1 879-] Prof. Hartt on ilie Brazilian Sandstone Reefs. 349 



Immediately upon the organization of the Geological Commis- 

 sion of Brazil, in 1875, under the leadership of Prof Hartt, active 

 field operations were commenced in the province of Pernambuco, 

 and another opportunity was thus afforded the late chief to exam- 

 ine that most noted of all these natural breakwaters. With the 

 larger force and more perfect appliances now at his command, he 

 proceeded to carefully map out the reef and its surroundings, and 

 to study in detail every feature connected with it. This extended 

 investigation did not materially alter his previously-formed ideas 

 as to the true character and mode of growth of the reef, but there 

 were added many facts not before observed. 



Other stone reefs, at Cape Sto-Agostinho, Parahyba do Norte, 

 etc., were explored by the Commission, and the conclusions 

 arrived at, after comparing the results of all these studies, are of 

 great interest and importance, demonstrating that on the coast of 

 Brazil conditions obtain for the united working of a set of simple 

 forces which, apparently, are not perfectly combined, at least with 

 the same results, in any other part of the world. As we can best 

 understand these phenomena after becoming acquainted with the 

 structure and appearance of a single reef, we will first describe 

 the one at Pernambuco, partly in the very words of Prof Hartt, 

 and then discuss the subject as a whole. 



• Just to the south of the city of Pernambuco, a little river 

 breaks through the land, and would open directly into the sea 

 were it not that a narrow wall of stone, running in a general way 

 parallel to the shore, carries its course some distance farther 

 north, to beyond the limits of the city. Another small river, near 

 its mouth flowing parallel with the sea, and separated from it by 

 only a narrow strip of land, runs through the city of Pernam- 

 buco, and opens also behind the reef 



There is thus formed a small and very shallow bay, which is con- 

 tinued northward past the city, as a rather narrow channel, having 

 a width of only a hundred rods or less, and a depth of but a few 

 fathoms. The outlet to this channel is close to the northern end 

 of the reef, and is so shallow as to admit only coasting steamers 

 and vessels of ordinary draught. It is, however, the protecting 

 reef and not the narrow harbor that interests us now. 



Standing upon some prominent point near the shore, in Per- 

 nambuco, we can plainly trace this narrow strip of stone from 

 its commencement, about opposite the ancient fortaleza do Brum, 



