76 
THE GllAND CASTON DISTRICT. 
be employed from time to time in the explanation of some notable features 
of the I'egion. 
BASE LEVELS OF EROSION. 
In his popular narrative of Explorations of the Colorado River, Pow- 
ell has employed the above term to give precision to an idea which is of 
much importance in physical geology. The idea in some form or other 
has, no doubt, occurred to many geologists, but, so far as known to me, 
it had not before received such definite treatment nor been so fully and 
justly emphasized. It may be explained as follows. 
Whenever a smooth country lies at an altitude but little above the level 
of the sea, erosion proceeds at a rate so slow as to be merely nominal. 
The rivers cannot corrade their channels. Their declivities are very small, 
the velocities of their waters very feeble, and their transporting power is 
so much reduced that they can do no more than urge along the detritus 
brought into their troughs from highlands around their margins. Their 
transporting power is just equal to the load they have to carry, and there is no 
surplus energy left to wear away their bottoms. All that erosion can now 
do is to slowly carry olf the soil formed on the slopes of mounds, banks, 
and hillocks, which faintly diversify the broad surrounding expanse. The 
erosion is at its base-level or very nearly so. An -extreme case is the State 
of Florida. All regions are tending to base-levels of erosion, and if the 
time be long enough each region will, in its turn, approach nearer and 
nearer, and at last sensibly I’each it. The approach, however, consists in 
an infinite series of approximations like the approach of a hyperbola to 
tangency with its asymptote. Thus far, however, there is the implied 
assumption that the region undergoes no change of altitude with reference 
to sea-level ; that it is neither elevated nor depressed by subterranean forces. 
Many regions do remain without such vertical movements through a long 
succession of geological periods. But the greater portion of the existing 
land of the globe, so far as is known, has been subject to repeated throes 
of elevation or depression. Such a change, if of notable amount, at length 
destroys the pre-existing relation of a region to its base-level of erosion. 
