We have already seen that the chief task of geology is to construct a history of the earth, to determine how and in what order the rocks were formed, through what changes they have passed, and how they reached their present position. The logical order of treatment might seem to require that we should first learn what the rocks are, of what they are composed, and how they are arranged, before attempting to explain these facts. In such a study, however, we should meet with so much that would be quite unintelligible, that a more convenient way will be to begin with a study of the agencies which are at work upon and within the earth, and which tend to modify it in one or other particular. In other words, we must employ the present order of things as a key by means of which to decipher the hieroglyphics of the past, and proceed from what may be directly observed to past changes which can only be inferred.

We might assume that the present was so radically different from the far-distant past, that the one could throw no light upon the other. Such an assumption, however, would be most illogical, for there is nothing to support it. There is no reason to imagine that physical and chemical laws are different now from what they have always been, and the more we study the earth, the more clearly we perceive that its history is a continuous whole, determined by factors of the same sort as are now continuing to modify it. Some geologists have assumed that these agencies have always acted with just the same intensity as they do to-day; but this assumption is neither necessary, nor in itself probable. There is, on the contrary, much reason to believe that while certain forces act with greater efficiency at the present time than they did in the past, others act with less.

An attentive examination of the changes which are now in progress on the surface of the earth, will show us that nothing terrestrial is quite stable or unchangeable, but that there is a slow, ceaseless circulation of matter taking place on the surface and within the crust of the globe. Matter, chemistry teaches us, is indestructible, and, disregarding the relatively insignificant amount of material which reaches us from outer space in the form of meteorites, the sum total of matter composing the globe remains constant. But while practically nothing is added to or taken away from the materials which make up the earth's crust, ceaseless cycles of change continually alter the position, physical relations, and chemical combinations of those materials. This circulation of matter may be aptly compared to the changes which take place in the body of a living animal, only, of course, they are of a different kind and are effected at an infinitely slower rate. In the animal body, so long as life lasts, old tissues break down into simpler compounds and are ejected, while new tissues are built up out of fresh material.

So, on the earth rock-masses decay, their particles are swept away, accumulate in a new place, perhaps far distant from their source, and are consolidated into new rocks, which in their turn are attacked and yield materials for further combinations. The study of the physical and chemical changes in the bodies of animals and plants constitutes the science of physiology, and by analogy we may call dynamical geology the physiology of the earth's crust. Analogies, however, must not be pushed too far, or they land us in absurdity. One essential difference between the earth and a living organism suggests itself at once; namely, that the former is self-contained, and neither ejects old material nor receives new, but employs the same matter over and over again in ever-varying combinations. The animal or plant, on the contrary, continually takes in new material from without, in the shape of food, and ejects the waste resulting from the breaking down of tissue.

Although the earth needs no fresh supplies of matter, its dynamical operations are, to a very large extent, maintained by energy from without; namely, from the sun. The circulation of the winds and waters, the changes of temperature, and the activities of living beings, all depend upon the sun's energy, and were that withdrawn, only such changes as are brought about by the earth's internal heat could continue in operation.

The study of dynamical agencies, subterranean and surface, necessarily gathers together an enormous mass of detail. But we need concern ourselves with only so much of this as throws light upon the earth's history, so that the sciences of dynamical geology and physical geography, though having much in common, are not coextensive. In order to make clear the operations of the forces which tend to modify the surface of the earth, it is necessary that we should classify and arrange them, so that they may be treated in a more or less logical order. However, in making such a classification' it is impossible to avoid entirely a certain arbitrariness of arrangement, since we must consider separately agencies that act together. Natural phenomena are not due to single causes, but to combinations and series of causes, and yet, to make them intelligible, they must be treated singly or in simple groups, else we shall be confronted by a chaos of uncorrelated facts. The career of a raindrop, from its first condensation to its entrance into the sea through the moutli of some river, is a continuous one, yet rain and rivers are distinct geological agencies and do different kinds of work.

Again, the very important way in which the various dynamical agents modify, check, or augment one another, must not be overlooked in a systematic arrangement of these agents.

Some of the agencies that we shall consider may seem, at first sight, to be very trivial in their effects, but it must be remembered that they appear so only because of the short time during which we observe them. For enormously long periods of time they have been steadily at work, and their cumulative effects must not be left out of account in estimating the forces which have made the earth what we find it.

Much as may be learned by the study of the operation of the forces which are still at work in modifying the earth, this method of study is yet insufficient to solve all geological problems. Many of the changes which have indisputably taken place are such as no man has ever observed, because they are brought about so slowly or so deep down within the crust that no direct observation is possible, and we can only infer the mode of procedure by examining the result. No human eye has ever witnessed the birth of a mountain range, or has seen the beds of solid rock folded and crumpled like so many sheets of paper, or observed the processes by which a rock is changed in all its essential characteristics; "metamorphosed," as it is technically called. All such problems must be discussed in connection with structural geology.

The dynamical agencies may, primarily, be divided into two classes: I, the Subterranean Agencies, which act, or at least originate, at considerable depths within the earth; and II, the Surface or Superficial Agencies, whose action takes place at or near the surface of the earth. The former are due to the inherent energy of the earth, and their seat is primarily subterranean, though their effects are very frequently apparent at the surface. These agencies are also called igneous (from ignis, fire), which is a misnomer; but the term is nevertheless in common use. The surface agents are those which are derived from the energy of the sun.

The logical order of treatment of these subjects is to begin with the subterranean agencies, because the most ancient rocks of the earth's crust were doubtless formed by these forces, and the circulation of matter upon and through the crust started originally from igneous rocks.

The subterranean agents originate, primarily at least, either below or deep within the lithosphere, while the surface agents originate at the surface and penetrate to varying depths. Thus it happens that at certain levels and along certain lines the two classes of agents have a common place of activity and cooperate to produce effects which neither could produce alone. We may regard the lithosphere as composed of a series of concentric shells, in each of which the conditions of pressure and temperature differ from those of the other shells and, in consequence, the characteristic chemical and mechanical processes are different in each. Depth is thus a controlling factor of great importance in the operation of the dynamical agencies. "Under any given set of conditions, minerals tend to form which remain permanent under those conditions "(Van Hise), but when new conditions arise a readjustment begins and new combinations are formed.

The outermost of the concentric lithosphere-shells (the thickness of which cannot be definitely stated, but may be as much as 20,000-25,000 feet) is characterized by a tendency of the minerals to change from more complex to simpler compounds. At greater depths, this tendency is reversed and the change is from simpler to more complex compounds. Changes of this class constitute the process of metamorphism, to which the metamorphic rocks owe their origin, but, locally, metamorphism may occur at much higher levels and even very near the surface. It may be doubted whether any chemical recombinations occur at very great depths.

The boundaries of the concentric shells are not always definite, one shell gradually passing into another, nor are they always fixed, but may fluctuate from time to time within certain limits.