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 Brazilian stone reefs is chiefly lime 
carbonate. 
The hardening of beach sands may be produced in the following 
ways: — 
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 processes 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 temporarily 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 
'gea-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 embayments. 
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