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 tliey are now doing, they would at once begin to erode their 

 channels. 



The case of Rio Sao Franci><co. — The Rio Sao Francisco appears to be 

 an exception to the rule that governs the other streams of this part of 

 the coast of Brazil. But the Sao Francisco is a large stream — the 

 largest one entering the Atlantic between the Araazonas 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 com- 

 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 sea. 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 Sao 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 clear 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 as the Rio Sao 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. 



I am 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. 



