Page:The Journal of geology (IA journalofgeology21894univers).pdf/87

This page has been proofread, but needs to be validated.
PHYSICAL GEOGRAPHY IN THE UNIVERSITY.
73

graphical cycle. Construction, destruction, baselevel and cycle are our primary terms. A full understanding of the destructional processes requires deliberate study of mineralogy and lithology, chemistry and structural geology; a good understanding of constructional forces and processes has not yet been gained, but a review of the advance made towards it carries the student through a wide range of geological theories, in which physics and mathematics are continually appealed to—perhaps sometimes with too great a confidence in the applicability of their conclusions concerning an ideal earth to the case of the actual earth.

If the cycle of destructive development is not interrupted, any constructional form will ultimately be reduced to a monotonous baselevel plain of denudation. This is a broad abstract statement. It is simply the first framework of the geographical scheme. It is a mere sketch in faint outline, needing all manner of finishing before its full meaning can be made out. It must be filled in by the gradual addition of details. The first step involves the recognition of the systematic sequence of topographic forms produced during the accomplishment of the destructive work. This should be considered before classifying the various kinds of constructional forms on which the destructional processes begin their tasks. Whatever constructional form exists at the beginning of the cycle, there is a certain general succession of features common to nearly all cases of geographical development. The understanding of this succession calls for the study of river systems and the general drainage of the land under their guidance; because it is so largely under the control of these processes that the destructive forces do their work.

Constructional drainage.—At the beginning of a cycle, there are relatively broad, massive forms, on which the carving of the destructive forces has made no mark. The unconcentrated drainage, or wet-weather wash, takes its way down the steepest slopes of the constructional surface, until the supplies from either side meet obliquely in the trough lines, forming constructional

This article is issued from Wikisource. The text is available under Creative Commons Attribution-Share Alike 4.0 unless otherwise noted. Additional terms may apply for the media files.