622 



DYNAMICAL GEOLOGY. 



islands, it is certain that this thickness is in some cases thousands 

 of feet. Within three-quarters of a mile of Clermont Tonnerre, 

 in a sounding made by Hudson, the lead struck and brought up an 

 instant at two thousand feet, and then fell off and ran out to three 

 thousand six hundred feet without finding bottom ; and seven miles 

 from the same island no bottom was found at six thousand feet. 



The barrier-reefs remote from an island must stand in deep 

 water. Supposing the slope of the bottom at the G-ambier Islands 

 only five degrees, we find, by a simple calculation, that the reef has 

 a thickness of twelve hundred feet. In a similar manner, we learn 

 that it must be at least two hundred and fifty feet at Tahiti, and 

 two or three thousand at the Feejees. 



3. Origin of the Forms of Reefs, — the Atoll and the distant 



Barrier. 



The origin of the atoll form of reefs was first explained by the 

 geological traveller Charles Darwin. According to the theory, each 

 atoll began as a fringing reef around an ordinary island ; and the 

 slow sinking of the island till it disappeared, while the reef con- 

 tinued to grow upward, left the reef at the surface a ring of coral 

 around a lake. 



The proofs are — 



1. As corals grow only within depths not greater than one hun- 

 dred feet, the bottom on which they began must have been no 

 deeper than this ; and, as such a shallow depth is to be found, with 

 rare exceptions, only around the shores of lands or islands, the reef 

 formed would be at first nothing but a fringing reef. 



2. A fringing reef being the first step in coral formations, slow 

 subsidence would make it a barrier-reef. 



In fig. 853 a section of a high island with its coral reefs is repre- 



Fig. 853. 



fe 



Section of an island bordered by a coral reef, to illustrate the effects of a subsidence. 



sented, the horizontal line 1 being the level of the sea,/ a section 

 of the fringing reef on the left, and f of the reef on the right. 



