80 CORAL-REEFS. 



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 so closely every 

 turn of the shore, 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 intelligent 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, 

 communicating with the sea. The bottom of the sea off 

 the coast of Brazil shoals gradually to between 30 and 40 

 fathoms, at the distance of between nine and ten leagues 

 from the land. 



From the description now given, we must 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 Gulf 

 and in parts of the East Indian Archipelago, the reefs lose 

 their fringing character, and appear as separate and irregu- 

 larly scattered patches, often of considerable area. From 

 the more vigorous growth of the coral on the outside, and 

 from the conditions being less favourable in several respects 

 within, such reefs are generally higher and more perfect in 

 their marginal than in their central parts : hence these reefs 

 sometimes assume (and this circumstance ought not to be 

 overlooked) the appearance of atolls ; but they differ from 

 atolls in their central expanse being much less deep, in 

 their form being less defined, and in being based on a 

 shallow foundation. But when in a deep sea reefs fringe 



1 See Baron Roussin's Pilote du Bristly and accompanying hydro- 

 graphical memoir. 



