BRANNER: THE STONE REEFS OF BRAZIL. Ch 
Many of the minor pools on top of the reef contain caleareous growths 
and seaweeds in cavities built up by Serpulae, etc. 
Sometimes these calcareous growths fasten themselves on points of 
sandstone. Here and there sea-urchins with their burrows cover con- 
siderable areas, especially on the seaward faces of the reef where the 
Fic. 46. Points on surface of sandstone reef covered with Serpulae and seaweed. 
waves constantly renew the water. These burrows are for the most part 
parallel with the currents that flow across them, and hence mostly at 
right angles to the trend of the reef. 
few of these burrows. 
These pits are only partially occupied by sea-urchins. They contain 
also corallines and some corals (Porites). They are cut in the hard 
surface sandstone, and are from 
three to ten centimetres deep. The 
. edges of these pits are generally 
Overgrown with Serpulae. In the 
pools of the reef's surface are occa- 
sionally found water-worn lumps of 
Serpulae tubes, apparently broken 
from the reef and rolled by the Fic. 47. Plan of the pits on the sur- 
Maia face of Cabo Santo Agostinho reef. 
The following is the plan of a 
There are some cracks in the surface of the reef, most of them run- 
ning lengthwise of it, 
varied where the 
tured area, 
The cracks become more abundant and more 
gaps occur, and they continue go clear 
In other portions of the reef they are less 
on the other stone reefs examined. 
The rock of this reef is, on the whole, finer of g 
average, 
across the frac- 
abundant than 
rain than that of the 
It contains some pebbles, some of them bl 
ones washed from the mottled Terti 
hard, light 
ack iron-colored 
ary plateaus. Most of the rock is 
» Sugar-brown sandstone cemented with lime 
Everywhere the body of the reef is covered with Algae ; on top they 
are mostly green. It is etched a little on the landward side, probably 
by the acid waters of the streams, just enough to bring out the bedding, 
carbonate. 
