BRANNER: THE STONE REEFS OF BRA.ZIL. 193 



It is not improbable, however, that the streams may be affected 

 physically as well as chemically by climatic conditions. 



Possible influence of climate. — In Chapter V., p. 143, it has been shown 

 that only the strong streams are able to enter the ocean boldly ; weaker 

 ones are compelled to seek outlets behind barriers, such as spits, reefs, 

 headlands, or other protection ; while the still weaker ones are often closed 

 altogether by the embankments of sand thrown across their mouths by 

 the ocean. 



In so far as precipitation is concerned, the climatic conditions of north- 

 east Brazil are very variable. As a rule, even in years of abundant rains, 

 the precipitation takes place in a few months, and the rest of the year is 

 very dry.i For this reason the streams are spasmodic at best. But 

 there are from time to time drouths that last for years. During these 

 prolonged dry periods, all but the larger streams are completely dried 

 up, or are converted into a string of stagnant pools. Streams in such 

 a region must have their mouths closed during these long dry periods, 

 even though they may be abundantly able to keep them open in seasons 

 of heavy rains. 



In all cases where the mouths of streams are closed, the 'waters are 

 held in pools behind the embankments, and obliged to escape to the sea 

 through the beach sauds. Xow the fresh-water pools thus shut out 

 from the sea are the homes of enormous quantities of aquatic plants, and 

 their margins are covered with a dense vegetation dropping its dead 

 leaves and fruits in the water. This organic matter, upon decomposition, 

 charges the water with organic acids. As the waters percolate seaward 

 through the sands of the beach, especially during low tide, when the 

 hydrostatic pressure of the water in the pools forces them forward, they 

 attack the calcareous matter with which they first come in contact, and 

 carry it along in solution until they encounter the sea-water. Hero 

 there would be a tendency for the less soluble lime to be precipitated, 

 especially if there were a checking of the movement of the water. 



Precipitation would thus take place along a beach-line, and would be 

 confined to a rather narrow belt, for its width would be something less than 

 that of the confining sand embankment, while its length avouM be deter- 

 mined by the coastwise length of the pools and of the restraining beaches. 



The mouths of streams closed in this manner would in time be open 

 again, and there would be established a long slender reef of hard sand 

 rock with a break in it, through which the stream would discharge. 



1 J. C. Branner. Decomposition of rocks in Brazil. Bull. Geol. Soc. Amer., 

 1896, VII., p. 309-312. 



vor.. xr.iv. 13 



