Chap. X.] IN LOWEST FOSSILIFEROUS STRATA. 87 



land have existed, subjected no doubt to great oscilla- 

 tions of level, since the Cambrian period. The coloured 

 map appended to my volume on Coral Eeefs, led me to 

 conclude that the great oceans are still mainly areas of 

 subsidence, the great archipelagoes still areas of oscilla- 

 tions 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 contin- 

 ents 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 formerly 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 superincum- 

 bent 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 meta- 

 morphic rocks, which must have been heated under 

 great pressure, have always seemed to me to require 

 some special explanation ; and we may perhaps believe 

 that we see in these large areas, the many formations 

 long anterior to the Cambrian epoch in a completely 

 metamorphosed and denuded condition. 



