet high, and present a magnificent set of precipices, which in some parts are nearly vertical. The Grand Cañon is about 220 miles long, and is really a valley within a valley, consisting of an upper trough or depression eight to ten miles wide, with a level floor and precipitous sides about 1,000 feet high, and an inner gorge or cañon which varies in depth, but is in several places from 4,000 to 5,000 feet deep, its walls being a succession of vertical precipices broken only by short steep talusslopes. One of the deepest and finest parts of this cañon is represented in fig. 37. The country on both sides of this enormous ravine is channelled by innumerable narrow, deep, and winding cañons, all similar in their general characters, and each one occupied by a tributary of the main river. These extraordinary ravines have been cut down through horizontal or gently-inclined beds of sandstone, shale, and limestone, and into the granite which underlies them, the limestones being especially hard and massive, with an aggregate thickness of from 2,200 to 2,500 feet. The shales and sandstones lie above these limestones, and it is |