SUMMARY OF ARGUMENTS. 
327 
I must confess, seems to me the more probable, especially 
when we remember that subsidence very commonly occurs 
in a district when it has recently ceased to be the scene of 
volcanic disturbances on a large scale. 
2. In regard to the lateral spreading of reefs, like a ‘ fairy 
ring,’ as it lias been happily expressed, there is no doubt 
that, as has been admitted by Mr. Darwin,' some augmen- 
tation may occur in this way ; but to regard this as a factor 
of prime importance in the development of a reef seems tome 
to import new and serious difficulties. Let us assume that 
the submarine mound or shoal on which the reef is founded 
remains at rest during the whole period of the growth of the 
latter, and that this commences on the area (regarded, for 
simplicity, as a plain) included within the bathymetrical 
contour line of 25 fathoms. For a considerable period, 
until the edge of the reef arrives within a few fathoms, pro- 
bably less than ten (see p. 315), of the surface of the sea — 
that is, for full three-fifths of its whole vertical growth — 
the exterior slopes will only be augmented by the accu- 
mulation of marine organisms, a process which cannot be 
rapid. Hence, for a considerable time, until the reef 
itself has completed the greater part of its growth, and 
begins to augment the talus with its own ruins, the process 
of laying the foundation for a new coral growth, and thus 
the lateral spreading of the reef, will be slow. 
Consider, then, the case of a reef where this process has 
begun, and for simplicity regard it as a cylinder cap- 
ping a flat-topped cone. Obviously, if the reef begin to 
spread laterally, the volume of the foundation required to 
support the new growth increases far more rapidly than 
the area from which material can be supplied. Hence, 
as the reef advances outwards, the rate of increase will 
rapidly diminish, unless we suppose either an extraordi- 
nary annual destruction of growing coral, or an increased 
1 See pp, 22, 67, 70 of this work. 
