Seite - 362 - in The Origin of Species

362 ORIGIN OF SPECIES nents nor continental islands existed where our oceans now extend; for had they existed, palaeozoic and secondary forma- tions would in all probability have been accumulated from sediment derived from their wear and tear ; and these would have been at least partially upheaved by the oscillations of level, which must have intervened during these enormously long periods. If then we may infer anything from these facts, we may infer that, where our oceans now extend, oceans have extended from the remotest period of which we have any record; and on the other hand, that where conti- nents now exist, large tracts of land have existed, subjected no doubt to great oscillations of level, since the Cambrian period. The colored map appended to my volume on Coral Reefs, led me to conclude that the great oceans are still mainly areas of subsidence, the great archipelagoes still areas of oscillations of level, and the continents areas of elevation. But we have no reason to assume that things have thus remained from the beginning of the world. Our continents seem to have been formed by a preponderance, during many oscillations of level, of the force of elevation; but may not the areas of preponderant movement have changed in the lapse of ages? At a period long antecedent to the Cambrian epoch, continents may have existed where oceans are now spread out; and clear and open oceans may have existed where our continents now stand. Nor should we be justified in assuming that if, for instance, the bed of the Pacific Ocean were now converted into a continent we should there find sedimentary formations in a recognisable condition older than the Cambrian strata, supposing such to have been for- merly deposited; for it might well happen that strata which had subsided some miles nearer to the centre of the earth, and which had been pressed on by an enormous weight of super-incumbent water, might have undergone far more metamorphic action than strata which have always remained nearer to the surface. Theimmense areas in some parts of the world, for instance in South America, of naked metamorphic rocks, which must have been heated under great pressure, have always seemed to me to require some special explana- tion ; and we may perhaps believe that we see in these large areas, the many formations long anterior to the Cambrian