BRANNER: THE STONE REEFS OF BRAZIL. 143 
elevation of the sea-bottom, I would reply that an elevation of the coast 
would give the streams greater fall and velocity, and that, instead of silt- 
ing up as they are now doing, they would at once begin to erode their 
channels. 
The case of Rio São Francisco. —The Rio São Francisco appears to be 
an exception to the rule that governs the other streams of this part of 
the coast of Brazil. But the Sáo Francisco is a large stream — the 
largest one entering the Atlantic between the Amazonas and the Rio de 
la Plata. It has already been pointed out that there is a never-ending 
conflict between the land and the sea forces. Along this coast there is a 
tendency on one hand for the silts to be thrown back into the mouths of 
the streams, and on the other hand for them to be forced out to sea by 
the waters of the streams. What the result is depends upon the сош- 
parative powers of these two agencies. If the sea is the more powerful 
agent the mouth of the river is closed and its waters are compelled to 
dodge behind a reef, island, bar, or bend in the coast-line in order to es- 
cape to the sen. Sometimes they must wander for kilometres in search 
of this protected outlet, Sometimes it happens that the energy of the 
stream is temporarily increased by floods to such an extent that it is able 
to break across these wave-constructed barriers and enter the ocean boldly. 
But when the floods subside the waves resume their control, the new 
passage is closed, and the stream returns to its former channel. 
In the case of the Rio Sáo Francisco the volume of water is at all 
times so large that it is able to flow squarely into the ocean and to keep 
its immediate mouth elear by throwing out bars well in front of it. 
A slight elevation or depression is not likely to produce changes about 
the mouth of such a stream ав the Rio Sáo Francisco that would be as 
readily recognized as they would be about a smaller stream. The lower 
valley has already been silted up far in advance of the highlands, and as 
one enters the river from the ocean he sees only great stretches of low, 
flat delta lands about its mouth. The high ground is reached several 
kilometres upstream —at Villa Nova on the south side, and near 
Penedo on the north. The valley above these points is broad and 
flat, but it ends abruptly against the rather steep hills on either side. 
Iam unable to say whether this topography of the lower river is that 
of a drowned river valley, or is produced simply by the ordinary floods 
that annually spread over a large part of it. 
Higher up stream — at Piranhas — the river flows over a rock bottom, 
but this is one hundred and eighty kilometres from the present river 
mouth, 
