branner: the stone reefs of brazil. 191 



The accompanying map taken from the chart of the "Challenger" Re- 

 ports, Physics and Chemistry, Vol. I., shows the areas of densest sea- 

 waters over the globe. The variation in density must be due to the 

 different rates of evaporation. The almost closed basins of the Medi- 

 terranean and Red Seas are in warm, arid, or partially arid regions, where 

 evaporation goes on very rapidly. Some of the streams entering these 



Fig. 95. Surface densities of sea waters ("Challenger"). 



basins are heavily charged with lime, so much so that fresh-water tufa 

 deposits are forming at many places on a large scale. On entering the 

 sea the dense salt waters tend to hasten the deposition of the lime, and 

 this takes place most readily upon the beaches where evaporation is 

 most rapid. 



The area of high sea-water density on the northeast coast of Brazil is 

 probably to be attributed to rapid evaporation on the surface of the 



