APPENDIX II. 
accumulation of other organisms. Moreover, unless we 
rely on solution for enlarging the lagoon, this will remain 
of its original size, and thus will be small in comparison 
with the ultimate area of the atoll. No doubt, for a time, 
as the reef is approaching the surface of the sea, the more 
rapid growth of the coral at its outer margin will cause 
it to be saucer-like in section, and thus somewhat enlarge 
the lagoon, but as soon as the upward growth ceases this 
process is arrested and the atoll can only spread laterally 
and thus must increase in breadth, while the lagoon, if 
there be no solution, tends rather to diminish in size. 
It is, however, stated on good authority 1 that coral 
growth, as a rule, is by no means entirely arrested in 
a lagoon, and we cannot suppose that so long as there is 
free passage for a considerable stratum of water above the 
reef — i.e. so long as there are soundings of 8 or 10 fathoms 
over it — the polyps on its inner part will suffer materially 
from want of food or properly aerated water. Hence the 
lagoon will not be formed at all until the reef has made 
some progress upwards, so that it should always be com- 
paratively shallow, not exceeding a few fathoms in maxi- 
mum depth. From the above considerations it appears to 
me that the ‘ fairy-ring ’ hypothesis is inadequate unless 
it be inseparably linked with that of ‘ solution.’ 
At this period we may not unfitly notice another con- 
sideration which has been urged, viz. that many shoals, 
chiefly of volcanic origin, which lie at too great a depth to 
be colonised by reef-building polyps, may be raised up to 
the proper level by the accumulation of marine organisms. 
That this may sometimes occur cannot be denied, but it 
must be remembered that, unless the shoal lie at a very 
moderate depth below the required level, the process 
of accumulation will be extremely slow. Mr. Murray’s 
estimate of the quantity of carbonate of lime present in the 
1 See pp. 302, 318 of this work. 
