REVIEWS 751 



17. The mangrove swamps have been important agents in building up the 

 newly formed land about estuaries and embayments. 



18. The sands of the coast are not of foreign origin, as has been surmised, 

 but are derived from the adjoining headlands, or they have been brought down 

 from the land by streams. 



The discussion of the consolidation of the reefs and of the origin of the 

 cementing material is very full and able, assembling and treating critically 

 and judicially a wide range of data and of phenomena, with ample references 

 to the literature of the subject. One of the most interesting points is the 

 relation of the density of the sea-water to the deposition of calcium car- 

 bonate, and the genetic connection of the stone reefs with the density of 

 the adjacent oceanic waters and with the surrounding climatic conditions. 

 The conclusions relative to the consolidation of the reefs are as follows: 



Stone reefs are formed where there are streams or lakes of fresh water entirely 

 or partially restrained by the beach sands. The new reefs may be formed either 

 in front of the old ones, or in the embayment and estuary behind the older ones. 

 For similar reasons, stone reefs may form behind or landward of the coral reefs. 

 This can only happen, 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 hthification 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 leads 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 consolida- 

 tion 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. 



