Изображения страниц

domain in order to place more definitely on record his opposition to all speculation that would rob the mountains and valleys of any portion of the dignity which a catastrophic origin is supposed to have conferred upon them.

The antagonists in this controversy start from a certain basis of agreement. It is admitted on all hands that the crust of the earth has been broken and contorted, and that these traces of underground movement are of all ages, some going back to pre-Cambrian times, while others are even taking place at the present moment. The one school of geologists maintains that the present irregularities of outline are directly due to these disturbances of the crust, and that consequently in our systems of mountains and valleys we possess in great measure the primeval contour of the surface. They admit that running waters have sometimes widened and deepened their channels; that frost and general atmospheric waste have lowered mountain summits ; that glaciers have helped to wear down the rocks over which they have moved; that the sea has eaten away large portions of the solid land. But they hold that the influence of all these agencies has been, on the whole, quite insignificant; and that it was the grand subterranean forces which truly gave, in very early ages, a leading impress to the broadly-marked features of mountain and valley-features which, however since modified by atmospheric agencies, have never been obliterated, and which are as eternal as the snows and glaciers of the Alps are, broad geological sense, casual and ephemeral.'

There can be no doubt that, as we have already remarked, this view is at once the obvious one. It naturally suggests itself to every one who contemplates a group of lofty mountains, or who finds himself in the deep shadow of a valley with a sweep of precipice and rocky scarp around him. And as it requires for its credence no geological knowledge-as indeed it existed long before geology had a being—it has naturally acquired an ascendancy which will not be easily overthrown. Nevertheless, its verisimilitude forms no reason why, after all, it may not be false. Its opponents maintain that this plausibility has given it a hold which has been hurtful to the progress of sound geology. They remark that it naturally misleads the dilettanti observers who have rushed into the fray; but that it should still sway the minds of men who are in other respects able geologists they regard as matter, if not for surprise, at least for regret.

Two fatal objections against it are urged at the outset. In the first place, it ignores the fact that above the present surface of mountain and valley there once lay hundreds or thousands of feet of solid rock, which has since been removed. Vol. 125.No. 249.


in a

If we


could restore this missing material we should fill up every valley and bury every mountain. It could not have been removed by subterranean movements, let these have been what they may. It has been worn off by some surface-action; and the only forces by which this could have been effected, so far as we know, are those same powers of waste which are wearing away the rocks still. Hence it is manifest, say the denudationists,' that whatever may have been the original contour of the surface, Hutton's dictum must be true that the present mountains have been left by the erosion of the valleys, and the valleys have been hollowed out by the water draining off the mountains. In the second place, this explanation wholly overlooks the denudation which is in progress. According to the view now combated, the existing features of mountain and valley were impressed upon the surface during 'very early ages;' they are referred to as the "aboriginal outline,' which can only have been but triflingly modified by atmospheric agencies. But as has recently been shown, the rate of waste by these agencies is even now so rapid that the mountains and valleys could not retain their present outlines even during the passing of a single geological formation. In somewhere about four or five million of years the present continents will be washed into the sea by atmospheric waste. There are but two

of escape from this dilemma: either the interval which has elapsed since the aboriginal outlines’ were impressed upon the surface is so short that the existing agencies of denudation have not had time to obliterate them, in which case the whole of geological history will be comprised within a few hundred thousand years; or the forces of denudation must have been idle until recent times, and the rate of waste must be infinitely more rapid now than it has ever been. It is needless to say that both of these deductions are opposed to the whole current of geological evidence, and must be rejected on all sides.

The objections presented by the facts of denudation have been supposed to be met by the admission of powerful translations of water, which suddenly swept away vast masses of rock and excavated valleys. It is evident, however, that those who offer this explanation in reality surrender the argument; for if once they admit that the valleys have been due to denudation, they yield the chief point for which their opponents contend. The question as to whether the erosion was sudden or gradual can be discussed on its own merits. That


conceivable rush of water sent over a country by an earthquake shock could dig out a valley may well be doubted. There are, indeed, objections of various kinds which seem to us to render this notion



of the effects of sudden debacles quite untenable. It may be enough to point out that, in spite of what has been often said to the contrary, there is really no proof in its favour to be found among the geological records. That deposition and denudation are processes inseparably connected,' is an axiom, the truth of which is disputed by no one. If, then, a gigantic rush of water could at once excavate a valley, the mass of material removed would be swept away into some place of deposit, where we may suppose that its tumultuous arrangement and great thickness would bear witness to its sudden and violent origin. Deposits of this kind, then, ought to be of frequent occurrence among the geological formations. But we are not aware of a single undoubted instance. All the so-called examples present no features which are not readily explained by phenomena within human experience.

If we reflect upon what denudation, or the removal of solid rock, really involves, we shall be led to perceive that all such hypotheses as those which invoke the agency of huge debacles, or even those which attribute the chief share to the abrading powers of the sea, proceed upon a total misconception of the true nature of this great process. Before it is removed by running water, the rock has been corroded and softened by atmospheric causes. The yearly tribute of silt borne from the land to the sea is the result of this rot and decay; the streams carry away the decomposed rock which is washed into them, or which they themselves derive from the sides and bottoms of their channels. The sudden excavation and removal of a deep mass of solid compact stone is a phenomenon which we venture to regard as a physical impossibility. Certainly no agency in nature at present known or conceivable could accomplish it.

