Atmospheric agencies must be admitted to have greatly co-operated in this result, especially if, as geologists suppose, there were grounds for believing these to have been more powerful in the earlier [or in some past] eras of the world, when the temperature was perhaps higher and the atmosphere in consequence more highly vaporous. Nor must we undervalue the eroding power of modern streams, or the volume of the disintegrated earthy masses which they transport away. It is past a question that modern rivers have cut their own channels through lava (Lyell,' Principles of GeoL'), through diluvial gravel and clay drifted from other regions (Phillips, Sections of the Yorkshire Coast'), and through trap thrown up by the Eifel volcanoes ((Phillips], MS.,1829). But in each of these latter instances the valley of diluvial gravel and clay lies in and conceals in part an older valley of ruder aspect, excavated in the stratified rocks of sandstone or limestone or argillaceous slate ; and we may often contemplate in the course of one stream the fragmentary state of the rocks as left by elevatory forces, the wasting of these when they formed part of an ancient shore, the obliteration of the old valleys"by some yet ill-imderstood cause of local accumulation, and the final adjustment of levels and slopes by causes which are still continuing this beneficent process, enlarging and en riching our meadows, contracting the areas of our lakes, and softening for the future wants of mankind the rugged features of hills which will not always defy the hand of industry.
(The reader who desires to follow out this large subject may consult with great advantage Do Lue's works--as, Letters on Geology,' Lettres sur l'Hist. de Is Terre et de l'Homme ; ' Playfair, ' Illustrations of Huttonian Theory ;' Buckland's Diluviatue ; ' Lyell, `Principles' and 'Manual of Geology ;' Murchison ; Darwin ; [John Phillips, ' Rivers, Mountains, and Seacoast of Yorkshire '); snd other modern writers. The article PARALLEL ROADS [NAT. HIST. DIV., with addition in the present article]. may also be read. M. Agassiz's Speculations on Glaciers,' have several points of important bearing on the subject of Valleys.) Allusion has been made in the article SURFACE OF TUE EARTH, of which subject valleys constitute so important a feature, to the researches on the subject of their excavation of Mr. G. Poulctt Scrope, V.P.G.S., a not less philosophical, if less popular advocate of the sufficiency of existing causes in geological dynamics than Sir C. Lyell, and to the views which his inductions from their results have led him to form. In the first volume of the Proceedings of the Geological Society of London,' p. 170, the reading (now above thirty years since) is recorded of a paper by that geologist, hitherto, we think, unappre ciated, in which attention is drawn to the value which would attach to a test by which any one valley could be ascertained to be the result either of a-rapid and violent, or of a slow and gradual exeavatory process; since the forces of aqueous erosion are of a general nature, and while in activity in one river channel, were probably not idle in others. Such a teat was previously pointed out by Mr. Scrope, in his
work on the ' Geology of Central France,' where lava-currents which have flowed into valleys at intervals of time appear now' at different heights above the actual river bed, marking the successive steps of the progress of excavation. In the paper hero cited he finds another, and an equally valuable test in the extreme sinnosities of some valleys.
Any sudden, violent, and transient rush of water of a diluvial character, that is, a flood of wide area, could only produce straight trough-shaped channels in the direction of the current, and could never wear out a series of tortuous flexures, through which some rivers now twist about, and often flow for a time in an exactly opposite direction to the general straight line of descent, which a deluge or &tack would naturally have taken. Curvatures of this extreme kind are frequent in the channels of rivers flowing lazily through flat alluvial plains ; these curves are gradually deepened and extended, till the extreme of aberration is corrected, and the direct line of descent restored, by the river cutting through the isthmus which separates two neigh bouring curves. There are occasional instances, where the bias of the river, or direction of its lateral force of excavation has remained so constant, as to give the valley itself the utmost degree of sinuosity. But such examples must be immensely rarer than those of the con figuration previously described ; because the frequent shiftings of the channels of streams tend to obliterate their windings, and reduce the sum of the several successive excavations—that is, the valley—to a more or less straight form.
The valley of the Moselle, between Berncastle and Roarn, excavated to a depth of from 600 to 800 feet through an elevated platform of later palmozoic—forinerly termed transition—rocks, constitutes a striking instance of the former class. Its windings are often so extreme, that the river returns after a course of seventeen miles in one instance, and nearly as much in two others, to within a distance of a few hundred yards of the spot it passed before ; wearing away on either side the base of the ridge-shaped isthmus separating the curves, and enclosing a peninsula of elevated land five or six hundred feet high ; but sloping towards the bottom of the curves, where it ie strewed with boulders, left there, it may be presumed, by the river, as it gradually deepened its channel and extended its lateral curvature. The valley of the Meuse near Civet offers, through a great distance, a number of similar windings, and the same character is seen at intervals in many of the other rivers of the same physical district of Europe. Parts of the Seine below Paris, and the valley of the Wye between Hereford and Chepstow, are examples nearer borne.