Page:The Journal of geology (IA journalofgeology21894univers).pdf/88
streams; these unite, forming constructional drainage systems. If the trough lines are systematically arranged, as among the corrugations of mountain folds, the initial drainage system is definitely located; if the trough lines are faintly marked and lead irregularly about, as on the nearly level surface of a plateau, the drainage is essentially vague and unsystematic. If the general descent of the trough lines is here and there reversed into ascent, lakes are accumulated in the basins thus determined; and this is very common. If the descent of the trough lines is locally intensified, constructional falls or rapids are developed, but this is relatively rare.
Consequent drainage.—The constructional streams run down their troughs, carrying along the waste that is washed into them, and trenching channels beneath the initial constructional surface; or filling constructional hollows; that is, degrading or aggrading their course, as the necessities demand. As soon as they thus depart from their initial constructional arrangement, they may be called consequent streams. It is true that the constructional phase of a drainage system endures only a moment; yet it seems advisable to recognize this phase by employing a special name for it, before introducing the term, consequent, which indicates the much longer phase that the next follows. At least, I am for the present experimenting on these two terms with my classes, and find them of value. As long as a stream flows on a line that is essentially the perpetuation of its original constructional course, it may be called a consequent stream; the trench that it cuts and the valley that is formed by the widening of the trench may be included under the name, consequent valley. Constructional features are encroached upon as the consequent features make their appearance. A constructional lake decreases in size by filling at the inlet and cutting down at the outlet; while thus dwindling away, it is a consequent lake. A fall or cascade recedes from its initial constructional position; but as long as it endures it is a consequent fall.
Subsequent drainage features.—As the consequent streams