THE CLASSIFICATION OF GEOGRAPHIC FORMS BY GENESIS.
BY W. J. MCGEE.
Scientific progress may be measured by advance in the classification of phenomena. The primitive classification is based on external appearances, and is a classification by analogies; a higher classification is based on internal as well as external characters, and is a classification by homologies; but the ultimate classification expresses the relations of the phenomena classified to all other known phenomena, and is commonly a classification by genesis.
The early geologic classification was based chiefly upon simple facts of observation; but with continued research it is found that the processes by which the phenomena were produced may be inferred, and, accordingly, that the phenomena may be grouped as well by the agencies they represent as by their own characteristics. Thus the empiric or formal laws of relation give place to philosophic or physical laws indicating the casual relations of the phenomena, and the final arrangement becomes genetic, or a classification by processes rather than products.
The phenomena of geography and geology are identical, save that the latter science includes the larger series: since the days of Lyell the geologist has seen in the existing conditions and agencies of the earth a reflection and expression of the conditions under which and the agencies by which its development has been effected; the far stretching vista of geologic history is illuminated only by knowledge of the earth of to-day; and the stages in geologic development are best interpreted in terms of geography. So a genetic classification of geologic phenomena (which is rendered possible and intelligible through geographic research) will apply equally to geography, whether observational or of the more philosophic nature which Davis proposes to call Systematic Geography, and which Powell has called Geomorphology. Such a classification is here outlined.
The various processes or movements with which the geologist has to deal fall naturally into two principal and antagonistic categories and five subordinate categories; and each category, great and small, comprises two classes of antagonistic processes or movements.
The initial geologic movements (so far as may be inferred from the present condition of the earth) were distortions or displacements of the solid or solidifying terrestrial crust, occurring in such manner as to produce irregularities of surface. These are the movements involved in mountain growth and in the upheavel of continents. They have been in operation from the earliest known eons to the present time, and their tendency is ever to deform the geoid and produce irregularity of the terrestrial surface. The movements have been called collectively "displacement" and "diastrophism," but in the present connection they may be classed as diastatic, or, in the substantive form, as deformation. Recent researches, mainly in this country, have indicated that certain diastatic movements are the result of transference of sediment—that areas of loading sink, and areas of unloading rise; but it is evident that the transference of sediment is itself due to antecedent diastatic movements by which the loaded areas were depressed and the unloaded areas elevated; and the entire category may accordingly be divided into antecedent and consequent diastatic movements. A partially coincident division may be made into epeirogenic, or continent-making movements (so called by Gilbert), and orogenic, or mountain-making movements. Though there is commonly and perhaps always a horizontal component in diastatic movement, the more easily measured component is vertical, and when referred to a fixed datum (e.g. sea level) it is represented by elevation and depression.
The second great category of geologic processes comprehends the erosion and deposition inaugurated by the initial deformation of the terrestrial surface. By these processes continents and mountains are degraded, and adjacent oceans and lakes lined with their debris. They have been in active operation since the dawn of geologic time, and the processes individually and combined ever tend to restore the geoid by obliterating the relief produced by deformation. The general process, which comprises degradation and deposition, may be called gradation.
The first subordinate category of movements is allied to the first principal category, and comprises, (1) the outflows of lavas, the formation of dykes, the extravasation of mineral substances in solution, etc., (2) the consequent particle and mass movements within the crust of the earth, and (3) the infiltration of minerals in solution, sublimation, etc.,—in short, the modification of the earth's exterior directly and indirectly through particle movements induced by the condition of the interior. These processes have been in operation throughout geologic time, though they perhaps represent a diminishing series; they have added materially to the superficial crust of the earth; and it is fair to suppose that they have modified the geoid not only by additions to the surface but by corresponding displacements in their vicinity. The category may be tentatively (but rather improperly) called vulcanism, and the antagonistic classes of movements constituting it are extravasation and its antithesis. The vibratory movements of seismism probably result from both deformation and vulcanism under certain conditions.