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bulletin: museum of comparative zoology. 



ward slope, rise about as high as the ])igh tides, and have flat tops, — 

 a topographic configuration in strong contrast with the rounded outlines 

 of the granites. The reef sands originally sifted into the crevices in 

 granites and hardened so as to enclose granite blocks here and there. 

 The rock is a rather fine-grained quartz sandstone of a light brown color. 

 It contains a few fossil shells. 



These fragmentary beach reefs and the Cape of Santo Agostinho itself 

 are separated from the great stone reef by a gap or break four hundi-ed 

 metres wide and, according to the hydrographic chart, 4|^ fathoms deep. 

 This gap is known as the Barra do Sudpe. It is the only break in the 

 great reef through which barcagas can enter the bay and the rivers that 

 flow into it. 



If the Su^pe break were restored, the total length of the Cape Santo 

 Agostinho reef from the cape to where it disappears beneath the sands 



Fig. 41. Section showing the relations of the stone reef and granite at Cape 

 Santo Agostinho. 



north of the cape would be thirteen kilometres. How much of its 

 southern end is buried beneath the beach sands we have no means of 

 knoAving. Even to-day, taking out the various gaps, we still have pre- 

 served a reef something more than twelve kilometres in length. 



A striking featuje of the Santo Agostinho reef is the long tide-pools on 

 its surface and running lengthwise of it. Toward the northern end 

 these pools are not so long or so deep, but toward the southern end there 

 is one pool a metre deep and nearly three kilometres in length. In the 

 photographs taken of the reef near Camboa, the reef has the appearance 

 of being double ; and in a sense it is double, for the pool is made by a 

 softer series of beds that dips beneath the harder ones that form the 

 reefs seaward face, and overlies another series of harder ones that forms 

 the landward face. 



From end to end the reef is nearly straight ; there is but one slight 

 seaward bend opposite Camboa Point. The surface is for the most part 



