[ _^y 
T/ie Pcneplai?i. — Davis. 215 
V 
which its present altitude has been gained as are the forms 
of the upland and the vaUeys. To explain this point more 
fully, a brief digression may be allowed. 
The p 'neplain is only one element in the theory of the 
geographical cycle. The systematic sequence in the develop- 
ment of land forms through the cycle is a much larger and 
more important principle than the penultimate development cf 
a peneplain, considered alone; for the former includes the lat- 
ter. One of the elements of the cycle is the development of the 
graded condition of streams of water during maturity, where- 
by an essential agreement is brought about between the abil- 
ity of a stream to do work, and the work that it has to do. 
Another element, less generally recognized, is the develop- 
ment of the graded condition in the streams and sheets of 
rock waste or soil on sloping surfaces, where no running 
streams of water occur. By following out the ideal scheme 
thus suggested, it must result that just as the graded condi- 
tion of water streams is normally propagated from the mouth 
towards the head, and in time reaches the source of all the 
branches, so the graded condition of soil-covered slopes is in 
time extended all over a land surface, from the valley floors 
to the divides. The supply of waste by the disintegration of 
the sub-soil rock is then everywhere essentially equal to its 
removal by all available agents of transportation' In a late 
stage of a cycle, when the surface slopes are small, agents of 
transportation are weak; hence the supply of waste must then 
be slow and the waste to be removed must be of fine texture. 
In order that the supply shall be slow, the waste comes to 
have a great depth.* and the upper parts greatly protect the 
rock beneath from the attack of the weather. At the same time, 
*It is not at first sight clear why the depth of soil should increase 
on a graded waste slope, if the supply and removal of rock waste are 
(essentially) equal. As a matter of fact, the supply exceeds the re- 
moval by a quantity of the second order. Then as the slope decreases 
and the agencies of removal weaken, the depth of soil increases by 
just such a measure as will suffice to reduce the agencies of weathering 
(supply) to equality with the waning agencies of transportation (re- 
moval). This is only one of the many natural examples of an (es- 
sential) equilibrium, maintained between varying forces and resistances. 
In elementary presentation, when the condition of a graded waste slope 
at a given stage of development is considered, and attention is not 
directed to the variation of the forces and the resistances with the ad- 
vance of the cycle, the equilibrium may be announced without qualifi- 
cation: in more advancedpresentation, the hedge-word, essential, is an 
assistance to clear understanding. 
