branner: the stone reefs of brazil. 91 



The sandstone reef lies at a higher level on the beach than the coral 

 reef, and has throughout most of its length a decided seaward dip. At 

 the southern end of its contact with the beach, however, the dip is re- 

 versed and the bedding looks very much as if it had been formed by 

 sand washed over and behind a low beach or spit. The rock is in some 

 places rather soft, in others it is quite hard and rings when struck with 

 the hammer. It contains many shells of living forms of mollusks, es- 

 pecially mariscos, and also calcareous Algae. 



Certain features about the southern end of these reefs are interesting 

 in connection with the coast history. 



There is here a mangrove swamp being encroached upon by the sea, — 

 a somewhat unusual process. The swamp, as will be seen from the 

 sketch, was formerly protected by the reefs, but the sea has gradually 

 broken down this barrier and, encroaching upon the shore about its 

 southwest end, has attacked the mangroves. In the embayment south 

 of the existing swamp and reefs the shallow bay contains a gi'eat many 

 dead stumps of mangrove trees still standing in place. 



Tlie stone reefs of the Prataay, Alagoas. — The Eio Pratagy is a small 

 stream, not more than sixty metres wide, entering the ocean eleven or 

 twelve kilometres north of Maceio. Where this stream enters, a small 

 sandstone reef lies square across its mouth. 



Both north and south of the Pratagy and within a short distance are 

 other reefs or fragments of stone reefs that may be considered as parts 

 of the one in front of the river's mouth. 



The chief features of the topography of most of this part of the coast 

 consist of a flat-topped plateau of Tertiary sediments rising abruptly 

 from near sea-level to an elevation of from fifty to a hundred metres. 

 The margin of this plateau is cut across here and there by streams com- 

 ing down from the interior and forming, especially near the coast, rather 

 steep-sided, flat-bottomed valleys. 



It is through one of these valleys that the Rio Pratagy enters the 

 sea. The narrow strip between the mouth of the river and the foot of 

 the Tei'tiary escarpment is low, flat, and mostly sandy, but near the 

 stream covered with mangrove swamps. Just back of the beach and 

 parallel with it is a somewhat higher bank of sand, apparently blown 

 up from the shore. 



The first or most northern of the Pratagy reefs begins on or near the 

 beach one kilometre northeast of the mouth of the river. It is here 

 only fragmentar}^, — one large piece after anotlier, but lying always in a 

 line, a little way out from the high-water beach and just covered at high 



