W. O. Crosby— Origin of Continents. 249 
A large proportion of the volcanoes of the globe are in the central 
portions of the ocean, nearly all the oceanic islands being either 
volcanic, or consisting of coral-rock resting upon old, submerged 
volcanoes. While of the submarine volcanoes which have never 
reached the surface, we of course know nothing, but it is probable 
that such exist, and possible that they outnumber those whose 
craters are dry land. Now on the land we observe no important 
exception to the rule that volcanoes are situated upon, or in the 
immediate neighbourhood of, thick deposits of recent sediments— 
Tertiary or Secondary. And we also observe that in the earlier 
periods of the earth’s history the same law held good. 
Are the oceanic volcanoes to be regarded as exceptions to this 
general law? If so, upon what grounds? If not, then the infer- 
ence is at least probable that the great volcanic archipelago of the 
Pacific, as well as the numerous voleanic islands of the Atlantic and 
Indian Oceans, rests upon extensive stratified formations of no great 
geological age. But the deep-sea sediments, as we have seen, are 
of very trifling thickness, with the exception of the coral-limestone, 
and this rests upon and is newer than the volcanoes. Hence the im- 
plication is plain that the floor of the central ocean has been at one 
time a marginal sea-bottom, forming the shoulders, if not the dry 
land of the continents. 
If, as those believing in stable continents and oceans virtually 
claim, the oceanic portions of the earth’s crust have been covered 
since the beginning of geological time with a sheet of cold water, 
the frigid zones extending along the ocean-floor through all latitudes 
to the equator; and if, as necessarily follows, during the whole of 
geological history, deposition has been almost entirely suspended 
over these vast areas; then since the strength and thickness of the 
earth’s crust are, in the main, due to, and are a measure of, the 
refrigeration which it has experienced, it must be admitted that the 
oceanic crust is very thick and stiff. That sediments are in general 
a source of weakness rather than of strength in the crust is the testi- 
mony of the ablest students of structural geology ; and this proposi- 
tion forms the basis of the generally accepted explanation of the 
origin of mountains. 
Now, if volcanoes are evidence of anything, they are evidence of 
weakness in the earth’s crust. They prove the existence of fissures 
reaching down to a plastic zone in or beneath the crust; and, as 
already noticed, they are, on the land, intimately connected with 
thick deposits of recent sediments—with what are generally recog- 
nized as weak places in the crust. But it is a logical deduction from 
the hypothesis here combatted that the numerous oceanic volcanoes 
do not stand on thick accumulations of sediments —for no deposits 
of sensible thickness are formed in the deep sea; and that they 
occur on the strongest, rather than the weakest, portions of the 
earth’s crust—for nowhere are the conditions more favourable for 
deep and permanent refrigeration than under the oceanic abysses, 
and according to Mr. Wallace and Prof. Dana the site of the deep 
sea has remained unchanged during all the changes of which geology 
furnishes a record. | 
