2 J. C. BRANNER — -STONE REEFS ON OOAST OF BRAZIL 



south latitudes 3 degrees 43 minutes and 16 degrees 30 minutes. Onl}^ 

 one of these reefs is where it is readily accessible to trans- Atlantic 

 steamers, and that is at the port of Pernambuco. The others are scat- 

 tered along a coast which has no commodious harbors, and are there- 

 fore but little known outside of Brazil. 



These stone reefs are unique or almost unique geologic phenomena. 

 Darwin, who saw the one at Pernambuco, says of it that he doubts 

 "whether in the whole world any other natural structure has so artifi- 

 cial appearance." Wonder at these structures is greatly increased when 

 one finds them repeated along the coast for more than a thousand miles. 

 Perhaps the clearest idea of them will be conveyed in fewest words if it 

 is stated at the outset that the reefs are lithified marine spits or beaches 

 that have been encroached on from both sides until their edges are more 

 or less broken and angular and their surfaces swept clean of the loose, 

 unconsolidated portions. This theorjr of their origin appears to be sup- 

 ported by the great bulk of the data collected. Only a few of the more 

 important features of the reefs, however, can be mentioned in this place. 



Where exposed at the surface the reef rock is sandstone, often almost 

 as hard as a quartzite and ringing under the hammer like a clink-stone, 

 but sometimes, and locally, it is only moderately hard. It contains 

 abundant shells and other remains of animals and calcareous plants, 

 apparently of the same species as those now living in the ocean along- 

 side. The beach sands in the vicinity of the stone reefs invariably con- 

 tain similar remains and abundant small fragments of shells, corals, and 

 of other lime-secreting organisms. Except in the matter of hardness 

 and compactness, the rock of the reef is therefore scarcely distinguishable 

 from the present beaches. The cementing material of the hard rock is 

 lime carbonate, which sometimes contains a little iron also. The rock 

 has a remarkably fresh appearance and the shells imbedded in it usually 

 retain their bright colors. 



A section across a reef invariably shows the beds dipping toward the 

 ocean at an angle varying from 2 to 20 degrees ; the lower angles are 

 the more common. In the gross structure no difference is apparent be- 

 tween the beds on the ocean side and those on the landward side. 



In 1874 three drill holes were put down by the Brazilian government 

 on the stone reef in front of the city of Pernambuco under the direction 

 of the English engineer, Sir John Hawkshaw. The records of these 

 holes furnish the only data we have of the thickness of the hard sand 

 rock and of the nature of the underlying strata. The deepest hole had 

 a depth of 17 meters. 



