Page:The Journal of geology (IA journalofgeology21894univers).pdf/96
hills. The third is a broad lowland for the most part; but low hills rise above the general level near the headwaters of the streams. The main river has essentially the same course in all three models and there is a manifest relation in the position of the streams and interstream hills of the series, plainly showing genetic relationship. The three models are different forms of the same region at certain stages in its cycle of development. Exercises are held in the simple description of these forms, and of other forms that might be interpolated in the series. It is suggested that the duration of a cycle should be divided into a hundred equal parts, and that the stages occupied by the three models should be designated by appropriate numbers. After some discussion, it is agreed that they may be represented by five, twenty and forty; thus impressing the idea that maturity is reached long before middle life; and that the passage through old age is extremely slow compared to the advance from youth well into maturity. These exercises are accompanied by others in which illustrations of actual geographical forms are presented, as will be explained later; but it is important that the different character of the two should be clearly kept before the mind.
Complications of the simple scheme.—The difficulty of finding examples of actual forms in the various stages of development of a single cycle suggests that the departures from the ideal uninterrupted cycle should be examined. These are of two kinds, which I am accustomed to call accidents and interruptions. Such departures as do not involve a change in the attitude of a land mass with respect to its baselevel may be classed under the first heading as accidents; those which do involve a change with respect to baselevel will fall under the second heading of interruptions.
The most important accidents are climatic and volcanic. Climatic accidents include changes from humid to arid, and from cooler to warmer conditions, independent of the normal climatic change due to loss of relief from youth to old age. A study of such a region as the Great Salt Lake basin, or as the glaciated