Page:The Journal of geology (IA journalofgeology21894univers).pdf/97
district of northeastern America assures us that these accidents may succeed each other rapidly; very rapidly compared to the rate of normal climatic change dependent on loss of relief from a constructional beginning to a destructional end. Volcanic accidents include the building of cones and the outpouring of lava flows. Both the glacial and the volcanic accidents may occur at any stage of a cycle. They both in a way involve constructional processes; both may be regarded as furnishing examples of new constructional forms; but when looked at with respect to the surface on which these accidents are imposed, and with respect to the relatively brief endurance of the effects of the accidents, they are seen in their relatively subordinate character. When sheets of drift are heavily spread over a country of low relief, or when heavy lava floods cover and bury some antecedent topography, the accidents assume such proportions that they may be considered as revolutions, after which a new start is made in the processes of denudation.
A cycle is interrupted when the land mass rises or sinks, or when it is warped, twisted, or broken. Like accidents, interruptions may happen at any stage of development. It is then convenient to say that the destructional form attained in the first incomplete cycle shall be called the constructional form of the new cycle, into which the region enters, more or less tilted or deformed from its former shape. Assuming for the moment that the constructional process is so rapid that its duration may be neglected, it follows that in cases of simple vertical movement, up or down, the rivers and streams at once proceed to adapt their activities to the new conditions. They are shortened and betrunked, if the interruption is a depression; they are revived and extended if the interruption is an elevation. These two special conditions are illustrated by paper models. One model exhibits a rolling country, into which a branching bay enters; a stream descending into the head of every branch of the bay. No flats occur at the head of the bays; no cliffs are seen on the headlands. Hence it is said, that on reaching maturity this country was depressed, and that the depression occurred very recently.