of Edinburgh, Session 1885-86. 
903 
on a misconception of tlie conditions that limit the depth of this 
zone. All observations go to show that the depths at which reef- 
corals thrive vary greatly in different localities, the variation being 
due to differences of local conditions, such as the degree of inclina- 
tion of the submarine slope, the presence of submarine declivities, 
the amount of sediment held in suspension, the force of the breakers, 
and other influences. The main determining condition is to be 
found in the injurious effect of sand and sediment rather than in 
the general influence of depth, and the distribution of these 
materials is dependent on the local conditions above referred to. 
Local conditions will usually restrict the reef-coral zone to depths 
less than 30 fathoms; but I have shown that where there is a 
gradual submarine slope, reef-corals are to be found in depths 
beyond the sand and gravel.* Since most observers, however, have 
regarded these materials as necessarily limiting the zone, they did 
not push their inquiries beyond. Under favourable conditions, 
reef-corals may thrive in depths of 50 or 60 fathoms, and thus we 
can readily explain the apparently abnormal depths inside some 
atolls and barrier reefs. The fact that I came upon some upraised 
massive corals resting in their position of growth on a partially 
consolidated ooze, which I never found in depths less than 50 
fathoms, goes to support this view. 
9. That proofs of the outward growth of barrier and other reefs 
on their own talus, a point to which Mr Murray attaches much 
importance, are to be found — 
(a) In the circumstance that massive corals may be commonly 
observed to increase in size as one approaches the lagoon 
from the outer margin of the reef-flat. 
(5) In the presence of old lines of erosion which have evidently 
been produced at the sea-level, but which have been cut off 
from the action of the waves by the advancing edge of the 
reef-flat. 
(c) In the characters and position of the wooded islets, situated 
on reefs, which would, in course of time, cover the whole 
reef-flat, were it not for one counteracting circumstance, the 
outward growth of the reef. 
* I found living reef-corals on one occasion at a depth of 40 fathoms. 
