82 THE JOURNAL OF GEOLOGY. 



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 develop- 

 ment. 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. 



Coinplications of tlie 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 uninter- 

 rupted cycle should be examined. These are of two kinds, which 

 I am accustomed to call accidents and interruptions. Such depar- 

 tures as do not involve a change in the attitude of a land mass 

 with respect to its baselevel may be classed under the first head- 

 ing as accidents ; those which do involve a change with respect 

 to baselevel will fall under the second heading of interrup- 

 tions. 



The most important accidents are climatic and volcanic. Cli- 

 matic 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 Sa;lt Lake basin, or as the glaciated 



