METHODS OF EROSION. 203 
agency in the production of these features is upheaval, i. e., upheaval in 
relation to the level of the sea, though it may possibly be down-throw in 
relation to the center of the earth. This movement in portions of the crust 
of the earth may be by great folds, with anticlinal or synclinal axes, and by 
monoclinal folds and faults. 
The second great agency is erosion, and the action of this agency is 
conditioned on the character of the displacements above mentioned, the 
texture and constitution of the rocks, and the amount and relative distribution 
of the rains. 
In a district of country, the different portions of which lie at different 
altitudes above the sea, the higher the region the greater the amount of rain 
fall, and hence the eroding agency increases in some well observed, but not 
accurately defined, ratio, from the low to the high lands. The power of 
running water, in corrading channels and transporting the products of 
erosion, increases with the velocity of the stream in geometric ratio, and 
hence the degradation of the rocks increases with the inclination of the 
slopes. Thus altitude ,and inclination both are important elements in the 
problem. 
Let me state this in another way. We may consider the level of the 
sea to be a grand base level, below which the dry lands cannot be eroded; 
but we may also have, for local and temporary purposes, other base levels 
of erosion, which are the levels of the beds of the principal streams which 
carry away the products of erosion. (I take some liberty in using the term 
level in this connection, as the action of a running stream in wearing its 
channel ceases, for all practical purposes, before its bed has quite reached 
the level of the lower end of the stream. What I have called the base level 
would, in fact, be an imaginary surface, inclining slightly in all its parts 
toward the lower end of the principal stream draining the area through 
which the level is supposed to extend, or having the inclination of its parts 
varied in direction as determined by tributary streams.) Where such a 
stream crosses a series of rocks in its course, some of which are hard, and 
others soft, the harder beds form a series of temporary dams, above which 
the corrasion of the channel through the softer beds is checked, and thus 
we may have a series of base levels of erosion, below which the rocks on 
