70 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the bay, another about one third, and the other two thirds of the way 
from Gaibü to Pedras Pretas. 
Lying along the beach of Gaibü Bay, but here and there a little way 
out from it, is a sandstone reef ex- 
tending nearly all the way from 
Pedras Pretas to the edge of the 
village of Gaibú. It is three kilo- 
metres or more in length and 
varies in width from forty-five to 
two hundred and forty metres, — 
a remarkable width for a stone 
reef, At low tide it stands two 
metres out of water at its highest 
Ponta de Pedras 
points. It has a gentle seaward 
dip. The rock is of the ordinary 
sugar-brown sandstone and con- 
tains an abundance of fossil marine 
shells. The surface has the etched 
appearance во characteristic of the 
stone reefs. 
The accompanying photograph 
(Plate 51) taken from the granite 
hills south of the bay shows the southern end of this beach reef. 
Fic. 97. Gaibú Bay. 
Fic. 58. Plan of a miniature bay behind a break in the Gaibú reef. 
South of the middle of this reef is a small break through which the 
waves have been able to encroach upon the beach and to form a minia- 
ture bay protected by the ends of the stone reef. 
