The superficial, or surface, agents, all of which are manifestations of solar energy, are those which act upon or near the surface of the ground; only one, circulating water, is able to penetrate to considerable depths within the earth. The work of the surface agents may all be summed up in two categories, the destruction and reconstruction of rock. These two processes are complementary; for, since matter is indestructible, and can have only its position and physical and chemical relations changed, it is obvious that what is removed in one place must be laid down in another. Neither process, therefore, can go on without the other, and reconstruction necessarily implies antecedent destruction to furnish the materials. Ceaseless cycles of change are everywhere in progress, new combinations continually formed, and older rocks worked over into newer. It is this circulation of matter upon and within the crust of the earth, which we have already compared to the physiological changes in the body of a living organism.

The work of destruction and reconstruction is in a continuous series of changes, beginning with the mechanical disintegration or chemical decomposition of an older rock, followed by the transportation of the material thus supplied, for longer or shorter distances, its deposition in a new place, and finally, if the series is complete, the consolidation of the loose debris into rock.

The processes of rock destruction and removal, which are grouped together under the general name of denudation, or erosion, are chiefly confined to the land surfaces, while those of reconstruction take place principally under bodies of water and, most of all, in the sea. An important work of reconstruction is also performed on the land, but on a less extensive scale than in the sea.

The surface agents all act both destructively and reconstruc-tively according to circumstances, but with very different degrees of efficiency. Certain agents are preeminently destructive, others as preeminently reconstructive, while others again operate most efficiently as agents of transportation. Again, as was pointed out in the Introduction, the depth below the surface at which operations take place has a very important bearing upon the effect produced. From this point of view, we may regard the earth's crust as being composed of a number of concentric shells, of somewhat irregular thickness and rather indefinite, or even fluctuating, boundaries. The superficial shell, which extends from the surface down to the level of the ground water (see p. 124), is called the shell of weathering (Van Hise's " belt of weathering "), and is characterized by the oxidation, carbonation, and hydration of minerals, and great quantities of material are removed in solution. As a result of these operations the rocks are decomposed, becoming soft and friable; the minerals produced are few in number, of simple composition, and are usually imperfectly crystallized.

The second shell, that of cementation, which extends downward from the groundwater level to a varying depth with undetermined lower limit, is largely saturated with water, and hence has but a limited supply of oxygen and carbon dioxide. Therefore, while oxidation and carbonation occur, they are less important than hydration, and the resultant minerals are more crystalline than in the shell of weathering. Solution goes on, but deposition becomes very important and fills the openings in the rocks with mineral matter, and the general effect of the various processes is to harden the rocks.

The work of the surface agents, in its threefold aspect' of erosion, transportation, and deposition, is profoundly affected by the dia-strophic movements of the earth's crust. In a given case the effects produced will vary greatly in accordance with the elevation, subsidence, or stationary character of the region. In general, elevation favours denudation, and subsidence favours deposition of transported material.

In studying the work of the surface agents, the logical order of treatment requires that the destructive operations be considered first. The agencies to be examined are: (1) the atmosphere, (2) running water, (3) ice, (4) lakes, (5) the sea, (6) animals and plants. Of these various agents the work is principally mechanical, but water, in its various forms, is a slow but extremely efficient agent of chemical changes.