the impressions became less numerous, but the adhering particles of
sand more and more numerous, until at last it was evident that the
bottom consisted of a smooth sandy layer: to carry on the analogy of
the turf, the blades of grass grew thinner and thinner, till at last
the soil was so sterile, that nothing sprang from it. From these
observations, confirmed by many others, it may be safely inferred that
the utmost depth at which corals can construct reefs is between 20 and
30 fathoms. Now there are enormous areas in the Pacific and Indian
Ocean, in which every single island is of coral formation, and is
raised only to that height to which the waves can throw up fragments,
and the winds pile up sand. Thus Radack group of atolls is an
irregular square, 520 miles long and 240 broad; the Low Archipelago is
elliptic-formed, 840 miles in its longer, and 420 in its shorter axis:
there are other small groups and single low islands between these two
archipelagoes, making a linear space of ocean actually more than 4000
miles in length, in which not one single island rises above the
specified height. Again, in the Indian Ocean there is a space of ocean
1500 miles in length, including three archipelagoes, in which every
island is low and of coral formation. From the fact of the
reef-building corals not living at great depths, it is absolutely
certain that throughout these vast areas, wherever there is now an
atoll, a foundation must have originally existed within a depth of from
20 to 30 fathoms from the surface. It is improbable in the highest
degree that broad, lofty, isolated, steep-sided banks of sediment,
arranged in groups and lines hundreds of leagues in length, could have
been deposited in the central and profoundest parts of the Pacific and
Indian Oceans, at an immense distance from any continent, and where the
water is perfectly limpid. It is equally improbable that the elevatory
forces should have uplifted throughout the above vast areas,
innumerable great rocky banks within 20 to 30 fathoms, or 120 to 180
feet, of the surface of the sea, and not one single point above that
level; for where on the whole surface of the globe can we find a single
chain of mountains, even a few hundred miles in length, with their many
summits rising within a few feet of a given level, and not one pinnacle
above it? If then the foundations, whence the atoll-building corals
sprang, were not formed of sediment, and if they were not lifted up to
the required level, they must of necessity have subsided into it; and
this at once solves the difficulty. For as mountain after mountain,
and island after island, slowly sank beneath the water, fresh bases
would be successively afforded for the growth of the corals. It is
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