SOLOMON ISLANDS. 
293 
thrive in the break of the trade-swell. ‘ They are only to be 
found in luxuriance on the slopes of the declivity that is 
situated in depths between five and fifteen fathoms, a decli- 
vity which may be truly termed the growing edge of the reef.’ 
At exceptionally low tides, when there is a heavy sea, large 
branches are apt to be torn off from corals growing beyond 
the usual reach of the breakers, and these are thrown up on 
the upper flat of the reef. But in cases where the reefs are 
protected from the heavier rollers, the corals living in the 
wash of the breakers are more numerous and in greater 
variety. The same rule holds good on the lee sides of small 
coral islands. Here the corals are often grouped in irre- 
gular patches or masses, which sometimes rise with wall-like 
sides from depths of twelve or fifteen feet of water. A lai’ge 
part of the interior both of lagoons and of their channels, 
is occupied by sandy and chalky mud ; but in the shallower 
portions, and especially in those situations which are near 
the breaks in the reef, corals, especially of foliaceous and 
branching habit, thrive in great profusion. As a rule 
corals are unable to sustain exposure to the air for long ; 
from one to two hours continuously appears the maximum 
of endurance, and that is reached only by a few species. 
Mr. Guppy considers that in this group the numerous 
detached submerged reefs or shoals, which lie at depths of 
from 4 to 10 fathoms (that is, at depths which vary with 
the amounts of disturbance produced by the breakers), 
represent the earliest conditions of coral reefs. Numerous 
instances of such reefs are given in this memoir : one, 
Lark Shoal, covered by water having a minimum depth of 
7 fathoms, rises from a depth of 200 fathoms. The shoal 
within the 20 fathom line measures H miles in one direc- 
tion and 1 mile in the other. There is no sign of a central 
depression, the summit being comparatively level and 
covered by from 7 to 10 fathoms of water. This general 
flatness of the upper surface is not peculiar to Lark Shoal, 
