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jo yards, and that of the Mississippi by 80 to 100 yards in the same time.
It is sufficiently obvious that the material carried seawards by rivers must afford some indication of the rate at which the surface of the land is being lowered by subaerial action. Having ascertained the annual amount discharged by any individual river, we learn, at the same time, to what extent the drainage-area of that river is being denuded. In the case of the Mississippi, for example, it has been calculated that the amount of sediment removed is equal to a lowering of- the whole drainage-area by ^-^^th of afoot. In other words, could we gather up all the material discharged in one year, and distribute it equally over the wide regions drained by that river and its tributaries,, we should raise the land-surface by )rq1c-g-th of a foot. That does not seem to be much, but at this rate of erosion one foot of rock will be removed from the Mississippi basin in 6000 years; and the Mississippi is not so active a worker as many other rivers. An average of many estimates of the similar work performed by rivers in all quarters of the globe shows that the rate at which drainage-areas generally are being lowered is one yard in 8000 to 11,000 years. It must not be supposed that this erosion is equal throughout any drainage-area. As a rule, denudation will take place most rapidly over the more steeply inclined portions of the ground. On mountain declivities and hill slopes rock-disintegration and the removal of waste products will proceed more actively than upon low grounds and plains. The work of erosion will be carried on most effectively in the torrential tracts of streams and rivers. Indeed, we may say that it is in valleys generally that we may expect to find the most cogent evidence of erosion now in action.
A little consideration will show that the estimates just referred to do not tell us all the truth concerning denudation. They show us only the amount of waste material which is swept into the sea. They afford no indication of the actual amount of rock-disintegration and erosion. Rock-rubbish gathers far more rapidly in mountain-regions than it can be removed by running water. Indeed, over a whole land-surface rocks are disintegrated and ddbris accumulates from year to year. Nor is the amount of material brought down by a river to its mouth an index even to the activity of the river itself as a denuding and transporting agent. Enormous volumes of detritus are deposited in valleys or come to rest in lakes and inland seas.
Hitherto we have been treating of the work done by the atmosphere and running water. Some reference has also been made to frost as a potent disintegrator of rocks. But we have still to consider the action of glaciers in modifying the surfaces over which they flow. It can be shown that valleys have been widened and deepened, and broad areas more or less remodelled, by flowing ice, so that glaciers must not be ignored in any general account of denuding agents. It will be more convenient, however, to leave them for the present; for however interesting and important their action may be, it is yet of minor consequence so far as the origin of surface-features as a whole is concerned. For similar reasons we may delay the consideration of marine erosion. The action of the sea upon the land is necessarily confined to a narrow belt, whereas that of the subaerial agents affects the whole surface of the land.
We may take it that the denudation of the surface, rendered everywhere so conspicuous by the discontinuity of strata, has been effected mainly by the atmosphere and running water. Other agents have, no doubt, played a part, but those just referred to must be credited with the chief share in the work of erosion. Such is the general conclusion to which we are led by the study of causes now in action. And observation and reflection combine to assure us that subaerial erosion has been equally effective during the formation of all the derivative rocks which enter so largely into the framework of the earth's crust. For these rocks are for the most part of sedimentary origin—they tell us of ancient lakes, estuaries, and seas. All their materials have been derived from the degradation of old land-surfaces, partly no doubt by the sea, but in chief measure by subaerial agents. And the great thickness and extent attained by many of the geological systems enable us to form some idea of what is meant by denudation. What, for instance, shall we say of a system composed essentially of sedimentary strata reaching a thickness of several thousand feet, and occupying an area of many thousand scquare miles? Obviously, the materials of such a system have been derived from the waste of ancient lands. Mountain-masses must have been disintegrated, and removed in the form of sediment, and gradually piled up, layer upon layer, on the floor of the sea. Every bed of sedimentary rock, in short, is evidence of denudation.
Further, it has been ascertained that in the building up of the various great geological systems the same materials have been used over and over again. Sediments accumulated upon the sea-bottom have subsequently, owing to crustal movements, entered
Fig. 6. Section Across Unconformable Strata.
a <s, beds of sandstone, shale, etc. ; b b, conglomerates and sandstone resting discordantly or unconformably upon a a; u u, line of unconformity.
into the formation of a new land-surface, and thereafter, attacked by the epigene agents of change, have again been swept down to sea as gravel, sand, and mud. The history of such changes is easily read in the rock-structure known as unconformity. In the accompanying section (Fig. 6), for example, two sets of strata are shown—the upper (b) resting discordantly or unconformably upon the lower (a). The lower series of sandstones and shales is charged with the remains of marine and brackish-water organisms and of land-plants. The overlying strata (b) are likewise of aqueous origin, and consist chiefly of conglomerates and sandstones below, and of somewhat finer-grained sedimentary beds above. Like the older series (a), they likewise contain marine and brackishwater fossils. The beds (a) introduce us to an estuary, or shallow bay of the sea, into which sediment is carried from some adjacent land. The whole series has evidently been deposited in water of no great depth, as is shown by the character of the rocks and their fossil contents. And as the strata attain a thickness of more than 2000 feet, we must infer that during their accumulation the sea-floor was slowly subsiding, the rate of sedimentation probably keeping pace with the subsidence. In other words, the bed of the sea appears to have been silted up as fast as it sank, so that relatively shallow-water conditions persisted during the deposition of the land-derived sediments. Then a time came when the sea-floor ceased to sink and another movement of the crust took place, which resulted in the folding of the sedimentary strata and the conversion of the sea-bottom into dry land. The folded rocks were now subjected during some prolonged period to the action of the various subaerial agents of erosion, whereby the whole land-surface was eventually denuded and planed down. When the work of erosion had been so far completed, the entire region again subsided, and formed the bed of a shallow sea. U nder these conditions the drowned land-surface became overspread in time with new ac