BKANNER: THE STONE REEFS OF BRAZIL. 241 



bend in the shore this reef connects with the beach. The part here 

 exposed is only about one hundred and fifty metres long by fifteen 

 metres wide. The great reef opposite this point stands well out of the 

 water at low tide. Five kilometres south of the mouth of the Rio 

 Persinunga the coral reef is double. 



At Ponta do Mangue the outer edge of the coral reef is just one kilo- 

 metre from the beach. The tide was not very favorable for an examina- 

 tion when this reef was visited. The width uncovered was one hundred 

 and fifty metres, but at the lowest tides the width is very much greater. 

 It stood 1.3 metres out of water, but it is considerably higher at the 

 lowest tides, — probably as much as three raeti'es. 



The surface of the reef here is exceedingly irregular and etched, and is 

 all dead save tlie Serpulae and barnacles that grow over the dead corals, 

 and the few Porites and Favias and such other forms as thrive in tide- 

 pools. Landward of the main wall of the reef the coral rocks rise in 

 patches that are separated from each other by a labyrinth of channels of 

 various widths. 



A chemical analysis has been made in duplicate of a typical specimen 

 of the rock of which this Ponta do Mangue reef is composed with the 

 following results : — 



Analysis of the Reef Rock from Poxta do Mangce, State 

 OF Alagoas, Brazil. 



Sample collected by J. C. Branner, Julij 28, 1899. R. E. Swain, analyst. 



No. 1 No. 2 



Calcium oxide (CaO) 46.01 46.03 



Magnesia (MgO) 6.95 7.05 



Iron (FejOg) 85 .88 



Alumina (AljOg) 56 .59 



Sodium (^Na) trace trace 



Carbon dioxide (CO2) 42.94 42.96 



Silica (SiOa) 1.27 1.29 



Phosphoric anhydride (P2*^5) ^" -26 



Chlorine (CI) 31 ,28 



Water (HaO) J9 ^ 



Total 99.95 100.11 



If all the calcium present were there as carbonate, and the rest of the 

 carbon dioxide as magnesium carbonate, it would be equivalent to : — 



Carbonate of lime (CaCOg) 82.19 



Carbonate of magnesium (MgCOg) 12.98 



1 Probably as chloride. 

 VOL. xi.iv. 10 



