4o8 PRINCIPLES OF STRATIGRAPPIY 



lation of organic or other debris, as in the case of the Florida 

 region, coral reefs may progressively develop seaward in what was 

 originally water of too great a depth for normal coral growth. 



Oceanic barrier reefs and atolls, on the other hand, rise from 

 depths greater than those at which reef corals flourish, and so these 

 types need further explanation. Two principal theories have been 

 proposed for their explanation: the subsidence theory of Darwin, 

 elaborated and strengthened by Dana, and the spreading ring theory 

 proposed by Murray and elaborated by Alexander Agassiz. 



Subsidence Theory of Darivin. Fringing' reefs surrounding 

 oceanic islands may be progressively converted into barrier reefs 

 and finally into atolls by a constant subsidence sufficiently slow to 

 permit the reef corals to grow upward and maintain themselves in 

 a relatively uniform depth within the normal limit. By such a sub- 

 sidence the older dead coral masses are carried constantly to lower 

 depths, while the top of the reef flourishing on this foundation 

 of dead coral constantly keeps near to the surface of the sea. The 

 channel between the reef and the rock island will become wider as 

 more of this island is submerged, while at the same time it becomes 

 filled by the coral sand, and broken coral masses thrown over the 

 reef and the land-derived detritus supplied by the streams from 

 within. Growing nullipores and other organisms will likewise help 

 in filling the constantly widening lagoon. When subsidence has pro- 

 gressed so far that the central land mass has become entirely sub- 

 merged, the barrier reef has changed to the atoll and the ring lagoon 

 to the open lake-like body of water within the ring of coral reefs 

 and islets. 



This explanation readily accounts for the observed gradation be- 

 tween the fringing reefs and atolls noted in the islands of the Pa- 

 cific, where every stage seems to be represented. It accounts for the 

 steep slopes and the occasional cliffs of the outside of the reef 

 which descend to abysmal depths, as well as the occurrence of dead 

 reef corals at depths where they cannot live — corals which have 

 been carried by subsidence of their foundation beyond their normal 

 depth and drowned. This theory accounts also in a satisfactory way 

 for the remarkable distribution of the various kinds of reefs in the 

 Pacific, as shown by Dana. In the middle of the atoll area of the 

 Pacific there is an extensive area 2,000 miles long by 1,000 miles 

 wide free from islands. This is roughly surrounded by a belt of 

 small atolls, outside of which occurs the region of large or grouped 

 atolls, then a belt in which the reefs are mostly barriers and finally 

 the belt of fringing reefs. This distribution is of course to be 

 traced only in a general way. Outside of the last belt there is even 



