Ch. III. FRINGING EEEFS. 77 



half a mile. Part of this reef is very symmetrical, and 

 has a uniform breadth of 200 yards. 



The coast of Brazil is in many parts fringed by reefs. 

 Of these, some are not of coral formation ; for instance, 

 those near Bahia and in front of Pernambuco ; but a 

 few miles south of this latter city, the reef follows 1 every 

 turn of the shore so closely, that I can hardly doubt it 

 is of coral. It runs at the distance of three-quarters 

 of a mile from the land, and within it the depth is 

 from ten to fifteen feet. I was assured by an intelli- 

 gent pilot, that at Ports Frances and Maceio, the outer 

 part of the reef consists of living coral, and the inner 

 of a white stone full of large irregular cavities com- 

 municating with the sea. The bottom of the sea off 

 the coast of Brazil shoals gradually to between thirty 

 and forty fathoms, at the distance of between nine and 

 ten leagues from the land. 



From the description now given, we may conclude 

 that the dimensions and structure of fringing-reefs 

 depend entirely on the greater or less inclination of the 

 submarine slope, conjoined with the fact, that reef- 

 building polypifers can exist only at limited depths. 

 It follows from this, that where the sea is very shallow, 

 as in the Persian Grulf and in parts of the East Indian 

 Archipelago, the reefs lose their fringing character, 

 and appear as separate and irregularly scattered 

 patches often of considerable area. As the conditions 



1 Baron Eoussin's Pilote du Bresil, and the accompanying lrydro- 

 graphical memoir. See also the supplement to this volume on a Bar 

 of Sandstone off Pernambuco. 



