Northeast Coast of Brazil. 389 



together by the remains of coralline animals. A smaller 

 amount of this same material was noted on the surface of a 

 wide reef of iron-cemented sandstone about two miles farther 

 northwest, within the reach of high tide. Similar material 

 forms a reef near Marcas, 16 miles farther west, but at this 

 latter place it is more firmly consolidated. 



Ponta Santo Christo is bordered by a low reef of compact, 

 pale-yellow coral that joins the beach and is uncovered for a 

 width of several hundred yards at low tide. No iron-cemented 

 sandstone was seen at the point, but within the cove westward 

 from it the usual sandstone is exposed. 



The point near the little settlement of Marcas is fringed by 

 a wide reef of hard, pale-yellow to cream-colored rock that 

 appears to be of quartz grains and pebbles cemented by coral- 

 line remains. No other rock was noted beneath it. At low 

 tide the material is uncovered for fully half a mile westward 

 from the beach, in a small cove or bay, and stands about two 

 feet above the water. Beyond the cove the reef reappears 

 along the shore for several hundred yards and extends 200 or 

 300 yards seaward at low tide. This western portion consists 

 of compact pale-yellow coral containing a few fossil shells'" 

 and a very little sand. It is the westernmost coral formation 

 that was seen along the coast. 



Summary. 



In addition to the observations on various reefs north of Rio 

 Grande do Norte which are given in the preceding pages, an 

 attempt has also been made to incorporate those points of wider 

 interest which were noted concerning the geologic features 

 exhibited. These may be summarized as follows : 



1. Off-shore reefs of calcareous sandstone are less well- 

 developed north of Rio Grande do Norte than they are south 

 of that place. The principal cause for this absence of promi- 

 nent calcareous reefs in the northern region is believed to be 

 absence of streams of considerable size entering the ocean. 

 Beach reefs or lithified beaches, however, are more common 

 northward, for the fresh water that slowly percolates from the 

 land to the shore encounters the salt water at the beach. The 

 tendency has therefore been for it to deposit its lime in the 

 beach sands, and this action has taken place on a considerable 

 scale where lithified beaches have been formed. 



2. Beyond where the coast swings from a northerly to a 

 westerly course, inland remnants of reefs and lithified beaches 

 become more common, as the prevailing westerly wind there 



* The two best-preserved shells were kindly identified by Dr. W. H. Dull 

 as Leptothyra albida Watson, and Botula cinnamomea Lamarck, both 

 being of species living at present in the Antilles. 



