branner: the stone reefs of brazil. 175 



II. Deposited from the ocean water after having been derived (through 

 the agency of carbon dioxide) from calcareous organic bodies in the sea. 



III. Brought down from the land by streams. 



IV. Dissolved from calcareous beach sands by fresh water streams 

 entering behind them, and redeposited while passing seaward through 

 these sands. 



These sources will be considered in this order. 



I. Cement from the beach sands by raiyi-water or spray. — The dis- 

 solving of lime carbonate by rain-water from the upper layers of cal- 

 careous sands, and its redeposition a little lower down, is a well-known 

 phenomenon. Woodward mentions instances of blown sands having 

 been hardened sufficiently by lime and iron to be used for building pur- 

 poses.^ This hardening is especially common in warm regions where 

 abundant molluscan life and coral-forming animals, calcareous Algae, 

 gorgonias, and the like so often contribute largely to the beach sands. 

 Aeolian sandstones and the sands upon the shores of coral islands are 

 often hardened by this process. But, so far as I can learn, and so far as 

 my own observations go, the rocks whose cements are derived from their 

 own beds in this fashion ai-e so highly calcareous as to be practically 

 pure limestones. 



The rocks of Bermuda are spoken of by Vetch, Nelson, Rice, Agassiz, 

 Thomson, and Heilprin as newly-formed limestones. Rice says, " The 

 cement which converts all these fragmental deposits into solid rock is 

 formed by the solution of the calcareous particles themselves," ' and he 

 points out that these rocks are almost exclusively limestones derived 

 from shells and other calcareous fragments. 



Sir "VVyville Thomson says the sand of which the white granular 

 aeolian limestone is made "consists almost entirely of carbonate of lime. 

 . . . When rain . . . falls upon the surface of the sand, it takes up a 

 little lime in the form of bicarbonate, and then, as it sinks in, it loses 

 the carbonic acid and itself evaporates, and it leaves the previously dis- 

 solved carbonate of lime as a thin layer of cement, coating and uniting 

 together the grains of sand. . . . The extreme result is a compact, 

 marble-like limestone." ^ 



1 H. B. Woodward, The geology of England and Wales, ed. 2. p. 54G-o47. Lon- 

 don, 1887. 



The same author cites several instances of the induration of marine deposits 

 (p. 550-551). 



- Wm. North Rice, The geology of Bermuda. Bull. 25. U. S. Nat. Mus., p. 10. 

 Washington, 1884. 



8 C. Wyville Tiiomson. The Atlantic. Vol. I., p. 291-205. New York. 187S. 



