Chap. X.J IN ANY SINGLE FORMATION. 75 



Formations rich in fossils of many kinds, and of 

 thickness sufficient to last to an age as distant in futu- 

 rity as the secondary formations lie in the past, would 

 generally be formed in the archipelago only during 

 periods of subsidence. These periods of subsidence 

 would be separated from each other by immense in- 

 tervals of time, during which the area would be either 

 stationary or rising; whilst rising, the fossiUferous for- 

 mations on the steeper shores would be destroyed, al- 

 most as soon as accumulated, by the incessant coast- 

 action, as we now see on the shores of South America. 

 Even throughout the extensive and shallow seas with- 

 in the archipelago, sedimentary beds could' hardly be 

 accumulated of great thickness during the periods of 

 elpvation, or become capped and protected by subse- 

 qu^.nt deposits, so as to have a good chance of enduring 

 to a very distant future. During the periods of sub- 

 sidence, there would probably be much extinction of life; 

 during the periods of elevation, there would be much 

 variation, but the geological record would then be less 

 perfect. 



It may be doubted whether the duration of any one 

 great period of subsidence over the whole or part of the 

 archipelago, together with a contemporaneous accumu- 

 lation of sediment, would exceed the average duration of 

 the same specific forms; and these contingencies are in- 

 dispensable for the preservation of all the transitional 

 gradations between any two or more species. If such 

 gradations were not all fully preserved, transitional 

 varieties would merely appear as so many new, though 

 closely allied species. It is also probable that each 

 great period of subsidence would be interrupted by os- 

 cillations of level, and that slight climatal changes 



