Page:The Journal of geology (IA journalofgeology21894univers).pdf/101

tion already seen on its surface. While the building of a volcanic cone is spasmodic, almost instantaneous, the uplift of a great mountain is rather slow; its uplift is brief only when compared to the duration of the destructive cycle on which it thereby enters. When first describing the cycle, it was implied that the destructive forces make no beginning until the constructional forces have completed their work. The view of St. Elias corrects that false idea. Several plains follow; all dead level; all ending in even sky lines. The Llano Estacado of Texas, the lava deserts of southern Idaho, the littoral plain of southern New Jersey, the lacustrine plain of the Red River of the North. The areas included in these views show no signs whatever of destructive processes; the surfaces are essentially as flat as when they were born. A pair of drumlins in Boston harbor, and a glacial sand-plain in Newtonville, Mass., as represented in a model by Mr. Gulliver,^[1] introduce examples of peculiar constructional forms; and as the more intelligent members of the class soon point out, these might be as fairly included under a consideration of destructional processes as of constructional processes; for they really belong among the "forms taken by the waste of the land on its way to the sea," under certain special conditions, and they will be reviewed in a later chapter of the course under that heading. The drumlins and the sand-plain may also be regarded merely as evidence of a glacial accident during the denudation of the New England plateau.

Passing next to illustrations of young destructional forms, Mt. Shasta is exhibited, with great gulleys worn down its flanks. It is at once pointed out that these gulleys follow lines of constructional slope; that they began as the paths of constructional streams, defined by some accidental irregularity in the form of the volcanic cone; and that they are now slightly advanced in their consequent growth. The Mancos canyon in Colorado illustrates the beginning of the dissection of a plateau; the consequent stream having here cut down a steep-sided consequent valley, but apparently not having yet graded its slope. A