In all cases then where lakes are so interposed in the path of a river that they must be believed to have received all the sediments which that river has brought, the cubic volume of these accumulations in the lake may be compared with the cubic space of the concavities between the hills along the lino of the rivers and rivulets above the lake ; and if found to be inferior in a notcable degree, wo may posi tively conclude that these concavities have not been produced, though doubtless they have been enlarged and modified, by the atmospheric agencies belonging to that particular area of drainage. Now this com parison has been often made, and generally with the poeitivo result, that the excavation of the valleys above the lakes Is not the effect of those watery agencies now exerted within them. Excavated however some of them have been by watery agency, and in all of them the surface slopes have been adjusted by this power, both in level and in direction, to the boundaries of the present lake; but we must avoid the error of assuming that no other currents having a different origin may have operated In those valleys before the existence of the lakes.
L. Beyond the region of the lakes, the rivers, flowing away from the rugged mountains, encounter ranges of stratified rocks, often very regularly inclined at a moderato angle, in parallel ridges and hollows which correspond to alternately hard and soft portions of the series of strata. If there were no gaps across these ridges, so as to connect their intervening hollows, each of these hollows would include one or many lakes, and tho river whose course we are tracing could not pass over the first of the ridges until the hollow space between It and the lake district (C) was filled with water, generally at a high level. If such a circumstance were supposed ever to have happened, the waters might be Imagined to make themselves a passage across this ridge, and, by like reasoning, across any lower ridges lying beyond. (See the figures above, and in art. Cgotoor, in NAT. DIET. DIV.) It sometimes !kappa's that more than one group of such parallel ridges and hollows—as the mountain limestone group (f), followed by the oolitic ridges (o), or those by the chalk•hills (c)—lie on the course of even one river, and require the repetition of such phenomena to account for the course of the valley. But s greater difficulty must be encountered. The very hollows themselves In which these sheets of water are imagined to have spread are valleys, and yield as plain proof that they also have been excavated and modified by watery action, as the river-channels which cross them. For in the midst of such hollows, insulated hills (the unremoved portions of the same or the nearest superjacent strata) remain variously distributed, to mark the ancient height of the land therein, and attest the enormous degradation which has been there occasioned. If, then, the supposed lake gave the force
to break over and cut through the 'naming barrier of rock beyond, so as to shape a course and descent to the river, the excavation of the space In which the lake W:13 gathered was the fruit of earlier and different watery action, This conclusion is again and again forced upon our attention as we proceed along the line of the valley.
E. In crossing through the parallel ridges and hollows of hard and soft strata, the river Is confined to steep, narrow, angularly bent passages among the hard rocks ; but in the softer strata between them it flows and winds more at liberty, through wider spaces, which open far on each aide, and bring additional supplies of water. In these hollows the velocity of the atream dice away, and the sediments derived from wasting of the adjoining high grounds fall on fertile meadows in floods or silt up their own channels in times of slack-water, while around appear Insulated hills, left by the ancient water-currents which swept away the materials around them.
P. The river, on emerging from these ranges of secondary strata, enters a wide region of plains and low hills of gravel (g), rising irregn larly amidst alluvial plains and marshes (m), amongst which, for a certain distance, the tide flows up the expanded river-channel.
Wherever these marshy plains and gravelly ridges are locally related by geographical situation and distribution to the main stream or smaller branches, so as to allow of the possibility of referring their formation to the action of the existing fluviatile and tidal currents, it would be false philosophy to look for a more remote or more general cause. This is often the case, perhaps generally so, with the alluvial sediments, for they contain often freshwater shells and other marks of limited lacuatrine or finviatile action ; but it is seldom the case with the gravel beds and ridges. These often lie across the path of the river (cc), and often rise to a great height above it ; often consist of stones not only beyond the present power of the river to transport, but such as do not occur in situ in any part of the area drained by its main stream or tributary waters.
Yet, from their form, distribution, and composition, there is no doubt that some have been wholly accumulated and all modified by water-currents ; so that here again we have proof of the waste and remodelling of the surface of the earth by other forces than the existing atmospheric agencies.