DYNAMICAL GEOLOGY. [Illustration] When we think of the ocean with its waves, tides, and currents, of the winds, and of the rain and snow, and the vast net-work of rivers to which they give rise, we realize that the energy or force manifested upon the earth’s surface resides chiefly in the _air_ and _water_—in the earth’s fluid envelope and not in its solid crust. And it would be an easy matter to show that, with the exception of the tidal waves and currents, which of course are due chiefly to the attraction of the moon, nearly all this energy is merely the transformed heat of the sun. Now the air and water are two great geological agencies, and therefore the geological effects which they produce are traceable back to the sun. Organic matter is another important geological agent; but all are familiar with the generalization that connects the energy exhibited by every form of life with the sun; and, besides, it is scarcely necessary to allude to the obvious fact that all animals and plants, so far at least as any display of energy is concerned, are merely differentiated portions of the earth’s fluid envelope. And so, if space permitted, it might be shown that, with the exception of the tides, nearly every form of force manifested upon the earth’s surface has its origin in the sun. Of this trio of geological agencies operating upon the earth’s surface and vitalized by the sun—_water_, _air_, and _organic matter_—the water is by far the most important, and so it is common to call these collectively the aqueous agencies. Hence we have _solar agencies_ and _aqueous agencies_ as synonymous terms. The aqueous agencies include, on one side, _air_ and _water_, or _inorganic_ agencies; and, on the other, _animals_ and _plants_, or _organic_ agencies. Let us notice briefly the operation of these, beginning with the air and water. I. AQUEOUS AGENCIES. 1. _Air and Water, or Inorganic Agencies._ CHEMICAL EROSION.—Attention is invited first to the specimens numbered 1, 2, 3, and 4. No. 1 is a sound, fresh piece of the rather common rock, diabase; and those who are acquainted with minerals will recognize that the light-colored grains in the rock are feldspar, and the dark, augite. This specimen came from a depth in the quarry, and has not been exposed to the action of the weather.