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. The immense 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 



