BRANNER: THE STONE REEFS OF BRAZIL. 167 



trated by the behavior of the silts about any extension of jetties about 

 harbors.^ 



Influence of the manr/ues. — The various forms of tide-marsh vegeta- 

 tion, especially the mangrove swamps, have played an important part 

 in the turning of shallow waters into marshes, and later into land. So 

 far as I have observed, the mangrove plants thrive only on new and 

 growing land deposits. The plants are influential in preventing scour 

 by the tides, and in holding the silts and other accumulations together 

 until they can be seized upon by other plants. Their wide and ever- 

 spreading roots, the new plants started both from seeds and from roots, 

 the protection they offer to various forms of amphibian life, make of 

 these mangroves a geologic agency of the first importance in the tropics 

 on the borderland between fresh and salt water— between land and 

 sea. Most of the mangrove plants one sees near the streams are only 

 from three to five metres high. At Cannavieiras, however, there are 

 enormous forests of mangrove-trees from fifteen to twenty-five metres 

 high, and with large straight trunks rising upon their straddling roots. 



Hyacinths. — In fresh waters the water hyacinth, known in Bi-azil as 

 Baronem, is an important geologic agent, choking up streams and lakes, 

 depositing organic accumulations over the bottoms of these water bodies^ 

 and even drifting out to sea, especially during the rainy season, in 

 enormous quantities. 



Origin of the coast sands. — It is not altogether germane to the present 

 discussion to consider the origin of the sands that form the dunes and 

 beaches of the northern coast of Brazil, and the subject would not be 

 referred to here if there were not erroneous theories current regarding 

 these sands. I have heard it maintained and have seen it stated that 

 they were brought to Brazil from Africa by the equatorial currents ;2 

 and also that they come from the Araazonas. Barao de Capanema 

 thinks the sands of the coast of Ceara come from the Serra do Araripe." 

 The carrying of the sands across the deep Atlantic is altogether out 

 of the question. Streams are unable to carry anv but lisht sediments 

 across deep portions of their channels, to say nothing ^of the South 

 Atlantic current that moves at a rate of from one to two kilometres an 



1 See cases of Calais and Dunkirk. Harbours and estuaries on sandv coa.t. 

 ay L.h. V ernon-Harcourt. Proc. Inst. Civ. Ensis., 1881-2 LXX p 3 G ' 



2 Thomaz Pompeo de Souza Brazil. Berthot and Moreau de Jonnes in Ensaio 

 -tstatistico da Provincia do Ceara'. p. 13, 49, 1863. 



8 Traballios da Commissfio Scicntifica de Explora^ao. I. Introduccao, C\X\V 

 Rio de Janeiro, 1862. » . * • 



