CORALS AND CORAL REEFS VAUGHAN. 225 



climatic conditions the great ice caps melted, and the water, thus re- 

 leased, flowed back to the sea, raising its level by an amount equal to 

 the quantity of water returned to it. The warmer waters were favor- 

 able for the growth of reef corals, and coral reefs grew luxuriantly 

 on flats, partly formed by Pleistocene wave-cutting, during the pe- 

 riod of moderate and gradual submergence following deglaciation. 



CRITICAL EXAMINATION OF THE DIFFERENT THEORIES OF THE FORMATION 



OF CORAL REEFS. 



The Semper-Murray theory will be discussed first, for it can 

 be eliminated from further consideration. By referring back to 

 page 217 of this article, it will be seen that present evidence is con- 

 vincing that neither a lagoon channel nor the lagoon of an atoll can 

 be formed by the solvent effect of sea water in coral reef areas, and as 

 lagoons in general are areas where the deposition of sediment pre- 

 dominates over its removal, they must be explained by an inclosing 

 and not by an excavating process. However, in small areas local 

 destruction may predominate over construction, but such localized 

 destruction will not explain the phenomena presented by lagoons. 



Both of the other two explanations are in agreement as regards 

 the part played by submergence in the formation of offshore coral 

 reefs, which include those of the barrier kind, but differ in that ac- 

 cording to the Darwin-Dana hypothesis the flat lying shoreward of 

 a barrier is due to infilling and leveling behind the reef, while ac- 

 cording to the other explanation the reef has grown upon the sur- 

 face, usually the outer edge, of a flattish area that antedates the 

 presence of the reef. 



The evidence bearing on submergence will be briefly reviewed, be- 

 ginning with some of the criteria used in inferring such a change in 

 position of land with reference to sea level. One of the first recog- 

 nized kinds of evidence indicating submergence of the land is the 

 presence of arms of the sea extending into the land area and oc- 

 cupying the lower parts of valleys to be accounted for only by stream 

 erosion operating at altitudes above present sea level. Plate 35, 

 figures D, C, illustrates submerged lower courses of valleys in the 

 islands of Antigua and St, Thomas, West Indies ; text figure 9 illus- 

 trates a part of the shore of Antigua, where it is deeply indented by 

 arms of the sea that as a result of submergence of the land extend 

 up valleys eroded when the land stood higher above the sea level than 

 it does at the present time. Figure B, of plate 35, illustrates a 

 view looking toward the head of Santiago Harbor, Cuba, and figure 

 A of the same plate is a view looking seaward through the harbor 

 mouth. Text figure 10 illustrates a cross section of Habana Harbor, 

 showing that within the harbor there is a filled channel, which must 



