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sculpture has been carried on. It is obvious, therefore, that a knowledge of the features produced by erosion, apart from other geological evidence, must often help us to determine- the relative antiquity of land-surfaces. We do not doubt that when the history of the hydrographic systems of the continents has been better worked out, when the evolution of surface-features has been more closely followed, our knowledge of land-development will acquire a precision to which it cannot at present lay claim. Geologists will then also be better prepared to attack and perhaps to solve the largest problem of all—the origin of our continental areas and oceanic basins. Not that we can expect or desire that students of nature should refrain from theorising and speculating in that direction until the fuller knowledge we desiderate has been acquired. Theory must often be in advance of the evidence. It may be that we are already in possession of the truth—that the continental plateau and the oceanic depression, as many maintain, are primeval wrinkles of the crust. At present, however, this view can only be considered probable, or, as some would say, possible—a brilliant suggestion which seems to explain much that is otherwise unintelligible.
Another question that will obtrude itself when we are investigating the origin of surface-features is that of time. Surely a very long period would be required for the completion of a cycle of erosion, for the upheaval of a great mountain-chain and its subsequent resolution to a plain of erosion, for the cutting up of a lofty plateau into hill and valley, and its final complete degradation. We find it difficult to conceive the lapse of time involved in the process, and the difficulty is increased when we remember that cycles of erosion have frequently been interrupted by long pauses, during which the regions involved have been submerged, and not only protected from denudation, but more or less deeply buried under new accumulations. Yes, assuredly, we must admit that many long ages have passed since the process of land-sculpture began. But physicists tell us that we can no longer draw unlimited drafts upon the Bank of Time. We have no immeasurable and countless aeons to fall back upon. Moreover, various estimates of the rate at which denudation is now being carried on, based as these are on the amount of materials carried seawards by rivers, have demonstrated that the demand for unlimited time is not justified. Even under existing moderate climatic conditions our own land is being levelled at a rate that will ensure its ultimate degradation within a period not so infinitely remote as geologists formerly supposed. In short, the cumulative effect of small changes is much greater than was at first realised. Further, their study of the past has taught geologists that the climate of the world has changed from time to time. And if so, then the rate of denudation must likewise have varied. In our own temperate lands we see how slowly erosion is effected—our streams and rivers are but seldom clouded with much sediment. Even after the lapse
•of many years their courses remain apparently unmodified. In less temperate lands, however, erosion often proceeds apace; watercourses are deepened and widened in an incredibly short time. During a tropical storm of rain as much erosion of soil and rock and transport of material are effected within a limited drainage-area as would tax a British river with all its tributaries to accomplish in a year or a number of years. Now these islands of ours have experienced many vicissitudes—tropical, subtropical, and arctic conditions have formerly obtained here— and we need not doubt, therefore, that the present rate of denudation has often been exceeded in the past. When streams and rivers began their work of erosion in the British area, it is probable that the climatic conditions were more favourable for that work than is now the case. In a word, although the wrork performed by geological agents of change has been the same in kind, it has necessarily varied in degree from time to time. The present rate of erosion in Britain, therefore, can be no infallible index to that of the past. But however rapidly denudation may have proceeded in former ages, the shaping out of our hills and valleys, even under the most favourable conditions, must have been a slow process. Nevertheless recent investigations leave little room for doubting that the time required for the evolution of all the multitudinous forms assumed by the land has been exaggerated. The tale told by our relict mountains and erosion valleys^does not support the claim for unnumbc
TABLE OF GEOLOGICAL SYSTEMS, AND THEIR PRINCIPAL
QUATERNARY Or (
Danian (not represented in England).
Senonian (Upper Chalk with Flints).
Turonian (Middle Chalk).
Cenomanian(LowerChalk and Upper Greensand).
,, .■ ■ / (Lower Greensand and Wealden Urgoman . > *
Bajocian (Inferior Oblite)
Toarcian (Upper Lias).
Liasian (Middle and Lower Lias
Sinemurian (Lower Lias in part)
Muschelkalk (not represented in England)
White Jura or Malm of Germany.
. ) Brown Jura or Dog) ) ger of Germany.
Black Jura or