The geologists of the Huttonian school, who maintain that in the formation of systems of valleys, of river-ravines, and of lakes, denudation has been the chief process, are charged with ignoring the effect of underground movements. The accusation is so far justified, we think, that they do not so frequently refer to the traces of these movements as perhaps they might do. This evidently arises not from their ignorance of the proofs of fractures and upheavals, but rather because they take these proofs for granted, and, proceeding upon them as indisputable, find that, after all allowance for the influence of such internal disturbance, it is in the main by surface action that the valleys and mountains, as they at present exist, have been carved out. They hold that no sooner is a mass of land upraised by subterranean agency above the ocean-level, than it begins to be attacked by rain, frost, streams, and the other subærial forces, and that these by



degrees chisel out for themselves a system of valleys whereby the drainage of the land is carried down to the sea. At first the running water, it is said, would naturally take the hollows that chanced to exist upon the upraised surface; but these hollows would eventually lose all resemblance to their original form, as age after age the land continued to be worn away, Glens and valleys would thus be excavated, and the valleysystems now in existence are pointed to as showing, by their nice adjustment to the grand end of carrying off the surplus water, that they can have been due to no other cause than to the erosive action of that water itself.

The objections wbich have been raised to the adoption of these views may for the present be grouped under two heads. In the first place, it is contended that the valleys and riverravines contain within themselves proofs of their connexion with subterranean movements. This, however, is an assertion which is stoutly denied by the believers in the powers of the denuding agents, who maintain that for one example of a valley which can be shown to coincide with a line of fracture, there are scores where it can be proved that no such coincidence exists. They further retort that it is simply a begging of the question to be proved when the catastrophists first assert that valleys are due to fracture, and then point to the existence of the valleys as proof of the assertion. The presence of a dislocation is not a matter which can always be made out at a glance, but which often demands much careful scrutiny. Some of the largest faults known in this country show no feature at the surface which would lead to a suspicion of their presence. They are marked by no long line of ravine or valley, while, on the other hand, even the deepest ravines and the largest valleys comparatively seldom happen to run along proved lines of dislocation.

In the second place, the opponents of the denudation theory contend that by no power of rain, frost, ice, streams, or the sea, could the phenomena in question have been produced. They assert that rivers merely deepen channels already made for them, and this to so trilling an extent, that in the long history of the past their influence may be disregarded ; that it is absurd to speak of the “gentle rain from heaven' as capable of working out any great geological change; and that even glaciers can do little more than polish and furrow the hard rocks over which they move. By much the most ingenious and carefully elaborated argument we have yet seen on one branch of this subject is that given so far back as the year 1843, by M. Elie de Beaumont, in his • Leçons de Géologie Pratique.' He cites number of human antiquities dating a thousand or two thousand



years back, and reasons that the fact of their perfect preservation affords good ground for believing that the surface of a country remains for an immense period without any appreciable alteration. The vegetable soil in his eyes becomes a kind of fixed point or zero by which to measure the changes that take place more rapidly.

A part, however, from the fact that every year lessens the number of remaining antiquities and impairs their freshness, it would not be difficult to show that the preservation of old forts and tumuli is in thousands of cases by no means so perfect as is alleged ; that the standing stones which are still erect do not furnish any proof that the soil around them has undergone no change-a statement, indeed, which seems sufficiently negatived by the number of stones lying prostrate ; and that for one legible inscription more than two or three centuries old it would be easy to furnish scores which have been obliterated after a few generations. But even if all these assertions were just, and if it could be conclusively proved that for a thousand, or two thousand years, certain human monuments had undergone no appreciable alteration, would the inference necessarily be just, that therefore rain, frost, streams, and the other meteoric agents of decay, exercise no material influence upon the surface of the earth? A process which in two thousand years has not effected any perceptible alteration on certain parts of the earth's surface may yet have been rapid enough in the course of the geological ages to have worked the most stupendous changes upon that surface as a whole.

There is one method of investigating this subject, first suggested a good many years ago, and only recently revived, which promises to furnish the geologist with accurate data. The loss which the surface of a country undergoes may be approximately measured by the amount of sediment removed from it by its different rivers. Measurements and calculations have, with more or less care, been made in various rivers, the most elaborate being those of the United States Survey of the Mississippi, whence it appears that the water of rivers contains about 3000 of its bulk of mud. The average discharge of water and mean amount of sediment being known, it is easy to calculate the total annual quantity of sediment carried by a river to the sea. If, inoreover, we know the extent of the area which the river drains, and from which of course it derives its burden of silt, we can ascertain by a simple piece of arithmetic how much the whole basin of drainage has its general level lowered in one year.

It appears, from the data which have already been collected, that on an average somewhere about oth part of a foot is annually


« ПредыдущаяПродолжить »