Common Minerals and Rocks

by William O. Crosby

The caverns which are so large and numerous in most limestone countries are a splendid example of the solvent action of meteoric waters, being formed entirely by the dissolving out of the limestone by the water circulating through the joint cracks. The process must go on with extreme slowness at first, when the joints are narrow, and more rapidly as they are widened and more water is admitted. We get some idea, too, of the magnitude of the results accomplished by these silent and unobtrusive agencies when we reflect that almost all the loose earth and soil covering the solid rocks are simply the insoluble residue which carbon dioxide and water cannot remove. In low latitudes, where a warm climate accelerates the decay of the rocks, the soil is usually from 50 to 300 feet deep.

Mechanical Erosion.—On the edge of the land.—Let us trace next the mechanical action of water and air upon the land. First we will consider the edge of the land, where it is washed by the waves of the sea. Whoever has been on the shore must have noticed that the sand along the water’s edge is kept in constant motion by the ebb and flow of the surf.

Where the beach is composed of gravel or shingle the motion is evident to the ear as well as the eye; and when the surf is strong, the rattling and grinding of the pebbles as they are rolled up and down the beach develops into a roar.

The constant shifting of the grains of sand, pebbles, and stones is, of course, attended by innumerable collisions, which are the cause of the noise. Now it is practically impossible, as we may easily prove by experiment, to knock or rub two pieces of stone together, at least so as to produce much noise, without abrading their surfaces; small particles are detached, and sand and dust are formed.

That this abrasion actually occurs in the case of the moving sand is most beautifully shown by the sandblast. We are to conclude, then, that every time a pebble, large or small, is rolled up or down the beach it becomes smaller, and some sand and dust or clay are formed which are carried off by the water.

But what are the pebbles originally? This question is not difficult. A little observation on the beach shows us that the pebbles are not all equally round and smooth, but many are more or less angular. And we soon see that it is possible to select a series showing all gradations between the most perfectly rounded forms and angular fragments of rock that are only slightly abraded on the corners. The three principal members of such a series are shown in specimens 5, 6, and 7 from the beach on Marblehead Neck; but equally instructive specimens can be obtained at many other points on our coast. It is also observable that the well-rounded pebbles are much smaller on the average than the angular blocks.

From these facts we draw the legitimate inference that the pebbles were all originally angular, and that the same abrasion which diminishes their size makes them round and smooth.

A little reflection, too, shows that the rounding of the angular fragments is a natural and necessary result of their mutual collisions; for the angles are at the same time their weakest and most exposed points, and must wear off faster than the flat or concave surfaces.

Having traced each pebble back to a larger angular rock-fragment, the question arises, Whence come these angular blocks?

Behind our gravel-beach, or at its end, we have usually a cliff of rocks. As we approach this it is distinctly observable that the angular pebbles are more numerous, larger, and more angular; and a little observation shows that these are simply the blocks produced by jointing, and that the cliff is entirely composed of them. In other words, our cliff is a mass of natural masonry, which chemical agencies, the frost, and the sea are gradually disintegrating and removing. As soon as the blocks are brought within reach of the surf their mutual collisions make them rounder and smaller; and small round pebbles, sand, and clay are the final result.