W. O. Crosby—Origin of Continents. 247 
cases, made it impracticable to trace their histories. There is nothing 
in our great formations of white crystalline limestone indicating that 
they are shallow-water deposits; and it is simply begging the 
question to set them down as such. Their purity and uniformity 
are favourable to the view that they have not been formed near the 
land. 
The Radiolarian and Diatom oozes and glauconite now accumulat- 
ing in the deep sea are fairly represented among the formations exposed 
on the land. And Mr. Wallace’s arguments against the identity 
of the Globigerina ooze and chalk have been, to a great extent at 
least, satisfactorily answered by Mr. Gardner and other writers. 
The chief objection which Mr. Wallace raises is that the chalk and 
ooze differ widely in composition. But the silica in the ooze is 
sufficiently accounted for in the flints of the chalk. And as for the 
other constituents, it may be said that the Cretaceous age closed 
several millions of years ago, a time long enough to permit con- 
siderable changes in the character of the deep-sea oozes. The 
alumina and iron in the ooze are chiefly the insoluble residue of the 
volcanic dust spread everywhere over the ocean-floor; they form 
a part of all marine formations, and the fact that they are conspicuous 
constituents of the calcareous ooze simply implies that the Forami- 
nifera shells accumulate with extreme slowness at the present time. 
To make the ooze chemically identical with the chalk, we have only 
to increase the rate of the organic deposition. But Mr. Wallace has 
already done this for the Cretaceous period; for he shows, first, that 
the abundance of the pelagic Foraminifera, of the calcareous tests 
of which both the chalk and ooze are mainly composed, is, other 
things being equal, proportional to the temperature of the water ; 
and, second, that the Cretaceous seas of Europe were very warm. 
He conceives that a land-barrier stretched from Scandinavia to 
Greenland, concentrating the Gulf Stream and directing it across 
the site of modern Europe. 
Mr. Wallace’s explanation of the chalk of Europe embraces 
propositions that are not easily reconciled. For he insists, and 
rightly, upon the great purity of the chalk, and yet holds that it was 
deposited in a shallow and narrow sea. He derives the chalk in 
large part from the comminution of coral-rock, and yet names only 
two points in Europe where coral-reefs of Cretaceous age may be 
observed; and refers to no modern coral-reef where chalk is now 
forming in this way. The Oahu deposit belongs to the past, and is 
very small, twenty to thirty feet across, and eight to ten feet thick, 
and entirely destitute cf Foraminifera. The chalk contains no 
corals nor fragments of corals, nor does it shade off at the borders 
into coarser caleareous rocks composed of broken coral; and in no 
modern ocean are the coral-reefs entirely converted, as fast as 
formed, to an impalpable slime or ooze. 
The truly abyssal deposit of the modern ocean is the red clay 
which is found at nearly all depths below 2,500 fathoms, and, 
according to Sir Wyville Thomson, covers not less than ten million 
square miles of the ocean-floor. We have been told by Sir Wyville 
