Page:The National geographic magazine, volume 1.djvu/51

 hofen, and other continental, as well as different British and Indian geologists, and Lesley in this country, indeed recognize this type of drainage, but they do not correlate it with the montanic types; and Löwl's designation, derived from the manner in which he conceives it to be generated ("rückschreitende Erosion"), does not apply to either the completed drainage or the coincident topography.

Although its subordinate phases are not yet discriminated on a genetic basis, this type or order of drainage is sufficiently distinct and important to be regarded as coördinate with the type represented by the entire group of categories recognized by Powell and clearly defined by Gilbert. Such hydrography (which either in its natural condition or superimposed characterizes many plains, some plateaus, and the sides of large valleys of whatever genesis) may be termed autogenous; while the drainage systems imposed by conditions resulting from displacement (which characterize most mountainous regions) may be termed tectonic. Gilbert's classification of drainage may then be so extended as to include topography as well as hydrography, and so amplified as to include the additional type.

Drainage systems and the resulting systems of topography (all of which belong to the degradational class of forms) are accordingly.—

Type 1, Autogenous. Type 2, Tectonic―
 * Order A, Consequent, upon
 * Class a, Displacement before emergence, and
 * Class b, Sudden displacement after emergence;
 * Order B, Antecedent; and
 * Order C, Superimposed, through
 * Class a, Sedimentation (when the superimposed drainage may be autogenous),
 * Class b, Alluviation or subaerial deposition, and
 * Class c, Planation (in which two cases the superimposed drainage may simulate the autogenous type).

In brief, the entire domain of geologic science is traversed and defined by a genetic classification of the phenomena with which the geologist has to deal; and the same classification is equally applicable to geographic forms, as the accompanying table illustrates: