branner: the stone reefs of brazil. 181 



We have no means of determining the age of the rocks of these islands, 

 but the freshness of the lavas and the rapidity with which the shores are 

 being eroded by wave action lead us to infer that they are new.^ 



The position of these eruptive islands affords a basis for the theory 

 that either at the time of the extrusion of the rocks that form the group, 

 or at some subsequent eruption, large quantities of carbon dioxide may 

 have escaped into the sea. The South Atlantic current at this latitude 

 flows westward, but during eight months of the year (October to May, 

 inclusive) the northeast trade winds shift it somewhat toward the south, 

 so that during those months the oceanic currents that pass Fernando 

 flow toward Cape St. Roque, and thence pass southward along the east 

 coast of Brazil, past Rio Grande do N^orte, Pernarabuco, Rio Formoso, 

 Bahia, etc. These currents would have thrown the carbon dioxide, had 

 they contained it, against the shores along the northeast coast of Brazil 

 from Cape St. Roque southward. 



But if submarine discharges of carbon dioxide have taken place, and if 

 beach sands have been hardened as here suggested, we may reasonably 

 expect to find beach sands similarly consolidated in other parts of the 

 world. There are indeed many cases known of the hardening of recent 

 beach sands. In only two instances, however, do they appear to have 

 been hardened by carbon dioxide escaping from submarine vents : these 

 are in the Straits of Messhia between Sicily and Italy, and on the sliores 

 of the Red Sea.'^ In both of these cases the consolidation has taken 

 place in the vicinity of volcanic activity. 



We seem wai-ranted in the conclusions that carbon dioxide of volcanic 

 origin is discharged beneath the sea ; that it is competent to cause the 

 hardening of beach sands ; and that the existence of a volcanic island 

 off the northeast coast of Brazil makes it possible that such discharges 

 may have taken place there. But inasmuch as the lithification of the 

 coast sands of the region is in various stages of development, and as 



^ Northwest of Fernando de Noronha and eighty miles away is another small 

 island known as the Rocas. This island is about two miles long (east-west) by one 

 and three-fourths miles wide, and was until lately uninhabited. I have never 

 visited the place, but from information kindly furnished me by Commandante 

 Huet Bacellar, of the Brazilian navy, who has visited the place, I conclude that 

 the rocks there are corals. But whether the corals have igneous rocks beneath 

 them, I have been unable to learn. Except some dunes on the southwest corner, 

 it is all covered by water at high tide. On the Rocas. see p. 2*26-'227 of this report 



2 Ilawkshaw, Quar. -Tourn. Geol. Soc, 1879. XXXV.. p. 242. 



Suess says history mentions many eruptions ou the Red Sea. Face de la Terre, 

 p. 478, note. 



