OBJECTIONS TO THE THEORY OF SUBSIDENCE. 295 
further, that the large channel in the main island of the 
group, “forty fathoms deep and many miles wide. . . . finds 
an easy explanation on the assumption of an upheaval ;”’ it 
became “ wider in proportion as the enclosed island, consist- 
ing of soft stone [tufa], was gradually eaten away, and during 
slow upheaval it would continue to grow deeper in propor- 
tion as the old porous portions of the reef and the rock in 
which it was forming were more and more worn down by 
the combined action of boring animals and plants, and of the 
currents produced by the tides and by rain.” ; Mr. Semper 
refers to the dead depressed tops of some masses of Porites 
near tide-level as the effects of the deposit of sediment over 
the top of the living coral and of erosion by the waves and 
exposure to rains while the sides continued to grow; and the 
fact is made an example on a very small scale of atoll-making. 
In addition, experiments on the solvent power of sea-water 
are appealed to. Examples of the action of currents, sedi- 
ment, boring species, and the solvent action of carbonic acid 
in the waters, are mentioned by Mr. Agassiz, in his excellent 
account of the “Tortugas and Florida reefs,’ and in his 
“Three Cruises of the Blake ;” but in his paper on the Coral 
Reefs of the Hawaiian Islands he concludes that the fact 
that “the majority of reefs are of great width goes to show 
also that solution alone is not active enough to remove great 
masses and form lagoons.” ! —_ 
b. The theory, if satisfactory, accounts not only for the 
origin of an atoll, but for the origin of atolls of all sizes, 
shapes, and conditions, and for great numbers of them in 
archipelagoes and chains; not only for channels through 
fringing reefs, like those that abrasion in other cases makes, 
but for all the irregular outlines of barriers, for the great 
1 Bulletin of the Museum of Comparative Zoology, 1ssd, vol. xvii. No. 3. 
