344. The immense quantity of _cailloux roulés_, or rounded gravel, collected in the immediate vicinity of mountainous tracts, has led some geologists to suppose the existence of ancient currents, which descended from the mountains, in a quantity, and with a _momentum_, of which there is no example in the present state of the world. Thus Saussure imagines, that the hill of Supergue, near Turin, which is formed of gravel, can only be explained by supposing such currents as are just mentioned, or what he terms a _debacle_, to have taken place at some former period.[172] If, however, we ascribe to the mountains a magnitude and elevation vastly greater than that which they now possess; if we regard the valleys between them as cut out by the rivers and torrents from an immense rampart of solid rock, neither materials sufficiently great, nor agents sufficiently powerful, will appear to be wanting, for collecting bodies of gravel and other loose materials, equal to any that are found on the surface of the earth. The necessity of introducing a _debacle_, or any other unknown agent, to account for the transportation of fossils, seems to arise from underrating the effects of action long continued, and not limited by such short periods as circumscribe the works, and even the observations, of men. [Footnote 172: Voyages aux Alpes, tom. iii. § 1303.] 345. The supply of gravel and _cailloux roulés_, for the plains extended at the feet of primitive mountains, is doubtless in many cases much increased by the pudding-stone, interposed between the secondary and the primary strata. The beds of pudding-stone contain gravel already formed on the shores of continents, that ceased to exist before the present were produced; and the cement of this gravel, yielding easily to the weather, allows the stones included in it to be washed down by the torrents, and scattered over the plains. I know not if the hill of Supergue above mentioned, is not in reality a mass of the pudding-stone which forms the border of the Alps, and of which the materials have suffered no transportation since the time of their last consolidation. This at least is certain, that Saussure, notwithstanding his accuracy, has sometimes confounded the loose gravel on the surface with that which is consolidated into rock; an inaccuracy which is to be charged, as I have elsewhere observed, rather against his system than himself.