537 



VALLEYS. 



VALLEYS. 



638 



giving passage to its great head-streams and tributaries, and almost all 

 to considerable rivers, the affluents of the former respectively. 



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 Tenochtitlau, 

 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 4o 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. Beke 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 mentioned, 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 G<5djam ; and while 

 forming this curve, or flowing in a valley of the same figure, it is joined 

 by numerous streams, having then 1 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 Kafia, in Abyssinia, around 

 which some of the rivers alluded to take their own curved course. 

 This remarkable subject will be noticed again in tha geological part of 

 the article. ('Sources of the Nile, 1 pp. 10,28; 'Journal of Royal 

 Geographical Society,' voL xvii., pp. 5, 81.) 



In the article HIVEIIS, col. 113, the Arabian wadiet have been noticed 

 as the winter-brooks of the countries of which they are characteristic, 

 lint this is a figurative use of the term. A wady, the correlative of 

 the Hebrew nachal (not nahar, with which it is confounded in the 

 authorised 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 

 i;Tg, col. 481, the 'mats 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 oatia, this description is correct. But the oasis itself, 

 like the wady, is properly a species of valley. What the oasis of 

 Amrnon, 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 w&dys, 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. 



To discuss the literary history of this celebrated question, and esti- 

 mate the degrees of truth attained in the conflicting hypotheses of 

 Dr. Hut ton and De Luc which may stand as the types of two great 

 classes of rival speculations, not yet completely reconciled would be 

 a long, intricate, and unfruitful labour. The problem to be solved has 

 seldom been seized by any but the most modern writers in all its 

 generality; and the partial solutions, really arrived at in particular 

 cases, were not permitted to have even the value which limited 

 truths often do possess, because they were unwisely made the basis of 

 what was called a general theory. Though De Luc could prove that 

 valleys on whose line were deep lakes could not have been excavated 

 by the streams now running in them, he was scarcely entitled to say 

 that " all the notions of the great ravages produced by the rain-waters 

 upon our continents since their existence have been mere illusions." 

 (Letter I. 'On Geology.') And Dr. Hutton might have carried his 

 pupils beyond the mechanical effects of " rivulets that run only in time 

 of rain," before he required them to admit " the great fact, that rivers 

 have in general hollowed out then- valleys." (Playfair's ' Illustrations 

 of Huttonian Theory,' Note xvi.) 



Theories thus supported were only successful in destroying each 

 other : modem geology has been advanced by a very different process. 

 Mr. Lyell, M. D'Halloy, Mr. Scrope, and other modem writers, have 

 contributed similar partial solutions of ' particular ca?es, by careful 

 investigation of the features of the valleys of Auvergne, Belgium, and 

 the Rhenish provinces (see cols. 541-2); but to obtain a general view 

 of the theory of valleys, we must add to these many other equally esta- 

 blished local results. This necessity is indeed virtually acknowledged 

 by the eloquent writer to whom the Huttonian hypothesis owes its 

 celebrity, for even while he declares the great hollow of the Valais to 

 be the work of the Rhone, he adds, this tract, when the Alps rose out 

 of the sea, may have included many depressions of the surface which 

 the river joined together, and, from being a series of lakes, became one 

 great valley. 



To take the problem of the formation of valleys in all its extent, let 

 us trace in imagination the course of a considerable river, which, com- 

 mencing in a mountain-ridge, runs to the eastward, namely, in the 

 direction of the dip of the strata, and, after traversing the usual variety 

 of ground, empties itself into a shallow sea full of powerful currents. 



The upper doping line (r) is upposed to be the bed of the river, 

 > the lower is the sea-level (). 



A. The summit of drainage between one river area and another 

 being supposed to be below the level of perpetual snows, we find, 

 above the permanent sources of many rivers, occasional feeders, which 

 depend on particular falls of rain, becoming dangerous torrents or 

 appearing as mere lines of pebbles according to the state of the weather. 



After heavy rains the hill-sides of the highland districts of Scotland, 

 Wales, and Cumberland are whitened by abundance of short-lived 

 torrents, which hurry down considerable heaps of the loosened materials 

 of the hills, and spread them into little deltas on the margin of the 

 valley below. Similar effects on a particular slope follow the bursting 



