C02 



FOEMATION OF COEAL EEEFS. 



[Ch.XLIX 



It will follow as one of the consequences of the theory 



bottom 



not sink too fast to allow the zoophytes to build upwards 

 at the same pace, the thickness of coral will be great in 

 proportion to the rapidity of subsidence, so that if one area 



mass 



movement 



first area will be double that in the second. But the down- 

 in general have been very slow and 

 uniform, or, where intermittent, must have consisted of a 



w 



amount, other- 

 sea would have been carried down 



bottom 



faster than the corals could build upwards, and the island 

 or continent would be permanently submerged, having 

 reached a depth of 120 or 150 feet, at which the efeective 

 reef-constructing zoophj^tes cease to live. If, then, the sub- 

 sidence required to account for all the existing atolls must 

 have amounted to 3,000 or 4,000 feet, or even sometimes 

 more, we are brought to the conclusion that there has been 

 a slow and gradual sinking to this enormous extent. Such 

 an inference is perfectly in harmony with views which the 

 grand scale of denudation, everywhere observable in the 

 older rocks, has led geologists to adopt in reference to up- 



also have been gradual and 



movements 



must 



continuous throughout indefinite ages to allow the waves and 

 currents of the ocean to operate with adequate power. 



^ The map constructed by Mr. Darwin to dis|)lay at one 

 view the geographical position of all the coral reefs through- 

 out the globe is of the highest geological interest, leading 

 to splendid generalisations, when we have once embraced 

 the theory that all atolls and barrier reefs indicate recent 



subsidence, while the presence 



of frinsrin^* 



reefs proves 



the land to^ be stationary or rising. These two classes of 

 coral formations are depicted by different colours: and 



one 



of the striking facts brought to 



classification 



formations 



light by the 



same 



the absence of active 



volcanos in areas of subsidence, and their frequent presence 

 in the areas of elevation. The only supposed exception to 

 this remarkable coincidence at the time when Mr. Darwin 

 wrote, in 1842, was the volcano of Torres Strait, at the 



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