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, 
— а 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. 
А topographic peculiarity often seen in connection with the stone 
reefs that lie on or near the beach is well illustrated at several points 
along the northern end of Ше Cunhahú-Sibaúma 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 Gaibü is illustrated at 
page 70. 
E 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 
