284 CORAL'S AND CORAL ISLANDS. 



IV, PERIOD OF THE SUBSIDENCE. 



The period during which these changes were in progress 

 extends back to the Tertiary era, and perhaps still farther. 

 In the island of Metia, elevated two hundred and fifty feet, the 

 corals below were the same as those now existing, as far as we 

 could judge from the fossilized specimens. At the inner margin 

 of shore reefs there is the same identity with existing genera. 

 AVe do not claim to have examined the basement of the coral 

 islands, and offer these facts as the only evidence on this point 

 that is within reach. We cannot know with absolute certainty 

 that the present races of zoophytes may not be the successors 

 of others that flourished, on the same sites, even before the 

 Tertiary era in Cretaceous and Jurassic times ; but as yet have 

 little reason, in facts observed, for such a conclusion. For a 

 long time volcanic action may have been too general and 

 constant over the Pacific for the growth of corals ; and this 

 may have continued to interfere till a comparatively late period, 

 if we may judge from the appearance of the rocks, even on 

 Tahiti. The subsidence has probably for a considerable period 

 ceased in most, if not all, parts of the ocean, and subsequent 

 elevations of many islands and groups have taken place. 



V. ELEVATIONS OF MODERN ERAS IN THE PACIFIC. 



Since the period of subsidence discussed in the preceding 

 pages, there has been no equally general elevation. Yet 

 various parts of the ocean bear evidence of changes confined 

 to particular islands, or groups of islands. While the former 

 exemplify one of the grander events in the earth's history, in 

 which a large segment of the globe was concerned, the latter 

 exhibit its minor changes over limited areas. The instances of 

 these changes are so numerous and so widely scattered, that 

 they afford convincing evidence of a cessation in the previous 

 general subsidence. 



