22 Annals of the Carnegie Museum. 



Crandall, shows the entire length of this reef, and gives a good idea of its 

 remarkable appearance. This reef is of hard sandstone, and varies in 

 width from ten to seventy meters. It is barely covered at times of high 

 tide, but at low tide it rises above the water like a broad flat wall against 

 the outer face of which the surf breaks. 



Reefs like this of Sao Miguel are found at several places along the north- 

 eastern coast of Brazil, but this is among the most beautiful of them all. 



The origin of these reefs of rock has been discussed at length elsewhere.® 

 and it only remains to explain this particular one. 



As pointed out in the paper on the stone-reefs of Brazil the existence 

 of these remarkable natural breakwaters is due to a peculiar combination 

 of circumstances. These are the distribution of the rain through the year, 

 the size of the streams, and the geographic conditions where the streams 

 enter the ocean. The Sao Miguel reef lies in front and across the mouth 

 of the Rio de Sao Miguel, a stream which is only about seventy-five kilo- 

 meters in length. 



The banks of this stream are everywhere covered with vegetation, and 

 along the lower part of its course the waters pass through grassy marshes 

 and mangrove swamps. During the rainy season there is water enough 

 in this river to cause its prompt discharge into the ocean, but during the 

 dry seasons the stream is so enfeebled that it has not always been able to 

 prevent the waves of the sea from completely stopping it by throwing 

 up the beach sands across its mouth. At such times the sluggish water of 

 the streams becomes highly charged with the acids derived from the or- 

 ganic matter decomposing in and along it, and this water has escaped 

 largely by seeping through the sand bank across the river mouth. The 

 acid water coming in contact with shell fragments and other limy materials 

 in the sand dissolves the lime and carries it forward until it comes in 

 contact with the dense sea waters within the sand bank, where the lime 

 is precipitated. The sands that were thrown across the river's mouth 

 are by this process turned into a sandstone so compact that it often rings 

 under the hammer like a bell. 



The shells found in the rock of the reef are those of mollusks which are 

 now living in the sea alongside, and may be found loose in the sands of 

 the beach. The accompanying photograph (Plate IV) shows a specimen 

 of the rock containing shells. 



8J. C. Branner, "The stone reefs of Brazil, etc.," Bull. Mus. Comp. Zool., XLIV. 

 Cambridge, 1904, 171-200. 



