36 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
and a half kilometres for the entire reef visible above water. North of 
the bar the surf breaks upon the concealed continuation of the reef in 
that direction. These breakers lie in the axis of the main reef. 
The various breaks and topographic relations of the reef are shown on 
the accompanying map made by my assistant, Mr. С. E. Gilman, and 
need not be verbally described. It will be seen that this reef is nearly 
straight. In width it varies from twenty to seventy-five metres. 
On that part of the reef that stands out from the land across the 
mouth of the river, and about six hundred metres south of the bar, is 
an old fortress now surmounted by a lighthouse. 
Seen from a distance the surface appears almost perfectly flat. It 
slopes gently toward the sea, but the bedding of the reef rock has a 
somewhat steeper though still a gentle slope. In detail the surface is 
in some places flat, in others it is etched in a manner characteristic of 
all the sandstone reefs, and well illustrated in the photograph of the 
etched surface of the Mamanguape reef. This etching is caused by the 
removal of certain portions of the upper beds of the rock, and the leav- 
ing behind of other and more resisting parts which stand out upon the 
surface as sharp points or irregular slabs supported by short columns. 
These jagged points are usually from a few centimetres to three decime- 
tres high, but sometimes they are a metre high and make it difficult to 
walk over the surface. 
Where the reef is broken and the surface has fallen in so as to be 
lower than the general level, it is uncovered but little as compared with 
the higher parts, and here the surface of the fallen blocks is overgrown 
with barnacles and is black with seaweeds and corallines and is some- 
times bored by sea-urchins. 
Along the outer or seaward face the reef is more or less protected 
from the force of the waves by enormous slabs that have been dropped 
where they now lie by the undermining of the original reef by the sea. 
These slabs lie tipped about at various angles, but generally with their 
outer ends dipping abruptly beneath the sea, and thus forming an 
effective breakwater against the onslaught of the surf. Here and there 
the outer face is broken off abruptly. The entire seaward face of the 
reef is covered with corallines and other Algae, while the somewhat 
protected parts are furrowed and bored by sea-urchins. The cavities 
over this outer face of the reef and the seaweeds that grow there abound 
in the forms of marine life that generally inhabit such places. 
The inner or landward face of the reef along its southern end lies 
against the land, or rather the sands of the shore come down upon and 
