468 AREAS OF SUBSIDENCE. [cmar. xx 
pelagoes, 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 ori- 
zinally 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 depo- 
sited 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, of 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 asingle 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 impossible here to enter into all the necessary 
details, but I venture to defy* any one to explain in any other 
manner, how it is possible that numerous islands should be dis- 
tributed throughout vast areas—all the islands being low—all 
being built of corals, absolutely requiring a foundation within 
a limited depth from the surface. 
Before explaining how atoll-formed reefs acquire their pecu- 
liar structure, we must turn to the second great class, namely, 
Barrier-reefs. These either extend in straight lines in front of 
the shores of a continent or of a large island, or they encircle 
smaller islands; in both cases, being separated from the land by 
* It is remarkable that Mr. Lyell, even in the first Edition of his ‘ Princi- 
as of Geology,’ inferred that the amount of subsidence in the Pacific must 
ave exceeded that of elevation, from the area of land being very small 
relatively to the agents there tending to form it, namely, the growth of coral 
and volcanic action. 
