The subterranean agencies are those which are due to the earth's own inherent energy and arise deep within the earth's interior, though they are often displayed at the surface in a most striking manner. No problems of geology are more difficult and obscure than those connected with the internal constitution of the earth, and satisfactory explanations of the subterranean processes have not yet been devised.
These agencies fall naturally into two great groups: I, Dias-trophism, or the movements of the earth's crust; and II, Vulcan-ism, or the phenomena of volcanoes, geysers, thermal springs, etc., while a third set of phenomena, Earthquakes, is intimately associated with each of the others, but, on account of its great importance, will require separate treatment. It is extremely probable that all of these so-called igneous processes are but different manifestations of the same forces, in ways that we cannot yet clearly understand, but, until the physical constitution of the earth's interior shall have been determined, such unity of origin cannot be definitely proved.
Diastrophism is the general term for differential movements of the lithosphere, whether upward, downward, or horizontal, and whether slow and imperceptible, or sudden and violent. These movements are of different kinds and may be classified as follows: I, Orogenic (Greek Oros, a mountain"), the upheaval of long and relatively narrow belts of land by compression and crumpling of the rocks. As will be shown in a later chapter, the orogenic processes take place at considerable depths below the surface and hence cannot be directly observed; they are included here merely for the sake of completeness, for the study of them belongs properly to structural geology. II, Epeirogenie (Greek Epeiros, a continent) , the broad uplift or depression of areas of the land or of the sea-bottom, in which the strata are not folded or crumpled, but may be tilted or may retain their original horizontal attitude. Movements of this class may be distinguished as (1) Warping, or Brady seism (Greek Brady s, slow, and Seismos, earthquake), which is a broad gentle curving of the surface upward, upwarp, or downward, downwarp; (2) direct upheaval or depression, with fracturing and dislocation of the rocks, which may be accompanied by a tilting of the strata.
Diastrophic movements of this class are almost, if not quite, invariably associated with earthquakes and can be most conveniently studied in connection with the latter. Warpings or bradyseisms manifest themselves most clearly as changes of relative level between land and sea, because even slight changes of that character are often easily detected, while in the interior of the continents they can be demonstrated only under exceptionally favourable circumstances.
Permanent changes of level frequently accompany earthquakes, bot these are sudden and appear to be nearly always the result of dislocation or faulting. By change of level, in the general sense, is meant the gradual elevation or subsidence of land, with reference to the sea, over considerable areas. Such movements are very slow and hence are apt to escape observation, so that there is much dispute as to the facts and still more as to their interpretation.
The change may be in the land or in the sea; any important and permanent change in the bed of the sea must affect its surface, but then such changes must be widespread. On the other hand, movements of the land may be either locally restricted or of great extent. The absolute direction of the movements we have no means of determining; that is, whether at a given point the earth's radius is shortened or lengthened. The movement may be always downward, but at different rates in adjoining areas, or may be sometimes in one direction and sometimes in the other. In view of these uncertainties, it has been proposed to avoid the use of the terms, "elevation and depression of land," and to substitute for them "negative and positive displacements of the coast-line," respectively. For the sake of convenience, it will be best to retain the older and more current terms, without insisting that in all cases the land moves rather than the sea.
It is by no means true that all displacements of the coast-line are diastrophic in origin; other processes that produce similar results must be carefully distinguished from actual changes of level. Thus, in many places the sea is advancing upon the land by cutting back its shore, and areas which once were covered with farms and villages are now permanently underwater; but this is not due to any sinking of the land. Another process which simulates depression is the settling of loose masses of sediment, which sometimes allows the sea to cover a flat coast. In other places the sea is building up the coast by depositing sand upon it, extending it seaward, and rivers build their deltas out into the sea, but such changes are not diastrophic.
Along coast-lines the evidences of elevation are more obvious than those of depression, because the traces of marine action are always present on land which has recently risen from the sea, while a submerged land-surface is soon changed and buried out of sight.