326 
APPENDIX II. 
fathomg, can only be explained by subsidence. It is sought 
to elude this difficulty by supposing that the reef builders, 
under specially favourable circumstances, may commence 
operations at depths considerably greater than the usual 
limit. It is indeed true that reef corals are sometimes 
dredged alive from depths much exceeding 25 fathoms, 
but the result of all recent researches has certainly been 
to confirm the general correctness of this bathymetrical 
limit, and the proposed evasion of the difficulty is at 
present a mere hypothesis, which bears a suspicious resem- 
blance to the epicycles devised to prop up the Ptolemaic 
system of astronomy. 
While the existence of ‘ continental rocks,’ as they may 
be called, in oceanic islands would have almost proved a 
general subsidence, I do not see that the frequent occur- 
rence of volcanic rocks is seriously opposed to it. The 
arrangement of the majority of coral islands, whether 
wholly composed of organic material, or incrusting a 
nucleus of volcanic rocks, is indicative of lines of weakness 
in the earth’s crust, which would give rise to movements 
in either direction, and in each case the islands would be 
connected with extruded masses of volcanic rocks, ejected 
at various points along these lines. 1 Thus, we have to 
consider which of two hypotheses is the more probable : 
(a) that mounds thus formed have, in the majority of 
cases, failed to reach the surface, but have nevertheless 
generally arrived within a comparatively short distance of 
that goal; or ( b ). that they, after having in many cases 
overtopped the surface, have again subsided. The latter, 
1 It must not be forgotten that though the peaks of mountain 
ranges are frequently composed of 1 continental rocks,’ instances are 
by no means wanting, as in the Andes, Caucasus, &c., where the 
higher portions are volcanic. In more insulated mountain masses, 
as those of Etna, Kilimanjaro, Ararat, of some of the islands of the 
Malay Archipelago and on the western coast of North America, we 
have instances of volcanoes forming the highest part of the land. 
