46 CORAL FORMATIONS. 
thick. At one of the hills large slate-like slabs may be obtained ; 
they have a sanded surface, but are so hard within as to clink under 
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BLUFFS OF CORAL SANDROCK, NORTH SHORE OF OAHU. 
the hammer. We reserve a particular description of these bluffs for 
the remarks on the geology of the Hawaiian Islands. 
One of the most interesting facts, observed in connexion with these 
drift hills, is the absence of shells, and even of fragments of shells or 
corals, sufficiently large to be referred definitely to either of these 
sources. The material is a fine sand, without organic remains, 
although situated on shores, off which, within a hundred yards, there 
are shells and corals innumerable. 
c. Thickness of reefs. 
We have considered in the preceding pages the peculiarities of 
form and structure characterizing the reef formations bordering islands 
and continents, and their influence upon the enclosed land. Could 
we raise one of these coral-bound islands from the waves, we should 
find that the reefs stand upon the submarine slopes like massy struc- 
tures of artificial masonry; some forming a broad flat platform 
ranging around the land, and others encircling it like vast ramparts, 
perhaps a hundred miles or more in circuit. The reefs that were 
near the water line of the coast would be seen to have stood in the 
shallowest water, while the outer ramparts rested on the more deeply 
submerged slopes. Indeed, it is obvious that with a given slope to 
