A very few words in the close of this chapter, as to the classifying of different kinds of Rocks, may not be amiss.
Ocean-deposits, or Ocean-buildings, are often roughly divided into three groups.
In “Deep-Sea Deposits” are included all that lie beyond the Hundred-fathom Line, or a depth of six hundred feet.
In “Shallow-water Deposits” are included all that lie between six hundred feet deep and low-water mark.
In “Littoral Deposits”—the word coming from the Latin for “shore”—are included all deposits that lie on shore-lines, between the highest and the lowest tides.
At first sight the tide-deposits might seem to be but a small matter. Not so small, really! Earth’s coast-lines reach to something like one hundred and twenty-five thousand miles in length. Since the medium breadth of ground affected by tides is about half-a-mile, the whole tidal area amounts to over sixty thousand square miles.
No mean workshop this, under the auspices of crushing and grinding billows. On shore-lines building takes place rapidly.
The “Shallow” zone is much more extensive, reaching to some ten millions of square miles. Its principal deposits are much the same as those of the tidal zone—muds, sands, gravels, pebbles. In parts of the Earth they also include volcanic and coral muds.
Greater, far greater, far more widely reaching, are the regions of “Deep-Sea Deposits,” which cover more than one-half of the whole surface of our globe.
In those cold and dark and silent workshops, hidden from the eyes of men, building goes on, very quietly, very slowly, through interminable ages. In those workshops are deposited divers materials—Red Clay, Red Mud, Blue Mud, Green Mud, Volcanic Mud, Diatom Ooze, Globigerina Ooze, and many others, forming Ocean’s carpet.