84 bulletin: museum of comparative zoology. 



The lowest two beds of this section, below the yellow clay seam, are 

 Eocene Tertiary, and similar to the beds exposed on Rio Formoso, south 

 of the Guadalupe church. The beds above the clay seam rest uncon- 

 forraably against the early Tertiary as shown on p. 30. 



The sandstone reef of Rio Formoso (not including here the southern 

 end buried by the sands and to be spoken of further on) begins on the 

 southwestern side of the river or estuary on the Praia dos Carneiros, and 

 extends northeast in an almost straight line across the mouth of this 

 stream, and at right angles to it.'^ 



The northern end of the reef curves westward slightly but distinctly ; 

 this curve, however, is not apparent on the small-scale map given 

 herewith. 



On an average, the reef is twelve metres in width, and stands a little 

 more than one metre above water at ordinary low tides. Its entire 

 length is* two thousand and seventy metres. Its general appearance 

 varies somewhat with the locality : near the south shore, to which it is 



Fig. 52. Profile of tlie etched sandstone reef at Rio Formoso. 



united, it is broad and flat, and comparatively smooth. Structurally 

 this reef has a slight but very apparent seaward dip. In surface 

 contour, the outer or oceanward side generally slopes at an easy angle 

 along the whole length of the reef, and as it dips beneath the water 

 becomes covered by corals and other marine animals and plants and 

 their skeletal remains. In many places the top is covered by rough 

 jagged points left by the wearing or etching away of the softer or more 

 soluble surrounding parts of the rock. These points ai'e a third of a 

 metre or less in height, and often so close together and so sharp as to 

 make walking through or over them almost impossible. They are 

 extremely hard, and can be broken only by a sharp blow with a lieavy 

 hammer. When struck, the larger ones ring with a clear metallic 



up from the seaward side, and heaped upon the original bed, until they are from 

 five to seven metres thick, and three or four hundred metres wide. If there should 

 be any demand for such materials in this part of Brazil, a short tramway along 

 the front of these beds could deliver the sands on board of barcacas at the foot of 

 the Guadalupe hill. 



1 Liais speaks of a triple line of reefs at Rio Formoso. (L'Espace Celeste, 549.) 

 There are no such reefs at this place unless one considers the patches of coral 

 outside the stone reef as such. 



