The largest single field of these ocean-bottom soils of North America is found in the lowland region of the southern United States, a wide belt of country extending along the coast from the Rio Grande to New York.

And now at length proof has been found, in the shape of mud dredged up from the ocean-bottom mud entirely composed of countless multitudes of these little shells, dropping there by myriads, and becoming slowly joined together in one mass.

When during a long epoch a continent, slowly sinking, gives place to a far-spreading ocean some miles in depth, at the bottom of which no deposits from rivers or abraded shores can be thrown down; and when, after some enormous period, this ocean-bottom is gradually elevated and becomes the site for new strata; it is clear that the fossils contained in these new strata are likely to have but little in common with the fossils of the strata below them.

Take, in illustration, the case of the North Atlantic. We have already named the fact that between this country and the United States, the ocean-bottom is being covered with a deposit of chalk a deposit which has been forming, probably, ever since there occurred that great depression of the Earth's crust from which the Atlantic resulted in remote geologic times.

But we have seen that the phenomena of glaciers, like those of currents, are in great part meteorological. The Gulf-Stream does not flow toward the English shore because the ocean-bottom slopes eastward; nor does the cold current of Baffin's Bay run down-hill when it pours its icy waters southward upon our northeast coast.