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:
_Representative Geographic Forms as classified by Genesis_.
-----------------------------------+--------------------------------- GENETIC PROCESSES. | GEOGRAPHIC FORMS. _Category_. _Class_. | -----------------------------------+--------------------------------- | ELEVATION | Continents, great islands, most | | mountain ranges, etc., not DEFORMATION | | classified in detail. | DEPRESSION | Oceans, great seas and bays, | some inland valleys and | lake-basins, etc., not | classified in detail. | DEPOSITION | Newly emerged ocean-bottoms | | (_e.g._, portions of the | | Coastal plain), playas and | | mountain-bound deserts, many GRADATION | | flood-plains, marshes, etc., | | not classified in detail. | DEGRADATION | Drainage-systems and resulting | topographic elements which | are-- | 1--Autogenous (not | classified in detail); | and | 2--Tectonic-- | Consequent, upon Displacement | before emergence, and Sudden | displacement after | emergence; | Antecedent; and | Superimposed, through | Sedimentation, Alluviation, | and Planation. | EXTRAVASATION | Volcanic peaks, craters, | | lava-fields, tufa-crags, | | sinter-cones, volcanic necks, | | mesas, dykes, some mineral VULCANISM | | veins, etc., not classified in | | detail. | (ANTITHESIS OF DO.) | Sinks, caverns, some fissures, | etc., not classified in | detail. | LITHIFICATION | Minor features of certain | | topographic forms, _e.g._, | | reefs, crags, pinnacles, | | salients, out-cropping veins, ALTERATION | | some cataracts, etc., not | | classified in detail. | DELITHIFICATION | Minor features of certain | topographical forms, _e.g._, | pools and basins, reëntrants, | some fissures and caverns, | etc., not classified in | detail. | GLACIAL | Drift-plains, moraines of | CONSTRUCTION | whatever character, drumlins, | | kames, aasar, drift-dammed | | lakes, loess-plains and GLACIATION | | ridges, etc., not classified | | in detail. | GLACIAL DESTRUCTION | Rock-basins, U-cañons, roches de | moutonnées, etc., not here | classified in detail. | WIND CONSTRUCTION | Dunes, sand-ridges, bars, spits, | | etc., not here classified in WIND ACTION | | detail. | WIND DESTRUCTION | Ponds associated with dunes, | "blow-outs," "purgatories," | etc., not classified in | detail. VITAL ACTION | (Not discriminated) | -----------------------------------+---------------------------------