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. 36. 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 

 nioUuscan 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 Gaibit stone reef. — Gaibii Bay is the erabayment 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 por- 

 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 

 a semicircular enclosure for the Bay of Gaibu. Between the hills and 

 the bay is a strip of fiat 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 discharires at the villacje of Gaibii at the extreme southern end of 



