THEORIES OF ORIGIN OF REEFS 407 



forming a barrier reef at a distance from shore where now lies the 

 outer rim of the Florida mainland. The lagoon behind this new 

 reef was of the character of the present outer lagoon, and received 

 normal marine sediments, until with progressive upbuilding the 

 outer reef was converted into a series of dead keys and a new line 

 of reefs came into existence upon the meanwhile extended sub- 

 marine platform. This new line was subsequently converted into 

 the present line of keys, the preceding row became the southern 

 coastal rim, and the old lagoon behind it was converted by suc- 

 cessive steps into the present Everglades. The present inner lagoon 

 channel between the mainland and the keys is gradually approach- 

 ing the same fate, and it and the line of keys will in turn be added 

 to the mainland, while the present living reef will gradually emerge 

 as a line of islands and the lagoon behind it suffer filling up. It is 

 not likely that a new line of reefs will form outside of the present 

 pne, as the force of the Gulf Stream will prevent further extension 

 on the Pourtales Plateau of the submarine platform which serves as 

 the foundation of the reef. (Le Conte-59; 60.) 



It is to be noted that as the building of the new reef progresses, 

 leading to the extinction of the preceding inner reef, a new type 

 of sedimentation — the mud-flat type — will come into existence be- 

 hind the older reef, while contemporary calcareous deposits will 

 accumulate in the outer lagoon and on the reef. Thus the top of 

 the limestone series formed by these shell accumulations and reefs 

 will rise progressively seaward, while mud-flat and later continental 

 conditions will progressively replace the marine conditions from 

 the landward side outward. Precisely such conditions existed in the 

 Onondaga (Middle Devonic) period of New York and Pennsyl- 

 vania, where the coral-forming conditions progressed toward the 

 northwest, while the black Marcellus muds replaced them on the 

 south and east. 



Theories of Origin of Types of Coral Reefs. Of all the 

 reef types, the fringing reef alone requires no special explanation 

 of origin, since here the corals grow in shallow water vmder normal 

 conditions of habitat. All continental or island shores in the tropics, 

 in which the submarine slope is a moderate one, are suitable for 

 coral reef development, provided mechanical detritus is absent or of 

 negligible amount and the other necessary conditions, clearness and 

 saltiness of the sea, absence of large streams, sufficient wave activi- 

 ties, absence of cooling currents, sufficiency of food supply, etc., 

 concur. Corals will grow under such conditions out to the zone of 

 depth limit (20-25 fathoms). Where the submerged platform on 

 which the corals grow is increased in extent seaward by the accumu- 



