VALLEYS. The term Valley, from the Latin Vallis or Valles, of the same signification generally, may be applied, iu its most compre hensive meaning, to any depressiou on the surface of the globe. " The largest valleys form the beds of the great oceans. Seas, bays, gulfs, &c., are all valleys below, or partially below, tho level of the sea." But in the common acceptation of the term, valleys are those depressions which are observable above the sea-level, separating or intersecting mountain-ridges, and in fact breaking them up into separate mountain masses, and in which are the basins of inland seas and lakes, the beds of rivers, &c. Mountains and hills, therefore, are the boundaries of valleys. Occasionally a valley is formed by a ridge of elevated land entirely, or almost entirely, surrounding the basin of an inland sea or of a lake. The word rale, sometimes employed synonymously, is the diminutive, but is more properly applied to the depressions between eminences of moderate elevation, or hills, which, together with them, form what is called undulating country. Valleys parallel to the mountain-ridges or chains which they separate, or which bound them, are said to be longitudinal ; while those the direction of which is transverse are so called. Both may be principal or lateral, the latter designation being usually applied to the smaller transverse valleys uniting the principal. But these epithets adroit of innumerable variations, and merge, in fact, into common descriptive language.
The citation above, in which the term valley is applied to the depressions filled by sea, is from an unquestioned authority, the Rev.
C. (I. Nicelay, in the ' Manual of Geographical Science.' But though the sea-b:utine or ocean-hollows may be included within the necessary geometrical definition of valleys, as beift depressions below the average level of the external surface of the solid mist of the globe, and continents and iehmtls within that of mountains, as being elevations above it, we conceive that some distinction in this respect should be established in the nomenclature of geology and physical geography. The surface of the globe above which continents and islands are raised, —that is, the bed of the sea, universally,--cannot be considered as identical, as a geographical or geological clement, with the surfaces of those masses of land themselves, on which are the minor, and in this sense secondary, elevations and depressions we commonly call moun tains and valleys. Many of the phenomena of configuration and physical state presented by the latter could only have been occasioned by causes connected with the agency of an aerial atmosphere ; and while many of them also have had a subaqueous origin, yet the con figuration of those parts of continents and islands which are below the level of the sae must have been received from operations exclusively subaqueous. The forms of the proper surface of the land were produced originally by its elevation through the surface of the ocean to a sube8rial position above it, and have been completed by atmospheric and fluvial erosion.
Many valleys, for example, were first excavated by marine currents during their elevation, and, have been reduced to their present figure by the rivers which have flowed through them from the period of the elevation above the sea-level of the eminences which bound them.
But the forms of the masses constituting continents and islands have been produced by the agency of elevatory and marine forces only, except in the cases of the subsidence of land previously fashioned more or lees by atmospheric action. However, the chief differences, probably, between the configurations of the surface of the land and those of the more extended surface consisting of the sea-bed and the land regained as one, depend on their relative magnitude and on the depth of the sea, which, in fact, is merely the measure of the vertical dimensions of the Letter. Around the islands of th:it part of the Indo
Australian archipelago which is physically a portion of Asia, the sea is so shallow that those islands may truly be regarded a&the summits of mountain-masses separated by valleys of very inconsiderable depth, inferior indeed to that of many of the terrestrial valleys upon them. Those islands are in fact united by a vast submarine plain, which abruptly terminates westward near the north-eastern coasts of Borneo, in the middle of the Straits of Macassar, and in the Strait of Lombock, in an unfathomable ocean. We then come to the island-masses of Lombock and the chain immediately to the west, the Moluccas, New Guinea, and the other Australian islands, rising abruptly again from the bottom of this ocean, and thus exemplifying one of the peculiar characters of the masses of land which rise from and bound ocean hollows ; and how characteristic of them is the form thus produced will appear in the sequeL Islands fundamentally volcanic—especially if, like Teneriffe, they contain one principal volcano—are probably altogether conoidal, the submarine portion being merely the continuation of the suba5rial. [Votc?.os.] Another difference, originating in the different process of formation, is this : while mountains proper have characteristically smaller dimen sion. above than below, and thus approximate more or less nearly to the figure of triangular prisms, of pyramids, or cones, their altitude being their principal dimension, the masses of continents and islands are tabula• In form, with sides of every degree of obliquity, and some times very nearly vertical, their upper surface being much greater in area than their aides, their altitude being their least dimension, and their form thus approximating to that of a low parallelopiped. Valleys proper, again, are widest at the top, and approach in general to the form of an inverted triangular prism, more or less obtuse or acute, or to that of an inverted pyramid, or to the frustum of such a prism or pyramid, the edge or apex being replaced by a plane—the floor of the valley ; while the intervals between continents are almost as tabular in form as the masses of the land, but their lower surface, while its area is much greater than that of the sides, being less than the upper formed by and at the level of the sea. [Soueetxas, DEEP Sea.] The configuration of land above the level of the sea which most nearly resembles the continental masses is that which is called Table land, such as the tablelands of Tibet, Eastern Africa, and Mexico; and the intervals which separate different portions of them most nearly answer to the ocean-hollows between continents. As an example of this may be cited the hollow between the table-land of Tibet on the south, and its reeurvature called the Thian-Shan, being a portion of North-Western China, on the north, of which the plain or plateau of Yarkend and Khotan, and what is termed the valley of Lake Lhop, form the ban, and which, if wo compare the two portions of the table-land to mountains, will answer to a valley, according to the com mon mode of description. [I'Laixs.] This comparison will hold good, and is illustrative in another respect. Colonel Strachey, in leis' Physical Geography of the Himalayas' (not yet published, but quoted In' PhiL Trans.; le59, pp. 774-776), represents that "the summit of the table land (of Tibet), though deeply corrugated with mountains and valleys in detail, Is In its general relief laid out horizontally." This description will accurately apply to the maw of elevated Land which forma a conti• nent or great island, of which it may always be said that while its summit Is corrugated with valley. and mountaine, it is in its general relief horizontal.