8 J. C. BRANNER — STONE REEFS ON COAST OP BRAZIL 



ward from near the coast of Africa and split on cape Saint Roque. In 

 the season of drouths the surface density must be considerably increased, 

 especiall}' near the land and the shallow continental shelf. 



Attention is called to the absence of reefs between Bahia and the 

 mouth of Rio Sao Francisco. This is attributed to the influence of the 

 large body of fresh water discharged by the Sao Francisco. The reefs 

 begin a short way north of the mouth of that stream, but the currents 

 set southward and shoreward along this part of the coast. This permits 

 the formation of reefs north and prevents it south of the stream. 



CALCAREOUS SANDS 



One more factor is this : The ocean alongside is warm and teems with 

 marine tropical life. There are many coral reefs along the coast, and 

 everywhere are shell-bearing mollusca, ichinoderms, crinoids, Crustacea, 

 worms, corals, calcareous algse, and other lime-secreting organisms. All 

 of these organisms contribute abundantly to the beach sands. 



The Course of Events 



With this data in hand, we may now observe the course of events on 

 the coast under consideration. We have an old coastline with long, 

 nearly straight, sanely beaches. The sands are rather coarse and are 

 commonly mixed with fragments of calcareous skeletons of the animals 

 and plants living in the ocean. Across old embayments the sands have 

 been thrown back landward by the waves until in many places the 

 weak drainage is partly or entirely shut off from the ocean by banks of 

 sand. In and about the pools, lakes, or sluggish streams thus formed, 

 abundant aquatic and semi-aquatic plants live and die. The fresh 

 water is thus rendered acid by the presence of large quantities of carbon 

 dioxide produced by organic decomposition. The acid water on the 

 land side percolating through the embankment of sand at low tide 

 attacks the calcareous matter in the sand and passes seaward with it in 

 solution, but as it comes in contact with the dense sea-water on its way 

 through the sand, the lime carbonate in solution is deposited in the 

 interstices between the sand grains. In time the interstices are com- 

 pletely filled, and the sand bank is hardened and so solidified that the 

 water can no longer soak through it. The process must then of a neces- 

 sity come to a halt at that particular place, and percolating waters must 

 either seek other loose sands or they must be turned aside by the now 

 hardened and impervious spit and compelled to flow through the open 

 channels. The percolation of fresh and salt waters would be very much 

 the same whether the fresh waters to landward were completely damned 

 in or were separated from the sea by a long spit, around which it had 

 to flow at ebb tide. Thus the streams by the help of the sea build of 



