О ааа 
BRANNER: THE STONE REEFS OF BRAZIL. 58 
tions are invariably of very hard rock that rings under the hammer ; 
they make walking over the surface of the reef in certain places almost 
impossible. Other points, rounded in outline, but standing at lower 
levels, are covered with barnacles. 
There are but few places over the surface of the entire reef that have 
the rock freshly exposed. On the outer face and wherever kept con- 
stantly moistened with salt water, corallines and other Algae grow in 
greater or less abundance. Barnacles abound in the gaps and depres- 
Fic. 25. Tide-pool formed by organisms ) 
on reef, 
sions in those places that do not receive 
N Be | 
the full force of the surf and yet are low 
enough or so situated as not to be long 
out of the water. Young barnacles speck 
the reef everywhere. The seaward edge, 
where the reef is flat, generally or always 
has a little low rim rising as a dam on 
its margin and enclosing shallow pools of 
water on top of the reef, Fig. 26. Sea-urchin burrows 
Oysters are found: in patches along the on the surface of a sand- 
inner face of the reef, especially where “агы reef; plan and sec- 
the ebbing tide brings the river water 
against it. There are occasional patches of 
to this inner part of the reef. 
A large part of the outer ‘surface and much of the top of the reef 
where it is kept constantly wet by the surf is full of holes 
occupied by sea-urchins. These holes are two or three 
of various lengths, from a few centimetres to two o 
They show a decided tendency to lie par 
direction of the waters that wash over tl 
cut, as shown in the profile herewith, 
These trenches are not full of sea-urchins, 
dead oyster shells clinging 
and channels 
inches deep and 
r three metres. 
allel to each other and with the 
lem. In section they are under- 
but have a few individuals 
Scattered through them, or, at most, are half full. 
