270 GROUPS OF ALLIED SPECIES [CHAP. X. 



support the view, that the older a formation is, the more in- 

 variably it has suffered extreme denudation and metamorphism. 



The case at present must remain inexplicable; and may be 

 truly urged as a valid argument against the views here enter- 

 tained. To show that it may hereafter receive some explanation, 

 I will give the following hypothesis. From the nature of the 

 organic remains which do not appear to have inhabited profound 

 depths, in the several formations of Europe and of the United 

 States; and from the amount of sediment, miles in thickness, of 

 which the formations are composed, we may infer that from first 

 to last large islands or tracts of land, whence the sediment was 

 derived, occurred in the neighbourhood of the now existing con- 

 tinents of Europe ai d North America. The same view has since 

 been maintained by Agassiz and others. But we do not know 

 what was the state of things in the intervals between the several 

 successive formations; whether Europe and the United States 

 during these intervals existed as dry land, or as a submarine 

 surface near land, on which sediment was not deposited, or as the 

 bed of an open and unfathomable sea. 



Looking to the existing oceans, which are thrice as extensive as 

 the land, we see them studded with many islands; but hardly 

 one truly oceanic island (with the exception of New Zealand, if 

 this can be called a truly oceanic island) is as yet known to afford 

 even a remnant of any palaeozoic or secondary formation. Hence 

 we may perhaps infer, that during the palaeozoic and secondary 

 periods, neither continents nor continental islands existed where 

 our oceans now extend; for had they existed, palaeozoic and 

 secondary formations would in all probability have been accumu- 

 lated 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 were continents now exist, large tracts of 

 land have existed, subjected no doubt to great oscillations of 

 level, since the Cambrian period. The coloured 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. 



