386 G. A. Waring — Reef Formations of the 



In a few places, notably at Touros, Ponta Peducto and Tres 

 Irmaos, the layer or zone that has been hardened by the concen- 

 tration of the iron rises a number of feet above sea level, but 

 in most places the hardened layer is only a little above high 

 tide limit, as is shown in a reef near Sao Jose. The preva- 

 lence of the hardened material and its presence only at about 

 sea level suggest that the concentration of the iron in the iron- 

 cemented sandstone has taken place in similar manner to that 

 of the lime carbonate in the calcareous sandstone reefs. In the 

 case of the latter Dr. Branner has shown* that the lime is 

 deposited along the zone of contact of the fresh with the salt 

 water. Similarly, when fresh water that has percolated down- 



Fig. 12. 



Fig. 12. Iron-cemeuted sandstone reef near Sao Jose. 



ward through the iron-impregnated sediments and dissolved a 

 slight amount of the iron comes in contact with the salt water 

 of the ocean, it probably drops its iron, which thus becomes 

 concentrated at about sea level. It seems that some such 

 action must have taken place in order to produce the extensive 

 fringe-reefs of the material that are in evidence at places where 

 landward the sediments are obscured by drifting sand. The 

 fact that such concentration and cementing is still going on 

 at about sea level appears to be shown at numerous reefs 

 which are dotted with fragments of sandstone, all firmly 

 cemented to the reef surface.f 



At a few places a layer that has been hardened by concen- 

 tration of the iron is also found in the red sediments some dis- 

 tance inland from the beach. Ponta Morcego, 2 miles southeast 

 of the center of Natal (Rio Grande do Norte), is of iron-cemented 

 sandstone in which the iron has been concentrated, and within 



*The stone reefs of Brazil, Bull. Mus. Comp. Zool., Geol. vii, p. 193, 1904. 

 \ This feature has been mentioned by Jenkins (Geology of the region about 

 Natal), Proc. Am. Phil. Soc, Sept.-Oct., 1913, p. 24. 



