BRANNER: TllK STONE REEFS OF BRAZIL. 261 



Eio Formoso, Maceio, and Tamandare may be taken as tlie type of this kind 

 of geographic relations of the reefs. The reefs in such places are usually 

 long and slender, and it is believed that they are younger than the large 

 reefs. These forms are so constant that one profile can be substituted 

 for another without modification of any of the essential features. In the 

 cases of the largest reefs, such as those of Cape Sao Eoque reefs, the 

 Parcel das Paredes and the Abrolhos reefs, the adjoining coasts are low, 

 and the coral reefs are probably older and thicker than they are off the 

 steep shores of the coast of Pernambuco and Alagoas. 



In at least one instance it seems probable that the coral reef (that at 

 Parahyba do Noi-te) has taken possession of and is now growing upon a 

 submerged stone reef. Briefly stated, the reasons for this opinion are : — 



I. A deep well sunk at Cabedello inside the reef penetrated only the 

 soft coastal sands. 



II. The reef lies across the ancient mouth of the Rio Parahyba do 

 Norte, — the position in which the stone reefs of the coast are usually 

 formed. 



The coral reef could live in its present position, however, only after 

 the formation of the Ponta da Matta spit, which turned the river waters 

 away from the reef. The coral of the Parahyba reef is probably less 

 than five metres in thickness. 



The actual thickness of the reefs can, in my opinion, be ascertained 

 with absolute certainty in but one way, and that is, by boring into the 

 reefs af a large number of x)! aces. Some idea of their thickness can be 

 had, however, by working out the geological and geographical history of 

 the coast. The first method it has not been convenient to employ ; the 

 second one has been made nse of in the present paper in so far as it has 

 been possible to make out the coast history. We must conclude, there- 

 fore, that the coral reefs of the Brazilian coast probably nowhere exceed 

 a thickness of one hundred metres. Most of them are much thinner 

 and do not exceed fifty metres. The greater part of them are even 

 thinner tlian this. 



The at/e of the coral reefs. — The existing coral reefs are necessarily de- 

 scended from the ancient ones. But the geologic and geographic history 

 of the Brazilian coast cannot be traced with much precision further back 

 than Tertiary times. 



Magnesian limestones and dolomites found among the Cretaceous 

 rocks of Sergipe and in the Cretaceous (or Tertiary T) rocks of Pernam- 

 buco, and Rio Grande do Norte show that coral reefs existed on this 

 coast in Cretaceous times and the present reefs must be descended from 



