246 W. O. Crosby—Origin of Continents. 
the ocean-floor moves as a unit, and that the entire crust of the 
earth is involved in the movement. But 10,000 feet subtracted 
from the depth of the Pacific would make it a very shallow ocean ; 
and its islands would be vastly more numerous and larger than they 
are now. In fact, the central Pacific, before the subsidence began, 
was probably as continental as the major portions of Europe and 
Asia during the early Tertiary epochs. Now, since this coral-island 
subsidence is not the result of radial contraction, what large element 
of improbability is there in the supposition that it may some day be 
reversed to the extent of 10,000 or even 20,000 feet? Nearly all 
the land bordering the Pacific is rising, and rising probably (as Prof. 
Dana has suggested) as a joint and complementary effect of the same 
great cause that produces the oceanic subsidence. It is safe to 
assume, however, that these continental movements will, sooner or 
later, be reversed; and when that happens, will not the Pacific 
subsidence be almost necessarily reversed too ? 
The formation of extensive deposits of sediments requires a con- 
tinent as well as an ocean. So far as our present purpose is 
concerned, we may say that the continents are entirely composed of 
stratified rocks, there being no igneous rocks, except such as have 
come up through the stratified series. In other words, no part of 
the primitive or unstratified crust is anywhere exposed. Further- 
more, the old erystalline or Eozoic formations, which, according to 
Prof. Dana, formed the first land, and the nuclei about which the 
present continents have been developed, are of enormous thickness. 
Where was the land whose waste afforded the material for building 
these tens of thousands of feet of strata? It is clear that extensive 
bodies of land, in other words, continents, were in existence before 
any part of the land of to-day had appeared above the sea. But, 
without pressing farther the question as to how, if the theory is 
correct, the modern continents ever came to have a beginning, let 
us advance a step and look for the source of the materials composing 
the subsequent additions to the continents, including the Paleozoic 
and all later formations. According to Professor Dana, they were 
derived entirely from the comparatively small Hozoic areas. This, 
however, means many miles of erosion, and necessarily implies 
either that this primitive land had originally an incredible height, 
or that during the course of geological time it has been constantly 
renewed by elevation as fast as worn away. But what are we to 
think of the original volume of formations which could suffer this 
enormous waste and still have a thickness measured by miles? We 
could not emphasize more strongly the absolute necessity of extensive 
Pre-Eozoic continents to serve as a source of Hozoic sediments. 
It is said that all the stratified rocks exposed on the continents 
are shallow-water deposits, and consequently that the floor of the 
deep sea has never been elevated to form land. ‘This proposition is 
more easily stated than demonstrated. Among the crystalline sedi- 
ments, especially, there are many kinds which, for aught that we 
can now determine, may very well have had a deep-sea origin; but 
the subsequent development of crystalline characters has, in most 
