The Elements of Geology eBook


Globigerina ooze.  Beyond the reach of waste from the land the bottom of the deep sea is carpeted for the most part with either chalky ooze or a fine red clay.  The surface waters of the warm seas swarm with minute and lowly animals belonging to the order of the Foraminifera, which secrete shells of carbonate of lime.  At death these tiny white shells fall through the sea water like snowflakes in the air, and, slowly dissolving, seem to melt quite away before they can reach depths greater than about three miles.  Near shore they reach bottom, but are masked by the rapid deposit of waste derived from the land.  At intermediate depths they mantle the ocean floor with a white, soft lime deposit known as Globigerina ooze, from a genus of the Foraminifera which contributes largely to its formation.

Red clay.  Below depths of from fifteen to eighteen thousand feet the ocean bottom is sheeted with red or chocolate colored clay.  It is the insoluble residue of seashells, of the debris of submarine volcanic eruptions, of volcanic dust wafted by the winds, and of pieces of pumice drifted by ocean currents far from the volcanoes from which they were hurled.  The red clay builds up with such inconceivable slowness that the teeth of sharks and the hard ear bones of whales may be dredged in large numbers from the deep ocean bed, where they have lain unburied for thousands of years; and an appreciable part of the clay is also formed by the dust of meteorites consumed in the atmosphere,—­a dust which falls everywhere on sea and land, but which elsewhere is wholly masked by other deposits.

The dark, cold abysses of the ocean are far less affected by change than any other portion of the surface of the lithosphere.  These vast, silent plains of ooze lie far below the reach of storms.  They know no succession of summer and winter, or of night and day.  A mantle of deep and quiet water protects them from the agents of erosion which continually attack, furrow, and destroy the surface of the land.  While the land is the area of erosion, the sea is the area of deposition.  The sheets of sediment which are slowly spread there tend to efface any inequalities, and to form a smooth and featureless subaqueous plain.

With few exceptions, the stratified rocks of the land are proved by their fossils and composition to have been laid in the sea; but in the same way they are proved to be offshore, shallow-water deposits, akin to those now making on continental shelves.  Deep-sea deposits are absent from the rocks of the land, and we may therefore infer that the deep sea has never held sway where the continents now are,—­that the continents have ever been, as now, the elevated portions of the lithosphere, and that the deep seas of the present have ever been its most depressed portions.


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The Elements of Geology from Project Gutenberg. Public domain.
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