284 CORALS AND CORAL ISLANDS. 
to a greater depth. It is the throw of an immense mass of 
water against the front, with the velocity increased by 
the tidal flow over a shelving bottom,—the rate some- 
times amounting, according to Stevenson, to 36 miles an 
hour or 52°8 feet a second, — together with the buoyant 
action of the water, that produces the great effects. 
A vertical surface below the sea-level of 20 feet made 
bare for the broadside stroke is probably very rarely ex- 
ceeded even in the case of earthquake-waves; and with 
storm-waves, or recorded earthquake-waves, the displace- 
ment of the water at a depth of 240 feet would be at 
the most only a few inches. I saw on atoll reefs no up- 
thrown masses of coral rock over ten feet in thickness and 
twenty feet in length or breadth. It is therefore plainly 
impossible that such a belt of debris should have been made 
at its present level, or even at a depth of twenty feet; and 
hence the debris affords positive proof of a large subsidence 
during some part of the reef-making era. 
ce. But if such accumulations of great blocks cannot result 
from the dropping of masses from the edge of the reef, the 
facts show that fine coral debris may be spread over the 
bottom for a mile outside of the surface reef, and contrib- 
ute thereby toward a rising basement. But the expression 
that they grow out on their own talus is wholly fallacious ; 
for there is nothing visible that is of this nature. Outside of 
the emerged reef there is a region of growing corals, as has 
been explained, and spots and areas of sand exist among the 
luxuriant groups and plantations. Outside of this there is 
more or less of deeper pelagic life and thinner deposits of 
sand from the reef; but nowhere a talus of debris. The sea 
makes debris as the waves and inflowing tides move toward 
the shores; but the action is almost. wholly landward, trans- 

