of Edinburgh, Session 1879-80. 
513 
merged banks could have been produced by subsidence, situated as 
they are in relation to each other and with respect to the perfectly- 
formed atolls of the groups. 
It is a much more natural view to regard these atolls and sub- 
merged banks as originally volcanoes reaching to various heights 
beneath the sea, and which have subsequently been built up to and 
towards the surface by accumulations of organic sediment and the 
growth of coral on their summits. It is a remarkable fact that, 
in all coral atolls which have been raised several hundred feet above 
the sea, the base is generally described as composed of solid lime- 
stone, or “ of various kinds of coral evidently deposited after life 
had become extinct.” * This base is probably often made up of such 
a rock as that brought by the missionaries from New Ireland, and 
described by Professor Liversidge,f as composed chiefly of Pelagic 
Foraminifera* the same as those taken by the (( Challenger ” in the 
surface waters of the Pacific. 
Microscopic sections of a rock taken from 50 feet below sea level 
at Bermuda show that a deposition of carbonate of lime is going on. 
The small shells are filled with, and the broken pieces of shells and 
corals are cemented by, calcite. The wells in coral islands rise and 
fall with the tide, so that the whole atoll is filled like a sponge with 
sea water. This water is very slowly interchanged, and by the 
solution of the smaller and thinner particles, becomes saturated, and 
a deposition of lime follows. In this way we may explain the 
absence of many of the more delicate shells from some limestones. J 
Barrier Reefs . — During the visit of the “ Challenger ” to Tahiti, a 
careful examination was made of the reefs by dredging, sounding, 
&c., in a steam pinnace, both inside and outside the reefs. Lieu- 
tenant Swire of the “ Challenger ” made a careful trigonometrical 
survey of the profile of the outer reefs on six different lines ; and while 
associated with him in this work, the author was indebted to that 
officer for many valuable suggestions. 
A ledge ran out from the edge of the reef to about 250 yards, 
where we got a depth of from 30 to 40 fathoms. It was covered 
with a most luxuriant growth of coral bosses and knobs. 
* TJ. S. Ex. Exp., vol. iv. p. 269. + Geol. Mag., Dec. 1877. 
+ Fuchs, liber die Entstehung der Aptychenkalke. Sitzb. der k. Akad. der 
"Wissensch. 1877. 
3 R 
VOL. X. 
