President's Address. 25 
origin or are formed of coral. This argument would un- 
doubtedly carry considerable weight, if it could be shown to 
be universally true; but it is far too sweeping, and is not 
borne out by the actual facts. Of many oceanic islands the 
geological structure is still unknown; and of those which 
have been examined by competent geologists, a fair number 
have been shown to have a core or nucleus of sedimentary 
or crystalline rocks, and thus to be fragments of larger land- 
masses now submerged. The archipelago of South Georgia, 
the Seychelles Islands, New Caledonia, and Barbados are 
cases in point. Of course, if it is made part of the definition 
of an “oceanic island” that it shall not contain any crystal- 
line or sedimentary rocks, then the argument will hold good; 
because when you find an island otherwise complying with 
the necessary conditions, but departing from them in this 
point, then you simply assert that it is not an “oceanic 
island.” It is also to be remembered in this connection, that 
the fact of an oceanic island being composed of volcanic 
materials in no way proves that it can not be part of an old 
land; it merely proves that it may not be so. 
RELATIVE MOVEMENTS OF OCEAN-BASINS AND CONTINENTS. 
Another argument in favour of the doctrine of the per- 
manence of the ocean-basins is that the disappearance of an 
entire continent by sinking would not give rise to the 
appearance of a new continent in a neighbouring ocean, since 
the mean height of the existing continents is only 2250 feet, 
whereas the mean depth of the ocean is 12,000 feet. The 
basis for this argument is the assumption that “on any large 
scale, elevation and subsidence must nearly balance one 
another, and thus, in order that any area of continental 
magnitude should rise from the ocean-floor . . . some 
corresponding area must sink to a like amount” (Wallace, 
Natural Science, vol. i., Aug. 1892). 
It is quite true, as insisted upon by Lapworth, that the 
processes which give rise to continents and ocean-basins are 
the same, in so far that they are parts of a single process. 
Both are primarily the result. of the folding of the earth’s 
