190 DISTRIBUTION OF CORAL-REEFS. Ch. VI. 



consist of upturned strata, or of erupted matter : and 

 from the wide spaces over which atolls are scattered, 

 although not one pinnacle of land now remains above 

 the level of the sea, we may conclude that immense 

 areas have subsided to an amount sufficient to bury not 

 only any formerly existing lofty table-land, but even the 

 heights formed by fractured strata and erupted matter. 

 The effects left on the land by the later elevatory 

 movements, namely, successively rising cliffs, succes- 

 sive lines of erosion, and great beds of shells and 

 pebbles, all requiring time for their production, prove 

 that these movements have been extremely slow. And 

 with respect to the whole amount of subsidence neces- 

 sary to have produced the many atolls widely scattered 

 over immense spaces, the movement, as already shown, 

 must either have been uniform and exceedingly slow, 

 or effected by small steps separated from each other by 

 long intervals of time, so as to have allowed the reef-con- 

 structing polypifers to bring up their solid frame-works 

 to the surface ; and this is one of the most interesting 

 conclusions to which we are led by the study of coral- 

 formations. We have little means of judging whether 

 many considerable oscillations of level have usually 

 occurred during the elevation of large areas ; but we 

 know from clear geological evidence, such as trees still 

 standing upright at successive levels and covered by 

 marine strata, that this has frequently been the case ; 

 and we have seen on our map, that some of the same 

 islands after having subsided, have been upraised ; and 

 that others after having been uplifted, have subsided. 



