BRANNER: THE STONE REEFS OF BRAZIL. 69 
signs of a submerged continuation of it beyond the northern end exposed 
at low tide. The axis of this reef points straight toward the lighthouse 
at Pernambuco. 
Near the northern end the reef has the profile given below. 
Fig. 86. East-west section across the stone reef at Venda Grande. 
The top of the reef is pretty well covered with barnacles and worm 
tubes. These barnacles and Serpulae seem to start most readily and to 
thrive upon the stone legs or sharp points left by the etching of the reef 
rocks, The surface pools contain heads of Porites, 
The rock is a yellowish, rather coarse sandstone, with many fossil 
molluscan shells in it. Shells of the same kinds are found in the sands 
in the breaks, and on the beach behind the reef, The rock is as hard as 
any seen on the reefs, — quite quartzitic in fracture. 
The stakes of an ancient fish-trap are still standing on this reef, the 
posts apparently driven in the hard rock. Upon inquiry it was learned 
that these stakes were not driven in sand which subsequently hardened, 
but that the holes for them were drilled in the rock. 
In some observations made upon the slope of wet sand behind this 
reef I found the steepest angle at which they stood to be 24°, This, 
however, was a face of false and not of true bedding. 
The Gaibá stone reef. — Gaibú Bay is the embayment immediately 
north of Cape Santo Agostinho. At its southern end rise the granite 
hills of the cape surmounted here and there by Tertiary sediments. 
At its north end is the promontory of Pedras Pretas, a rocky point of 
black porphyry hills not shown on the hydrographic chart. The рог- 
phyry hills stand boldly out in the ocean; inland they are capped by 
the Tertiary sediments; nearer the sea these sediments have been 
removed by denudation, and only the quartz and other pebbles left 
scattered over the surface of the porphyry. 
Between these two prominent points runs a line of Tertiary hills 
more or less notched on their edges, but Swinging inland so as to form 
а semicircular enclosure for the Bay of Gaibü. Between the hills and 
the bay is a strip of flat land, partly mangrove swamps, partly fresh- 
water marshes, and near the beach dry sands and some dunes. 
There are three small streams flowing from these flat lands: one of 
them discharges at the village of Gaibú at the extreme southern end of 
