624 



DYNAMICAL GEOLOGY. 



Supposing the slope of the bottom at the Gambier 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 thou- 

 sand 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 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 disap- 

 peared, while the reef continued to grow upward, left the reef at the 

 surface, a ring of coral around a lake. 



The proofs are — 



1. As reef-forming corals grow only within depths not greater than 

 one hundred 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 sub- 

 sidence would make it a barrier-reef. 



In Fig. 965, a section of a high island with its coral reefs is repre- 

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

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

 growing reef depends for its upward progress on the growth of the 

 coral, and on the waves. The waves act only on the outer margin of 

 a reef, while the dirt and fresh water of the land directly retard the 



Fig. 965. 



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



inner part. Hence the outer portion would increase the most rapidly, 

 and would retain itself at the surface, during a slow subsidence that 

 would submerge the inner portion. The first step, therefore, in such 

 a subsidence, is to change a fringing reef into a barrier-reef (or one 

 with a channel of water separating it from the shore). The continued 

 subsidence would widen and deepen this channel ; then, as the island 



