140 CORALS AND CORAL ISLANDS. 
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 favorable 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-re- 
gion such united coral-heads cover acres in extent, being joined 
together above and supported by their pillars. A case is re- 
ported of a whale having gone through one of these under 
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.” 





















































































































































































































































































































































































THE LIXO CORAL REEF, ABROLHOS. 
Professor C. F. Hartt, in his ‘‘ Geology, ete., 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 
speaks of it as giving simply a general view of the region with- 
