STRUCTURE OF CORAL REEFS. in 



two-thirds of its height (or of the fifty feet), was only fifteen 

 feet in diameter along its upper half; and it supported above a 

 great tabular mass one hundred feet in diameter, whose top was 

 bare at low tide. The tide at this place is but two feet, and 

 this is favourable to the preservation of such top-heavy struc- 

 tures. In many places, he says, these tops have joined together, 

 leaving arches between them ; and in some parts of the reef- 

 region such united coral-heads cover acres in extent, being 

 joined together above and supported by their pillars. A case is 

 reported of a whale having gone through one of these under 



THE LIXO CORAL REEF, ABROLHOS. 



passages after being struck with a harpoon. Mr. Whipple 

 also states that there are cavernous recesses in some of these 

 heads, some that are 200 to 300 feet across ; and " when there 

 is a heavy swell on, the water is one entire sheet of white foam, 

 caused by its being forced through them and the air entering 

 as the heavy sea recedes from them." 



Professor C. F. Hartt, in his " Geology, &c., of Brazil " 

 (1870), describes very similar coral-heads in his account of the 

 reefs of the Abrolhos, and represents a scene of coral-head tops 

 in a sketch, of which the preceding is a copy. Professor Hartt 



