XX 
AREAS OF SUBSIDENCE 
497 
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 Oceans, 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 the 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 pro- 
foundest 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, innu¬ 
merable 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 face 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 
2 K 
