PHYSICAL GEOGRAPHY IN THE UNIVERSITY. 83 



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 CO. structional beginning to a destructional end. Volcanic acci- 

 dent: include the building of cones and the outpouring of lava 

 flows Both the glacial and the volcanic accidents may occur at 

 any st..ge 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 topog- 

 raphy, 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, inter- 

 ruptions 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 be- 

 trunked, 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 head- 

 lands. Hence it is said, that on reaching maturity this country 

 was depressed, and that the depression occurred very recently. 



