PROCEEDINGS OF THE ACADEMY AND AFFILIATED 



SOCIETIES 



THE GEOLOGICAL SOCIETY OF WASHINGTON 



At the 243d meeting, at the Cosmos Club, April 12, 1911, the following 

 paper was presented and illustrated by lantern slides: 



The physical conditions under which Paleozoic coral reefs were formed. 

 T. Wayland Vaughan. 



As the physical conditions prevalent during the formation of fossil 

 coral reefs cannot 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 strati- 

 fication of the sediments in which the fossil reefs are embedded. The 

 distribution of reef corals and other organisms that form calcareous 

 reefs were considered with reference to depth of water and intensity of 

 light, temperature, motion of the water, character of bottom, composi- 

 tion of the oceanic salts and the specific gravity of the water. Especial 

 attention was paid to the diminishing strength of light with increase in 

 depth of the sea as limiting the downward extension of reef-forming cor- 

 als. In this connection attention was called to the fact that a1 the gov- 

 ernment wharf at Fort Jefferson, Tortugas, which is supported by iron 

 piles coated with cement, there are many corals on all the peripheral 

 piles while on those in the permanently shaded areas there arenone. 



The results obtained by Sir John Murray from studying the effects 

 of temperature on the distribution of lime secreting organisms were cited 

 as bearing on the problem under consideration. According to this in- 

 vestigator it appears "that the most favorable conditions are met with 

 in the warm, equable, tropical waters of the ocean, and here as a matter 

 of fact we find the greatest development of corals, and the largest num- 

 ber of lime secreting pelagic organisms. In the polar areas and in the 

 cold water of the deep sea there is, as is well known, a feeble development 

 of all carbonate of lime structures in marine organisms." 



The data on conditions under which modern coral reefs are formed were 

 summarized as follows: (a) depth, maximum, 25 fathoms: light, strong; 



(b) temperature, annual minimum 68° F., annual menu above 70 F.; 



(c) water, agitated and circulating; (d) bottom, firm or rocky, without 

 silty deposits; (e) composition of the oceanic salts, as for the oceanic waters 

 as a w r hole; (f) specific gravity, as for the ocean in general, range 1.02405 

 to 1.02748. Of these conditions shallow water, strong light, high tem- 

 perature, circulating water, a comparatively clean sea-floor, and a chem- 



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