Page:The National geographic magazine, volume 1.djvu/47

 and classified empirically, and can be referred to their proper places in a genetic taxonomy without change in terminology. The volcanoes, craters, calderas, lava fields, tuff fields, tufa crags, mesas, volcanic necks, dykes, etc., however modified by degradation, alteration, glaciation, or wind action, exhibit characteristic forms which have often received names indicative of their origin. The glacial drift with its various types of surface, the moraines, drumlins, kames, roches de moutonnées, rock basins, kettles, lacustral plains, aqueo-glacial terraces, loess hills and plains, etc., have been studied in their morphologic as well as their structural aspects, and the elements of the configuration commonly assumed have been described, portrayed, and appropriately named; and they take a natural place in the classification of products by the processes giving rise to them. The dunes, dust drifts, sand ridges, etc., and the wind-scooped basins with which they are associated, are local and limited, but are fairly well known and fall at once into the genetic classification of forms and structures. But all of these geographic forms are modified, even obliterated, by the ever prevailing process of gradation, which has given origin to nearly all of the minor and many of the major geographic forms of the earth. The forms resulting from this second great category of geologic processes have generally engaged the attention of systematic students, but their prevalence, variety and complexity of relation are such that even yet they stand in greatest need of classification.

Lesley thirty years ago regarded the mountain as the fundamental topographic element; Richthofen recognizes the upland and the plain ("aufragendes Land und Flachböden") as the primary classes of configuration comprehending all minor elements of topography; Dana groups topographic forms as (1) lowlands, (2) plateaus and elevated table lands, and (3) mountains; and these related allocations are satisfactory for the purposes for which they are employed. But the implied classification in all these cases is morphologic rather than genetic, and is based upon superficial and ever varying if not fortuitous characters; and if it were extended to the endless variety of forms exhibited in the topography of different regions it would only lead to the discrimination of a meaningless multitude of unrelated topographic elements.

In an exceedingly simple classification of geographic phenomena, the primary grouping is into forms of construction and forms of destruction; but it is evident on inspection of the table intro-