192 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



equatorial current that sets westward from the coast of Africa and 

 crosses the whole width of the Atlantic under a tropical sun. By the 

 time these waters reach the Brazilian coast, the density is greatly 

 increased by evaporation. 



If the pi'ocess on the Brazilian coast were the same as that in the 

 MediteiTanean and Eed Seas, the lime would be derived from the 

 streams. This may or may not be the case. Unfortunately I have no 

 analyses of the waters of the coast streams; but the geology of the 

 region leads to the conclusion that some of the streams are heavily 

 charged with lime, while others contain very little lime. The Creta- 

 ceous areas of Brazil are, or have been, largely limestone areas. From 

 the Abrolhos to Ceara sedimentary rocks cover most of the coast. 



In the introductory geological sketch of the coast it has been pointed 

 out that where the sedimentary beds are unaltered, they contain con- 

 siderable lime, even when they are not limestones. It has also been 

 noted that the upper portions of these rocks along the whole length of 

 the coast where the stone reefs occur, have been profoundly weathered 

 and chemically altered. It seems probable, therefore, that during the 

 process of the alteration of these upper beds large quantities of lime 

 have been removed from the land and carried by the streams into the 

 already heavily charged sea-water.^ It is a striking and significant fact 

 that the area of high density of the sea-water along the coast of Brazil 

 corresponds closely with the distribution of the stone reefs. 



Whether the cementing material was brought down from the land or 

 was thrown upon the shores from the ocean, it seems that the unusual 

 amount of hardening of beach sands along the northeast coast of Brazil 

 is to be attributed indirectly to the density of the sea-waters in that 

 region. 



IV. The seaward 2}ercoIafion of acid land-water. — The density of 

 the sea-water is, or may be, an element of controlling importance in the 

 hardening of the reef rocks. It is also evident that this density is 

 directly attributable to climatic conditions. But climatic conditions 

 affect the land and its streams even more than they do the ocean. 



No analyses of the waters of the streams of northeast Brazil being avail- 

 able, it cannot be said whether or not those streams are heavily charged 

 with lime. But speaking from a general knowledge of those streams, I 

 should say that outside of the State of Sergipe their waters are not 

 so charged. 



^ Murray shows that sea-water contains 2,000,000 tons of calcium to the cubic 

 mile. Nature, February 28, 1889, XXXIX., p. 426. 



