196 bulletin: museum of comparative zoology. 



however, in places where marine currents prevent the land-water from 

 interfering with the growth of coral reefs. 



The local lithification of the sea beaches is not uncommon, but the 

 most noteworthy instances of lithification on a large scale are those of 

 the northeast coast of Brazil and of the Levant. 



The cementing material of the Braziliaji stone reefs is chiefly lime 

 carbonate. 



The hardening of beach sands may be produced in the following 

 Avays : — 



1. By carbonated rain-water dissolving out the lime carbonate in the 

 upper portions of calcareous sands and depositing it in the lower portions. 



2. By the escape of carbon dioxide from the sea-water when the surf 

 breaks upon the beaches. 



3. By the escape of carbon dioxide from sea-water where it is warmed 

 by the tropical sun. 



4. By the submarine escape of carbon dioxide about volcanic vents. 

 These pi'ocesses may have contributed somewhat to the hardening of 



the Brazilian reefs, but they do not seem competent to account for them 

 altogether. These theories are especially incapable of accounting for 

 the lithification of beaches behind older reefs. 



The distribution of the consolidated beaches of northeast Brazil lead 

 to the inference that the consolidation is directly related to the density 

 of the sea-water. The geology and climatic conditions over the adjacent 

 land are, however, important factors in the hardening of the reef sands. 

 It seems probable that the consolidation of the reef sands would not 

 take place if the rainfall were large enough and constant enough to keep 

 the mouths of the streams open and the water of the streams fresh. 



In a region of concentrated rainfall and long drouths the river mouths 

 become temporai'ily closed, and the abundant aquatic and other life in 

 the lagoons thus formed contributes to the organic acids of the waters, 

 ■which, upon penetrating the wall or dam of beach sand, first dissolves 

 the lime, and then redeposits it when it comes in contact with the dense 

 sea-water on the ocean side. In this manner some portions of the 

 beaches have been hardened, while others have remained incoherent. 



The density of the ocean water is in all probability considerably 

 greater during the dry than during the rainy season, and this would 

 still further hasten the consolidation of the beaches during dry seasons. 



The process of beach hardening is not a continuous one, but varies 

 with geographic and climatic conditions. New reefs may be formed 

 behind the older ones on the shores of the estuaries and embavments. 



