bkanner: the stone reefs of brazil. 197 



VII. 



The Age of the Sandstone Reefs. 



If the conclusion reached from the study of the process of consoli- 

 dation is correct, we may reasonably suppose the formation of the stone 

 reefs has been going on, wherever the local conditions were favorable, 

 ever since the depression of the coast in early Pliocene times. The 

 principal facts that support this view are : — 



1st. The stratigraphic relations of the reef rocks to the other rocks 

 of the region. 



2d. The relations of the reefs to the adjacent shores. 



It is possible that the fossils in the reef rock, in connection with a 

 study of the living fauna of the coast, would bear out this theory, but 

 no study either of these fossils or of the coast fauna has ever been made. 



Stratigraphic relations. — The rocks exposed along the coast on and 

 against which the stone reefs are built vary in age from Archaean to 

 recent. 



At Bahia, the stone reef rocks abut against ver}'' old crystalline 

 schists and eruptives ; at Cape Santo Agostinho they rest directly 

 against granite.s assumed to be of Archaean age. At many places water- 

 worn and subangular fragments of the ix'on-cemented sandstones, so 

 common in the weathered Cretaceous and Eocene Tertiary rocks, 

 are common in the reef rocks. Mention will be found of some of these 

 instances under the detailed descriptions of the reefs in Chapter III. 



At Riacho Doce in the State of Alagoas, Tertiary (?) shales are 

 exposed upon the beach. These shales, and the granite boulders accom- 

 panying and underlying them in some places, have patches of coral 

 reefs lapping over them, and at two places on this beach I observed the 

 reef sandstone on top of the coral reef. One of these exposures is two 

 hnndred metres long. At another place the sandstone reef lies directly 

 upon the Eocene rocks. 



