A form of valley to which the convenient appellation of valley-plain has been given is that which exists when an extensive horizontal sur face of land at any elevation, and generally of greater length than width, is surrounded by continuous ranges of mountains rising from and above it. Of this form the so-termed Vale of Tenochtitlan, usually called, after Humboldt, the Valley of Mexico, and the plain, valley, or " Vale " of Cashmere, are examples. The latter is, in fact, the valley of the river Behat or Jelam (by which, however, at least in its present form, it was Clearly not excavated), and the inclosed plain, as in many cases of this description, has once been the bed of a lake, and consists, itself, of a lacustrine formation. It depends on the extent of the plain, its degree of unevenness, and the inclination of the slopes, whether or not it shall become and deserve to be called a river-basin.
Another form of valley is the circular spiral, alluded to in the article RIVERS, col. 115. This has been noticed, hitherto, in Eastern Africa only, in the basins and valleys of the eastern affluents of the Nile. Though plainly shown, in one instance at least, in the maps of Abys sinia constructed at the beginning of the 17th century by the Portu guese Jesuit missionaries, Dr. Beka appears to have been the first geographer of modern times to call attention to it, and that from his own explorations. In reference to the Mareb, one of the tributaries of the Atbara, already meutioned, he points out "the remarkable pecu liarity which it possesses, in common with many of the rivers of the Abyssinian table-land, of returning on itself, so as to perform a sort of spiral course." The river Abai, or the Nile of Bruce, for example, has a circular spiral course round the peninsula of Godjam ; and while forming this curve, or flowing in a valley of the same figure, it is joined by numerous streams, having their sources in the mountains forming the conoidal core of the peninsula. Dr. Beke has recorded his opinion of the probability that the head-stream of the Nile itself has such a circular spiral course, and therefore flows through a valley of that figure, around a lofty mountain-mass, similar in character to the snow-capped mountains of Samien and Ksffa, in Abyssinia, around which some of the rivers alluded to take their own curved course. This remarkable subject will be noticed again in the geological part of the article. (' Sources of the Nile,' pp. 10, 28 ; ' Journal of Royal Geographical Society,' vol. xvii., pp. 5, 81.) In the article ItivEss, col. 113, the Arabian wadies have been noticed OS the winter-brooks of the countries of which they are characteristic. But this is a figurative use of the term. A wady, the correlative of the Hebrew nae/ud (not nahar, with which it is confounded in the authorized version of the Scriptures, but which is a river proper), is originally a valley of a peculiar kind. It is a depression, more or less
deep or wide or long, worn or washed by the mountain torrents, or winter rains for a few months or weeks in the year. In the article DESERTS, col. 481, the oases have been described, after Malte-Brun, as rising in the midst of the sands like islands in the ocean. If we con sider the sources of the springs of water which supply them, and to which they owe their existence, to be included in the locality designated an oasis, this description is correct. But the oasis itself, like the wady, is properly a species of valley. What the oasis of Ammon, in the western desert of the Nile, " is on a great scale may be seen on a small scale elsewhere; namely, deep depressions of the high table-land, which thus became the receptacles of all the rain and torrents, and, consequently, of the vegetation and the life of the whole of that portion of the desert. These oases, therefore, are to be found wherever the waters from the different wftdys, or hills, whether from winter streams," or from the few living perhaps perennial springs of the country, " converge to a common reservoir." We are indebted for these explicit characters of geographical features much oftener alluded to than understood, to the Rev. A. P. Stanley, who, in his Sinai and Palestine,' has so well described the details of the physical geography of those countries.
The preceding statements and views relate principally to the subject of valleys geographically considered : their geological history is discussed in the following essay, which originally constituted the article. We have now appended to it some facts and considerations disclosed by and arising from the subsequent progress of geology.
" Why has the earth any mountains ?" is the question from which De Luc, writing in 1792, sets out to expound his whole geological system ; and to answer at the present time the corresponding question, Why has the earth any Valleys ? requires reference to almost the entire series of general truths which have been established by investigation into the structure of the crust of the globe. For in these hollows on the surfaces of plains, hilly slopes, and steep mountains, we behold not only the results of atmospheric agencies, both chemical and mechanical, and of the flowing of streams, operating under the actual conditions of nature on materials of unequal induration, but also the earlier effects of other watery agencies, under other physical conditions, on materials differently circumstanced, both as to their consolidation and their position in reference to the general curve of surface of the globe and the relative level of the sea. The origin of valleys ascends to the earliest geological eras, but their completion includes the latest phe nomena produced in our own days.