GEOLOGY AND PALEONTOLOGY OF THE CANAL ZONE. 241 



Wherever there are well-developed fossil coral reefs it seems safe 

 to infer that the physical conditions above enumerated prevailed. 

 It is unnecessary to discuss separately each item entered under No. 

 11 of the summary statement of the periods of coral reef develop- 

 ment on page 226. During upper Ohgocene time (the time of the 

 deposition of the upper part of the Chattahoochee formation) tropical 

 conditions extended in Georgia as far north as latitude 32° 4.5'. 



Hypotheses of the Formation of Coiial-Reefs. 



During the past few years elaborjite reviews of theories of the forma- 

 tion of coral reefs have been published by Prof. W. M. Davis, the 

 larger of which are referred to in tlie footnote below.^ These reviews 

 are valuable in presentmg most of the important coral-reef theories, 

 as they are understood by a physiographer, who is convinced of the 

 adequacy of the Darwinian hypothesis. Numbers of complex phe- 

 nomena associated with coral reefs are not considered, and his pre- 

 sentations are not in all respects satisfying. Prof. R. A. Daly has 

 reviewed the literature on coral-reef theory, particularly from 

 the standpoint of an adherent of the glacial-control hypothesis.^ 

 The literature on coral reefs is so enormous, that in the present paper 

 consideration can be given only to certain papers that largely deal with 

 coral-reef hypotheses or that contain information on areas herein dis- 

 cussed. The limitations of space cause me to omit references to 

 many papers of much merit. 



Three kinds of coral reefs are generally recognized, namely: (1) 

 Fringhig or shore reefs which, as the n£ime indicates, occur along the 

 strand line; (2) barrier reefs which occur at variable distances 

 off shore and have lagoons from 1 or 2 to as much as 30 or even 40 

 fathoms in depth between them and the strand line; (3) atolls, which 

 are ring-like and inclose lagoons above whose surface no land-masses 

 of importance protrude. 



As the relations of barrier reefs and atolls to the platforms above 

 which they rise constitute m the opinion of geologists the essential 

 part of the theory of the development of Recent reefs, the warfare 

 of coral reef theory has been waged over the interpretation of these 

 relations, which are the conditions of changmg or changed position 

 of the strand line and the part played by reef-forming organisms 

 as constructional agents. 



1. According to Darwin ^ and Dana/ corals first form a frmging 

 reef off the sloping shore of a subsiding land area; the reef grows 



1 Davis, W. M., Dana's confirmation of Darwin's theory of coral reefs, Amer. Joum. Sci., ser. 4, vol. 35, 

 pp. 173-188, 1913; The home study of coral reefs, Amer. Geogi-. Soc. Bull., vol. 46, pp. 561-577, 641-654, 721-739, 

 1914; A Shaler memorial study of coral reefs, Amer. Joum. Sci., ser. 4, vol. 40, pp. 223-271, 1915; Problems 

 associated with study of coral reefs, Scientific Montlily, vol. 2, pp. 213-333, 479-501, 557-512, 1916. Also 

 several short articles in Nat. Acad. Sci. Proc, vols. 1, 2, 1915-1917. 



2 Daly, R. A., The glacial-control theory of coral reefs, Amer. Acad. Arts and Sci., vol. 51, pp. 157-251,1915. 

 "3 Darwin, C. R., Structure and distribution of coral reefs, ed. 3, fig. 5, p. 134, fig. 6, p. 137, 1899. 



* Dana, J. D., Corals and coral reefs, ed. 3, pp. 263, 267, 1890. 



