44 bulletin: museum of comparative zoology. 



beach and finally merges into it. It appears to be perfectly straight ; 

 the slight curves of the entire reef are brought out only when one can 

 get a view of it from some high point from which he can see lengthwise 

 of it, or by a survey on a large scale. 



The landward face of the reef is in places broken square off, but for 

 the most part it slopes down rather abruptly but not at right angles. 

 The outer face is in most places broken squarely off. 



The top is flat, covered with small shallow pools, especially on the 

 outside, and with low etched points studding the inner or landward 

 margin. 



The rock of the reef is like that of most of these northern stone reefs, 

 — a light-brown sandstone of medium fineness, and varying slightly 

 from place to place in hardness, coarseness, and the abundance of fossil 

 shells. It contains many pebbles made of the red or black iron-stained 

 sandstone so common in and characteristic of the Tertiary beds of the 

 vicinity. It has also the usual fossil shells, though they are probably 

 not so abundant as they are in some of the other reefs. 



The sand-covered flat behind the reef is flooded at high tide ; when 

 the tide is out many big angular fragments of the reef rock are un- 

 covered and left projecting from the sand. I did not see on top of this 

 reef any loose blocks thrown up and left by the surf. The surf outside, 

 however, is very severe at times, for there are no outside coral reefs 

 to break the full force of the waves coming in from the deep ocean. 

 I am disposed to think that the angular blocks partly buried in the 

 sands behind the reef are pieces broken by the surf from the outer face 

 and thrown completely across it. 



A topographic peculiarity often seen in connection with the stone 

 reefs that lie on or near the beach is well illustrated at several points 

 alono- the northern end of the Cunhahu-Sibauma reefs: where these 

 inshore or beach " reefs are broken clear through the sea is able to 

 encroach upon the land, but only to a limited extent. The result is 

 that semicircular bays of sizes proportional to the width of the open- 

 ings are cut in these shores. A similar bay at Gaibti is illustrated at 

 page 70. 



The rocks of the southern end of the inner reef are covered with great 

 quantities of a sandstone of organic origin, — a kind of rock I have seen 

 only on this northern coast of Brazil. These rocks are formed by worms 

 that cement together sand grains in masses resembling sandstone boulders. 

 They appear always to be built upon hard rock bases. 



The material is not hard where found on the beaches, but can be 



