To discuss the literary history of this celebrated question, and esti mate the degrees of truth attained in the conflicting hypotheses of Dr. Hutton 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 Do 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 their valleys." (Mayfair's ` Illustrations of Huttonian Theory,' Note xvi.) Theories thus supported were only successful in destroying each other : modern geology has been advanced by a very different process. Mr. Lyell, M. D'Halloy, Mr. Scrope, and other modern writers, have contributed similar partial solutions or particular cases, 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 be 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.
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 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 of a waterspout (nigh Pike, in Cumberland) (Warensrour), or the intumeerauce of a wet prat-bog (above I:eighley, Yorkshire). Frost
and the heat are felt in extreme in the high regions which give birth to rivers, and by their alternation the rocks are broken and disintegrated. To these regions Hutton and I'layfair rightly propose to carry their pupils for the purpose of impressing upon their minds the extensive waste produced on the earth's surface by modern causes in action. Examples •are everywhere abundant : Glen Coe, Borrow dale, Snowdonia, may be cited. (N.B. The arecra/ features of the higher puts of mountain-valleys are nearly the same around glaciers, and these features are liable to change by the violent alternations of temperature.) B. The second stage of valleys is that which admits of the union of permanent natural springs to the occasional hill-side floods, and of the gathering of these streamlets into a rapid and agitated river. The now augmented water is often confined in a narrower glen than any of its tributaries, and rushes and cascades among rocks and mounds, which are so disposed as to show proof that the course of the stream has varied from time to time, as the levels changed, in consequence of eroding action.
C. At length the glen opens in a pebbly plain, or sinks into a hroad and quiet lake. Such lakes, or plains which seem to have been lakes, are of very general occurrence along the line of rivers, while they are engaged in the midst of their parent mountains (Derwent R titer, Lhanberis, Loch Tay). They even appear at the foot of particular mountains, receiving only occasional streams (lied Tarn, under Hel vellyn), and in a very great variety of cases appear• to be irregular hollows left after great disturbances of the stratification amongst the angularly posited masses of broken ground. Their depth is from a few feet to a thousand feet (Lake of Geneva) below the level of the valley ; and as the rivers which enter the upper ends there lose their force in the expansion of water, and drop their transported sediments, the growth of sew land In that part of such lakes is proportioned to, and is in truth a measure of, the whole effects of those rivers in trans florting away the detritus of the mountains round their source. Such lakes then are 'lateral dynamometers, which may with proper caution be used to determine the amount of transported materials delivered into them in given times by rivers; they also give the sum of all the effects of this kind performed by such rivers; and thus finally they are natural chronometers ; for by dividing, in any particular case, the integral effect or mass of deposited materials by the rato of annual progress, an approximate answer in years is given to the question of the length of time which has elapsed since that river began to flow. lty this argument De Luc arrived at the conclusion that the desicca tion of our continents by elevation above the sea is a phenomenon of no very great antiquity, belonging to an epoch only a few thousand years removed from our own. Though geologists cannot, from the evidence of particular lakes in certain districts, adopt this conclusion for other districts where quite different phenomena appear, Professor Sedgwick and other eminent persons have declared the argument of De Luc, within its proper limits, to be unanswered and unanswerable.