Common Minerals and Rocks

by William O. Crosby

We have already noticed incidentally the powerful disintegrating action of water where it freezes in the joints and pores of the rocks; and it is probable that it thus facilitates the destruction of the rocks in cold countries nearly as much as the higher temperature and greater rain-fall do in warm countries.

Our observations up to this point show us that erosion, by which we mean the breaking up by chemical and mechanical action of the rocks of the land and the transportation of the débris into the sea, is one great result accomplished by the inorganic aqueous agencies.

Mechanical Deposition.—Next let us notice what becomes of all this vast amount of clay, sand, and gravel after it is washed into the ocean. By taking up a glass of turbid water from our roadside rill, and observing that as soon as the water is undisturbed the sand and clay begin to settle, we learn that the solid matter is held in suspension by the motion of the water. But it does not remain in suspension long after being washed into the sea, for otherwise the sea would, in the course of time, become turbid for long distances from shore; and it is a well-known fact that the sea-water is usually clear and free from sensible turbidity close along shore and even near the mouths of large rivers, while at a distance of only 50 or 100 miles we find the transparency of the central ocean.

Putting these facts together, we see that the ocean, nothwithstanding the ceaseless and often violent undulations of its surface, must be as a whole a vast body of still water; and to the reflecting mind the almost perfect tranquillity of the ocean is one of its most impressive features. For it is in striking contrast, in this respect, with the more mobile aerial ocean above it.

We have got hold, now, of two facts of great geological importance: (1) The débris washed off the land by waves and rivers into the still water of the ocean very soon settles to the bottom; and (2) it nearly all settles on that part of the ocean-floor near the land.

And now we have in view the second great office of the inorganic aqueous agencies,—deposition, the counterpart or complement of erosion.

The land is the great theatre of erosion and the sea of deposition; the rocks which are constantly wasting away on the former are as constantly renewed in the latter.

We will now observe the process of deposition a little more closely. Each of these two bottles contains the same amount of fine yellow clay, but in one the water is fresh, and in the other it is salt. At the beginning of the lesson, as you may have observed, I brought the clay in both bottles into suspension by violent agitation, and since then they have remained undisturbed. The main point is that the salt water has become quite clear, while the fresh water is still distinctly turbid, showing that the salt favors the rapid deposition of the clay. At the second lecture, a week later, these two bottles, yet undisturbed, were exhibited, and the fresh water seen to be still sensibly turbid. The fact is, the clay is not held in suspension wholly by the motion of the water; but, just as in the case of dust in the atmosphere, a small portion of the medium is condensed around or adheres to each solid particle, i.e., each clay particle in our experiment has an atmosphere of water which moves with it and buoys it up. Now the effect of the salt is to diminish the adhesion of the water to the particles, i.e., to diminish their atmospheres, and consequently their buoyancy. The diminished adhesion of the salt water is well shown by the smaller drops which it forms on a glass rod.

The geological importance of this principle is very great; for it is undoubtedly largely to the saltness of the sea that we owe its transparency, and the fact that the fine, clayey sediment from the land, like the coarse, is deposited near the shore.

This bottle of fresh water contains some fine gravel, coarse sand, fine sand, and clay. By agitating the water, all this material is brought into suspension. Now, suddenly placing the bottle in a state of rest, we observe that the gravel falls to the bottom almost instantly, followed quickly by the coarse sand, and very soon afterward by the fine sand; and then there appears to be a pause, the fine particles of clay all remain in suspension; but finally, when the water is quite motionless, they begin to settle; they fall very slowly, however, and the water will not be clear for hours.