Page:The Journal of geology (IA journalofgeology21894univers).pdf/376

at the cañon-mouth be not relatively increased in a considerable degree.

It follows that the second feature of the typical glacial cañons may naturally result from temporary occupation of water-cut cañons by ice, and that it does not necessarily argue profound glacial erosion.

In obedience to the law of varigradation,^[1] all and particularly smaller streams tend to depart in a minor degree from uniform gradient, and to develop in their channels a longitudinal profile

Fig. 3.

of slightly variable declivity; this law finding expression in the alternating pools and rapids of mountain brooks and in the always perceptible and often conspicuous alternations of greater and less declivity in the courses of water-cut cañons.

If now an otherwise uniform V cañon of irregular gradient become occupied by a glacier, the flow, varying as it does with the declivity, will become unequal and the ice will tend to accumulate on the planes of low declivity until it approaches a uniform surface slope; when the weight of ice at different points in the medial or other longitudinal plane of the glacier will become variable, and will reach a maximum over the greatest depression (fig. 4). With such increased weight will go (a) direct increase of intensity with the augmentation of its principal factor, (b) indirect increase of intensity in virtue of the office of weight as a function of the down-stream impulse, and (c) direct diminution