Mineralogy and Geology. 293 
commonly the case, by lateral channels and rills down the sides of the 
gorge, as well as by the main source ; and the amount er depth of water 
is thus in constant increase, as it flows onward. Denudation is conse- 
quently most rapid the farthest from the head, or towards n! ; the valley, 
therefore, increases in depth in this part till the slope has become so 
n1n? becomes the bottom of the lower valley, and Cn? the steeper 
portion above it. In the same manner the valley bottom continues to 
prolong at nearly the same slope, and Cn°, Cn*, Cn® become suc- 
cessively the course of the stream descending into it. And even 
n°, is no exaggeration of possibilities; for many examples of it are 
met with. 
But the results explained are but a part of the actual course of things 
in these regions of horizontally stratified rock. As on Oahu and else- 
where, when the denudation at bottom has reached its limit, the waters 
necessary result of the action. 
produce below a flat-bottomed valley. The consequence is, that they 
increase the width and extent of the main valley-plain ; for whenever 
they become thus flat-bottomed, they contribute to its lateral enlarge- 
ment. At the same time, the bluffs at the lower extremity or embou- 
chure of the main valley remain without much change, as the denuda- 
tion is mostly confined to the vicinity of the streamlets alluded to, and 
these streamlets are most abundant above, since they are produce 
