PALEOZOIC CORAL REEFS 239 



primarily a limestone formed through the activity of organisms secreting 

 carbonate of lime. 



Since the physical conditions prevalent during the formation of fossil 

 coral reefs can not be ascertained by direct observation, it is necessary to 

 resort to the process of deduction. Evidence for establishing criteria 

 may be derived from two sources: The first, through the study of the 

 conditions under which modern reefs are formed, and the determination 

 of the factors necessary for the physiologic activity to which large accu- 

 mulations of calcium carbonate are due ; the second, through the investi- 

 gation of the physical character and the nature of the bedding or stratifi- 

 cation of the sediments in which the fossil reefs are embedded. An 

 attempt will be made to derive criteria from both these sources and to 

 apply them in the elucidation of the problem. 



Eecent Coral Eeefs 

 composition 



A modern coral reef is not entirely composed of the skeletons of corals, 

 the remains of nullipores, mollusks, echinoids, and littoral foraminifera 

 forming important constituents. Eeef corals do not exclusively belong 

 to the Madreporia, the Alcyonaria and Hydrozoa both contributing a cer- 

 tain quota of niaterial. In the succeeding remarks the distribution of 

 these groups of organisms with reference to depth and intensity of light, 

 temperature, motion of the water, character of bottom, composition of 

 the oceanic salts, and specific gravity of the water will be considered. 



DEPTH OF WATER AND INTENSITY OF LIGHT 



The literature on the maximum depth at which reef-building Madre- 

 poraria may grow vigorously is extensive, and there is practical unanim- 

 ity among all investigators that 25 fathoms is the greatest depth at 

 which they work effectively, although an occasional reef species may ex- 

 tend downward to a depth of 40 fathoms. The most luxuriant growth, 

 however, is in shallower water, from just below low tide level to perhaps 

 10 or 15 fathoms. These bathymetric limits of the Madreporaria 

 usually apply to the Alcyonarian Heliopora and Tuhipora, the Hydroid 

 Millepora, and the N'ullipores, although Heliopora and Millepora in the 

 Maldives are important constructional agents to depths between 35 and 

 40 fathoms, and Nullipores extend to a depth of 35 fathoms.^ 



Several factors besides pressure are correlated with increasing depth. 

 One is the intensity of light. In this connection the following remarks 



2 J. Stanley Gardiner : Fauna and geography of the Maldive and Laccadive archipela- 

 goes, pp. 342-326. 



