ELEVATION AND DENUDATION. 
225 
turn, involves the correlative fact that no elevating force has acted upon the 
region for a long period of time.* For the most part base-levels are preva- 
lent during a cessation of the uplifting force. The recurrence of upheaval 
terminates the condition of base-level. The declivities of the streams are 
increased, their energy augmented, and their corrasive power renewed. 
New features are then carved out of the topography, or older ones are em- 
bossed in higher relief. A period of upheaval, then, is one in which the 
sculptural features of the land are generated and increased ; a period of 
quiescence or cessation of vertical movement is one in which these features 
are obliterated.f Now, in trying to form some conception of the pi'ocess by 
which the great denudation of the Mesozoic was accomplished, we may 
suppose that the uplifting of the region went on (1) either at a constant or 
a slightly varying rate, or (2) through alternating periods of activity and 
quiescence. The results would be widely contrasted in the two cases. The 
foi’mer would give us an exceedingly rough and hilly country at all periods 
of the erosion ; the latter would give us just such a country as we see at 
present. The inequalities produced during a period of upheaval would be 
smoothed off during the period of repose. As a matter of fact we may be 
confident that the upheaval in its later stages has been of the paroxysmal 
character. Of this the proofs are abundant. 
We may then conjecture the reason for the somewhat remarkable fact 
spoken of in Chapter VI, that the same stratum or geological horizon is 
almost eveiy where the surface of the interior platform of the Grand Canon 
district. Before the last upheaval we may conceive of the region occupying 
the situation of a base-level in which the inequalities which may have existed 
were obliterated. We shall see more of this subject of base-level hereafter. 
*Otber causes might be suggested, but this no doubt is the predominant one. 
t If it be suggested that a long time might have to elapse after the cessation of upheaval before 
the river would find its base level, I reply to the contrary. Corrasion at any level notably above base 
level is a very swift process. Our concrete ideas of corrasion are apt to be drawn solely from ordinary 
rivers, which are almost always close down to base level. But any marked increase in the declivity of 
a large stream would (^ceteris paribus) enormously accelerate the rate of corrasion. After the periodic 
uplifts of the Grand Canon district I doubt not that the corrasion of the river bed has been exceedingly 
rapid, and that the river has recovered its base level as often as disturbed in very short spaces of time, 
though grinding through many hundreds of feet of strata. This will appear more fully in the next 
two chapters. 
15 G C 
