Page:The National geographic magazine, volume 1.djvu/49

 the divides; and, although these geologists have not dwelt upon and perhaps have failed to perceive the relation, the same classification is as applicable to every feature of the local relief as to the streams by which the relief was developed.

In a general classification of the topographic forms developed through gradation, it would be necessary to include the forms resulting from deposition as well as degradation, and also to discuss the relation of base-level plains to antecedent and consequent relief; but in a brief résumé it will suffice to consider only the modifications produced by degradation upon a surface of deposition after its emergence from beneath water level as a regular or irregular terrane; and the influence of base-level upon the topographic forms developed upon such a surface may be neglected in a qualitative discussion, though it is quite essential in quantitative investigation.

The hydrography developed upon terranes affected by displacement both before and after emergence has already been satisfactorily classified. Powell, years ago, denominated valleys established previous to displacement of the terrane by faulting or folding, antecedent valleys; valleys having directions depending on displacement, consequent valleys; and valleys originally established upon superior and subsequently transferred to inferior terranes, superimposed valleys; and these valleys were separated into orders determined by relation to strike and again into varieties determined by relation to subordinate attitude of the terranes traversed. Gilbert adopted the same general classification, and so extended as to include certain special genetic conditions. Tietze, in the course of his investigation of the Sefidrud (or Kizil Uzen) and other rivers in the Alburs mountains of Persia, independently ascertained the characteristics of the class of waterways comprehended by Powell under the term antecedent; Medlicott and Blanford observed that many of the Himalayan rivers are of like genesis; and Rütimeyer, Peschel and others have recognized the same genetic class of waterways; but none of these foreign geologists have discussed their taxonomic relations. Löwl, who upon a priori grounds denies the possibility of antecedent drainage, has recently developed an elaborate taxonomy of valleys which he groups as (a) tectonic valleys, and (b) valleys of erosion (Erosionsthäler). The first of these categories is separated into two classes, viz: valleys of flexure and valleys of fracture, and these in turn into several sub-classes determined by character of the displacement and its relations to structure; and the second,