VALLEY, a hollow tract on the earth's surface between hills or mountains. Valleys are generally parallel to the direction of the ridges of elevated ground; but some are transverse, cutting through the mountain-chain. They have a watercourse at or near their lowest level. The main valley is that which has the river of the drainage-system to which it belongs flowing through it, while the tributary streams which feed this river flow through lateral valleys. The terms upper and lower valley define parts of the same valley, as related to the source or to the mouth of the river which flows through it. In a narrow valley, the river always occupies the lowest part; but in wide valleys, especially in those in which waters run that are largely charged with sediment, the river often builds up a channel for itself, that is higher than the ground at the foot of the hill. The river, in its floods, bears a large amount of mud, which it continues to carry as long as the water is retained within its bed; but whenever it overflows its banks, the velocity is re duced, and the heavier particles, which form the bulk of the sediment, are deposited near the river's course; while, flowing over the surface of the level ground, even the particles fall to the bottom, until, as it reaches the limits of the valley, the water gradually becomes clearer. The Rhine, the Nile, and indeed almost all great rivers in wide val leys, illustrate this phenomenon. The river seldom flows through the middle of the valley, but is generally nearest to that side where the slope to the high ground is steep est; the opposite side oT the main valley presenting a more gradual rise to the mountain summits, supplies the chief lateral valleys and feeding-streams to the river.
The origin of valleys has been a subject of considerable controversy, and this ques tion continues to occupy the attention of geologists. At the time when a universal deluge was used to explain whatever was inexplicable iu geology, it was considered to have been the agent which furrowed the earth's surface with valleys; and this opinion was entertained so lately, as to have been advocated by the late clean Buckland in his Reliquia Diluvianee, until prof. Fleming showed the untenableness of these opinions.
At the present day, geologists are very much divided as to the origin of valleys. Some hold that they are the result of the operation of that internal agency which has, at different periods, so broken the crust of the earth, and changed its surface; while others maintain that various agents now operating more or less favorably in disinte grating and removing the solid materials of the exposed portion of the surface of the earth, produced the inequalities that now exist. There can be no doubt that all these have been active, and that the special advocacy of individual agents, as the sole pro ducers of these phenomena, is the source of error, and the cause of controversy. Each and all have done their part; and in a satisfactory explanation, they must all be taken into account. That internal force has been a principal agent in producing the diversity
of hill and valley, seems beyond doubt. This force acted by raising the surface perpen dicularly from below upward; by producing great faults, which presented facilities for the action of running water; or by pushing a portion of the crust forward, so as to pro duce immense folds, alternating with mountain ranges. The Appalachians of North America, and the associated valleys, have been produced, as has been shown by prof. Rogers, by the last-mentioned method; and the tertiary strata of the Alps were carried up a 1000 ft., while the valley-beds of the Adriatic and the Mediterranean either remained stationary or subsided to a lower level. The fact that some valleys are wily the syncli nal axes between the bounding mountain systems, like the basin of Switzerland between thk elevated ridges of the Alps and the Jura, also confirms the opinion that sonic valleys owe their origin to the operations of an internal force, which operated iu geologic ages in a more powerful manner than it has been known to do in historical times. In the face of such facts, it is surprising to hear practical geologists so influenced by pet theo ries as to assert that the action of internal force has direct effect on the external features of the ground." But this is the position of men who adhere to the strict Lyel lian doctrine, that all the past changes on the earth's surface have been produced by agents now operating, and at the same rate, but through enormously protracted periods of time. But as these agents are various, so we have almost as many theories as there are agents. Lyell insists that ocean currents, and the wear and tear of the waves, have produced the inequalities. Jukes will have it that the atmosphere has disintegrated, and the rivers carried off the materials which formerly filled up the hollowed-out valley to a level with the surrounding hills; while Ramsay declares that glaciers were the impor tant agents in the process. That any one of these alone has produced the great changes on the surface of the earth, is a position that would be maintained only by those who are blinded by their idol of a favorite hypothesis which they have to defend. But that all of them. in addition to the operation of an internal force, have been agents, more or less, in producing the present conformation of the earth's surface, cannot be doubted. While the advocates of superficial agents so completely ignore the influence of internal force, as in the statement of prof. Jukes quoted, those who maintain the opposite view are equally open to condemnation when they declare that " the wear and tear due to atmospheric sub-aerial erosive agency never could, even after operating for countless ages, have originated and deepened any of the valleys which occur in fiat countries."— Murchison's address at British association, 1865.