a 
BRANNER: THE STONE REEFS OF BRAZIL. 167 
trated by the behavior of the silts about any extension of jetties about 
harbors.! 
Influence of the mangues. — 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 Brazil ав 
Baroneza, 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 у 
and also that they come from the Amazonas. Barão de Capanema 
thinks the sands of the coast of Ceará 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 any but light 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 sandy coasts. 
Ву Г. Е. Vernon-Harcourt. Proc. Inst. Civ. Engs., 1881-2, LXX., p. 8, 6. 
? Thomaz Pompeo de Souza Brazil. Berthot and Moreau de Jonnes in Ensaio 
Estatistico da Provincia do Ceará. р. 18, 49, 1868. 
8 Trabalhos da Commissão Scientifica de Exploragäo. I. Introducção, OXXXV, 
Rio de Janeiro, 1862. 
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