200 W. O. Crosby—Origin of Continents. 
Prof. Dana says the oscillations of the sea-floor are slight compared 
with those of the land, the principal movement being a slow sub- 
sidence running through the ages, which may be reversed to the 
extent of a few thousand feet, but never sufficiently to convert the 
sea-bottom into dry land. Yet this cold, thick, stable, oceanic crust, 
which has never been weakened by thick sedimentary deposits, is 
an area of wide-spread and intense volcanic activity; while the 
continental interiors, which, according to the theory in question, 
have experienced far greater changes of level, and are covered by 
immense thicknesses of stratified rocks, are almost entirely free from 
active volcanoes. 
Volcanoes have burned, and poured out their floods of rock, over 
nearly all parts of the continents. But all land volcanoes are, in 
a geological sense, short-lived; and, ere the sediments through 
which they reach the surface have become old, their energy 1s 
exhausted. There can be little doubt that active terrestrial volcanoes 
follow the sea-shore simply because it is there, chiefly, that thick 
deposits of recent sediments are found. It is a natural inference 
from these considerations that the volcanoes of Polynesia, for ex- 
ample, are piled upon thick sedimentary formations, deposited, 
perhaps, during the slow subsidence of a great Pacific continent. 
But, according to Professor Dana, they are quite unlike terrestrial 
volcanoes, having no necessary connexion with sediments, and being 
as old as the earth’s crust. 
The submarine mountain-ranges are, equally with the oceanic 
volcanoes, an argument against the immutability of oceanic con- 
ditions. Few geological theories are now more generally accepted 
than the theory that mountains are formed by the horizontal mashing 
up of thick deposits of sediments. These stratified formations of 
immense thickness—five to ten miles for most important mountain 
systems—can only be formed on a marginal sea-bottom. Hence it 
is impossible to avoid the conclusion that mountains are of sea-shore 
origin. But an application of this theory to the submarine mountain- 
ranges is fatal to the notion that the oceanic abysses are permanent. 
As an argument in favour of the permanence of continents and 
oceans, Mr. Wailace attaches great importance to the supposed fact, 
first mentioned by Darwin, that, with the exception of New Zealand 
and the Seychelles Islands, none of the truly oceanic islands contain 
either Palzeozoic or Mesozoic rocks; the inference being that during 
the Palzozoic and Mesozoic eras neither continents nor continental 
islands existed where the oceans now extend, for, had they existed, 
Palzeozoic and Mesozoic formations would in all probability have 
been accumulated from sediment derived from them. 
This argument is not so formidable as it at first appears. Mr. 
Wallace thinks it is doubtful if New Zealand can be properly called 
a true oceanic island. But it is difficult to see how it can be differ- 
ently classified, since the ocean between it and Australia is one 
thousand miles broad and three miles deep. But there are other 
exceptions to the law which he formulates. New Caledonia is an 
oceanic island, over 700 miles of deep water separating it from 
