ON CORAL AND CORAL REEFS 133 



In the absence of any evidence that the sea has ever 

 risen to the extent required to give rise to the encircling 

 reefs and the atolls, Mr. Darwin adopted the opposite 

 hypothesis, viz., that the land has undergone extensive 

 and slow depression in those localities in which these 

 structures exist. 



It seems, at first, a startling paradox, to suppose that 

 the land is less fixed than the sea ; but that such is the 

 case is the uniform testimony of geology. Beds of sand- 

 stone or limestone, thousands of feet thick, and all full 

 of marine remains, occur in various parts of the earth's 

 surface, and prove, beyond a doubt, that when these 

 beds were formed, that portion of the sea-bottom which 

 they then occupied underwent a slow and gradual 

 depression to a distance which cannot have been less 

 than the thickness of those beds, and may have been 

 very much greater. In supposing, therefore, that the 

 great areas of the Pacific and of the Indian Ocean, over 

 which atolls and encircling reefs are found scattered, 

 have undergone a depression of some hundreds, or, it 

 may be, thousands of feet, Mr. Darwin made a suppo- 

 sition which had nothing forced or improbable, but 

 was entirely in accordance with what we know to have 

 taken place over similarly extensive areas, in other 

 periods of the world's history. But Mr. Darwin sub- 

 jected his hypothesis to an ingenious indirect test. If 

 his view be correct, it is clear that neither atolls, nor 

 encircling reefs, should be found in those portions of 

 the ocean in which we have reason to believe, on inde- 

 pendent grounds, that the sea-bottom has long been 

 either stationary, or slowly rising. Now it is known 

 that, as a general rule, the level of the land is either 

 stationary, or is undergoing a slow upheaval, in the 

 neighborhood of active volcanoes; and, therefore, 



