298 
APPENDIX II. 
positions of 40, 50 and even 60 fathoms are mentioned. 
This difficulty Mr. Guppy overcomes by the hypothesis that 
the limit for the development of reef-building coral is really 
determined, not so much by actual depth as by the con- 
dition of the water, especially in regard to the presence or 
absence of suspended mud (p. 888). 
Another consideration confirms Mr. Guppy in his 
opinion that reefs are often begun at a much greater depth 
than 25 fathoms. The usual foundation, so far as his 
observations go, is composed of partially consolidated 
volcanic mud or ooze, more or less foraminiferous, and 
generally abounding in recent shells, and is not a layer of 
detrital sand and gravel. But in all the soundings about 
the reef, which often extended down to 50 fathoms, the 
armings never brought up any indication of the nature of 
the bottom other than sand and gravel. Hence it may 
be presumed that such reefs — as, for example, those in 
the Shortland Islands — began at depths greater than 50 
fathoms. But if it be urged that in this case the reefs 
should be more than 100 feet thick — and this amount is 
rarely exceeded in the Solomon Islands — he replies that, 
as a rule, reef corals will be confined to depths of 25 or 
30 fathoms, and the beginning of a reef in deeper waters 
will be an exceptional thing. It must also be remembered 
that the rapid subaerial denudation which occurs in these 
regions may, in some cases, have reduced the vertical 
thickness of the reef. 
In support of Mr. Murray’s view that reefs spread by 
an out-ward growth, Mr. Guppy states that he found the 
corals inside the lagoons to be much larger than those 
which occur near the outward border of the reef, and in 
the barrier reefs the corals were largest near the inner 
edge of the flat, and diminished in size as the outer edge of 
the reef was approached. ‘ These facts are of importance, 
since, according to the theory of subsidence, the central 
