1879.] Prof. Hartt on the 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, 
