5/6 The American Naturalist. [July, 
Certain elements of importance yet remain to be considered. 
If the plain be raised to a moderate height over sea-level, it can 
never acquire great intensity of relief ; for the streams are then 
allowed but a small depth to which they can cut. If, on the 
other hand, the elevation is great, and rapid enough to be for the 
most part acquired before the destructive processes have made 
great headway, then the vertical element is strong, the topographic 
relief is intense. Our coastal plain is an example of a region of 
mild form; it has but slight elevation, and hence however long 
the rivers flow across it they can never cut out deep valleys. The 
plateaus of Utah and adjacent parts of the west are of another 
sort ; here the elevation is excessive, and the depth of cutting 
allowed to the rivers is correspondingly great. Marvelously 
have they taken advantage of their opportuity. The valley cut 
by the Colorado and its tributaries is in some places a mile deep, 
and yet, when we see the enormous mass of land still lying on 
either side of the valley above base-level, and waiting to be 
carried down to the ocean, we cannot doubt that the time thus 
far employed in doing so great a piece of work is a small part 
of the whole cycle of growth. The upper plateau surface is 
still broadly level, except for certain irregularities to be re- 
ferred to later on ; the valley is narrow even to notoriety, and 
must therefore be called young. It is a case of precocious 
adolescence. Intensity or faintness of relief are therefore varia- 
tions on the general scheme, and it is my intention that these 
variations shall also be represented by models when new mem- 
bers are added to complete the present series : a young plateau of 
intense relief, a middle-aged plain of mild relief, will thus become 
definitely intelligible terms to our mind. Along with this, it 
must be perceived that two mature plains need not be of the 
same age, if measured in years : for the development of maturity 
in a high plateau requires more time than in a lowland. 
There is another element of variation that must be considered. 
Sometimes the simple cycle of development that has been de- 
scribed is interrupted : the land does not lie quiet long enough 
to pass through a complete series of changes without disturb- 
ance. Indeed, this interruption is, except in ver>' young plains, the 
