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 process on the Brazilian coast were the same as that in the | 
Mediterranean and Red 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 Ceará 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 limo 
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 percolation 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. 
= 
1 Murray shows that sea-water contains 2,000,000 tons of calcium. to the cubic 
mile, Nature, February 28, 1889, XXXIX., p. 426. 
