Chap. X.] IN ANY SINGLE FORMATION. 75 



Formations rich in fossils of many kinds, and of 

 thickness sufficient to last to an age as distant in 

 futurity as the secondary formations lie in the past, 

 would generally be formed in the archipelago only 

 during periods of subsidence. These periods of subsi- 

 dence would be separated from each other by immense 

 intervals of time, during which the area would be 

 either stationary or rising; whilst rising, the fossili- 

 ferous formations on the steeper shores would be de- 

 stroyed, almost 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 within the archipelago, sedimentary beds could 

 hardly be accumulated of great thickness during the 

 periods of elevation, or become capped and protected 

 by subsequent deposits, so as to have a good chance of 

 enduring to a very distant future. During the periods 

 of subsidence, 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 accumula- 

 tion of sediment, would exceed the average duration of 

 the same specific forms; and these contingencies are 

 indispensable 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 

 oscillations of level, and that slight climatal changes 



