branner: the stone reefs of brazil. 183 



That the deep cold ocean waters dissolve lime deposits readily is dis- 

 tinctly shown by the fact that deep dredging brings up little or nothing 

 in the way of calcareous shells and bones, and of sharks' teeth only the 

 hard dentine, and only the hard earbones of whales, while the larger 

 bones have been dissolved. The chemical process involved seems to 

 be the formation of bicarbonates, and later the deposition of carbonates 

 upon the liberation of carbon dioxide. 



In ordinary chemical laboratory work it is the practice to precipitate 

 carbonates from solutions by raising their temperatures. In such cases 

 it is understood that a part of the carbon dioxide is driven off by the 

 increase of temperature, while the lime or magnesia is precipitated as a 

 carbonate. This increase of temperature is not necessarily great, — not 

 near the boiling point. Similar increase of the temperature of ocean 

 waters is produced by the waters breaking in surf or rolling upon the 

 warm sandy beaches of the tropics. The whole process results, therefore, 

 in a general tendency for the carbonates to accumulate in the tropics. 

 But this accumulation must take place at or near the surface of the 

 waters, for the coldness and the pressure at great depths would keep the 

 carbonic acid free.^ Only when the pressure is relieved, and the temper- 

 ature raised near the surface, is there any opportunity for combination 

 and for the formation of carbonates. 



Waters carried from a cold sea into a warm one, or from the cold 

 depths to the surface and warmed suddenly, would yield up their car- 

 bonic acid contents. The form of the continental shoulder of northeast 

 Brazil, and the on-shore direction of the cui-rents, favor this process. 

 The accompanying map (Plate 1) gives in contours the relief of the sea 

 floor along the Brazilian coast out to the one-thousand-fathom line. 

 This map shows that the continental margin lies only from twenty-five 

 to thirty-five miles off the shore. When the oceanic currents strike this 

 steep submarine escarpment, the colder waters are swept up from the 

 depths, quickly warmed at the surface, and made ready to yield up their 

 carbonic acid contents. Observations upon the waters themselves bear 

 out this view. 



Waters of the Atlantic Ocean between Fernando de Xoronha and Per- 

 nambuco vary in temperature from 30° at five hundred fathoms to 78° 

 at the sui-fiice.- In genci-al the surface layer of warm water is thicker 



^ The Prince of Monaco says Dr. Jules Richard found that gases "are not dis- 

 solved in the depths at any other pressure than they are at the surface." Nature, 

 June 30, 1898, LVIIL, p. 201. 



2 Challenger Reports. Piiys. and Cheni. I. pt. I., Plate 78. London, 1884. 



