m LO WEST FOSSILIFEIiO US STRA TA. 34 5. 



the oscillations of level, which must have intervened 

 during these enormously long periods. If, then, wo may 

 infer anything from these facts, we may infer that, whoro 

 our oceans now extend, oceans have extended from the 

 remotest perioc] of which we have any record; and on 

 the other h^md, that where continents 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 Keefs, led 

 me to conclude that the great oceans are still mainlv areas 

 of subsidence, the great archipelagoes still areas of oscilla- 

 tions of level, and the continents areas of elevation. i^>nt 

 we have no reason to assume that things have thus re- 

 mained 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. Bu't 

 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 nov/ spread out, and clear and open 0(,'eans 

 may have existed where our continents now stand. Xor 

 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 

 recognizable 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 center of the earth, and which had 

 been pressed on by an enormous weight of superincumbent 

 water, might have undergone far more metamorphic action 

 than strata which have always remained nearer to the sur- 

 face. The immense areas in some parts of the world, for 

 instance in South America, of naked metamorphic rorks, 

 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 {interior to the Cam- 

 ■^rian epoch in a completely metamorphosed and denuded 



condition. 



The several difficulties here discussed, nanu'ly, that, 

 though we find in our geological formations niany links 

 ^fitween the species which n^w exist and which formerly 



