CLOSED STREAMS / 



be expected, there were what are known in that country as " rios tapa- 

 dos" or shut up streams — that is, streams whose mouths are completely 

 closed by bars across them. At one place a stream was waded that was 

 cutting through a sand bar and running from the ocean landward ; at two 

 places gaps were found that had been cut in this way, but as they were 

 passed at low tide, there was no water flowing through the breaks. The 

 disposition of the sand, however, left no doubt about the water having 

 flowed from the ocean toward the stream. 



VEGETATION 



Another important fact is that the region under consideration is in the 

 tropics, and wherever there is fresh water, vegetation is rank. The banks 

 of streams are everywhere overgrown with a dense jungle, bodies of fresh 

 water are covered and filled with aquatic plants, while the densest of man- 

 grove swamps cover the tide flats of the region of salt and brackish water. 



DENSITY OF THE SEA WATER 



The other element of the problem is the density of the ocean water 

 along the part of the Brazilian coast on which the stone reefs occur. In 

 volume I of the Challenger reports, " Ph} T sics and chemistry," data have 

 been brought together and a chart constructed showing the oceanic areas 

 of different densities of the surface waters. This chart shows that the 

 areas of the highest densities are in the Red sea and the Mediterranean 

 sea; these, however, are land locked basins. The highest densities in 

 the open ocean form two areas in the Atlantic : one being near the mid- 

 dle of the ocean, lying between northern Africa and the West Indies ; the 

 other being in the south Atlantic and hugging the coast of Brazil from 

 cape Saint Roque to south of Rio de Janeiro. 



It is worthy of note that the area of high Atlantic density as repre- 

 sented in the Challenger chart does not quite fit the area of the stone 

 reefs. The map seems to show the dense area to be too far to the south 

 to suit the theory of the reefs here put forward. This is probably due 

 in part to a lack of density data, especially for the dry season, along the 

 coast of Brazil northwest of cape Saint Roque, and extending half way 

 from there to Para. The southern limit of the high density lies south 

 of Rio de Janeiro and far beyond the southern limit of the stone reefs. 

 The absence of reefs in this direction is readily explained by the fact 

 that rainfall on this part of the coast is very much larger than it is far- 

 ther north, and the streams are therefore able to keep their mouths open. 

 In connection with the subject of climatic conditions it should be noted 

 that the long dry season which enfeebles the streams and permits the 

 waves to dam them back must likewise be the season of the highest den- 

 sity of the sea water. The South Atlantic equatorial currents flow west- 



