370 G. A. Waring— Beef Formations of the 



sands. The solvent action of the fresh water for lime carbo- 

 nate was increased by organic acids derived from decaying 

 vegetation, and the redeposition of the lime was mainly due to 

 the inability of the increasingly salty water to hold it longer 

 in solution. 



The conditions necessary to such an hypothesis are all 

 present along the coast where the reefs exist. The annual 

 rainfall is so irregular in amount and distribution that the 

 streams carry little or no water during a large part of the 

 year and their outlets become partially or wholly closed by 

 drifting sand. Vegetation, especially the mangrove and the 

 water hyacinth, grow luxuriantly in the tidal portions behind 

 many of these closed river mouths, and the decay of dead 

 leaves and other fragments of vegetation doubtless charges the 

 water with organic acids. This water gradually escapes to 

 the sea by percolation through the sands that block the way, 

 and at times by flowing long distances around the ends of 

 sand bars. The sand contains many fragments of shells, coral 

 and other calcareous animal remains, so that it may easily 

 yield the requisite lime carbonate for the cementing process. 

 Studies made by the Challenger expedition have shown that 

 the ocean water is unusually dense in an area that borders the 

 Brazilian coast approximately along the portion where the 

 reefs are found.* This greater density of the ocean water 

 tends to cause more rapid deposition of the lime, due to the 

 inability of the dense water to readily hold it in solution, and 

 is believed to be another important factor which caused the 

 formation of the reefs. 



Reefs north of Rio Grande do Norte. 



The structure and the mode of formation of the reefs have 

 been thoroughly discussed by Dr. Branner, and the present 

 paper is not written in an attempt to add anything to his con- 

 clusions in those respects. Dr. Branner's studies, however, 

 were confined to the reefs at and south of Rio Grande do 

 Norte, and as in February, 1913, the writer traversed a portion 

 of the coast north of that port, the following notes concerning 

 the region are presented. These notes are intended mainly to 

 record the calcareous sandstone reefs noted, but observations 

 on the other kinds of reefs seen are also given. 



The harbor at Rio Grande do JS : orte is formed by an especi- 

 ally well-developed reef, descriptions of which have been pub- 

 lished, f Since no reefs equalling it in development were seen 

 farther north, the illustration (fig. 1) is here presented, for 



* Challenger Reports, Physics and Chemistry, vol. i. 



f The stone reef at the mouth of the Rio Grande do Norte, by Dr. J. C. 

 Branner and C. E. Gilman, American Geologist, Dec, 1899, pp. 342-344. 

 The stone reefs of Brazil, by J. C. Branner, Bull. Mus. Comp. Zool., Geol. 

 Series vii, pp. 35-39, 1904. 



