STRUCTURE OF CORAL REEFS. Ley 
quite extensive. On Oahu, they occur at intervals around the 
eastern shores, from the northern cape to Diamond Point, 
which forms the south cape of the island,—the part exposed to 
the trades ; and they are in some places twenty to forty feet 
in height. They are most remarkable on the north cape, a 
prominent point exposed to the winds that blow occasionally 
from the westward, as well as to the regular trades. They 
also occur on Kauai, another of the Hawaian Islands. But 
at Upolu (Samoa), where the protecting reefs are broad, the 
author met with no instance worthy of mention. 
These sand-banks, through the agency of infiltrating wa- 
ters, fresh or salt, become cemented into a sand-rock, more or 
less friable, which is frequently odlite. The rock consists of 
thin layers or lamine, which are very distinct, and indicate, 
generally, every successive drift of sand which puffs of wind 
had added in the course of its formation: and where a heavier 
gale had blown off the top of a drift, and new accumulations 
again completed it, the whole history is distinctly displayed. 
On northern Oahu, the elevated bluffs of coral-made 
limestone near Kahuku Point, eighty feet or so high 
above the sea-level, have a top of drift-sand rock, charac- 
teristic in its structure, resting on a much thicker coral- 
reef rock made up in part of large corals, some of them 
in the position of growth. Views of the bluffs showing 
the division between the two formations are given in the 
author's notes on Oahu, in his work on “ Volcanoes and 
the Hawaiian Islands.” 
The thickness and extent of drift-sand deposits depends 
on the character of the wind, the agent that makes them. 
Winds from one direction add a little to the height of a 
beach; from two or more, by sweeping off sands for con- 
tributions from a wider range of surface, make the height 
