PEEIOD OF THE GREAT DENUB ATIOK 
221 
time when the great erosion was begun until it reached a certain stage (to 
be spoken of speedily) not a single detail can be pointed to beyond the 
principal facts of elevation and erosion. We are, so to speak, passing a 
long interval of time in the dark. We must, therefore, stride at once from 
the middle Eocene to an epoch which may be provisionally fixed at the 
close of the Miocene. From this epoch looking backward the total change 
wrought upon the region up to that time breaks into view. But we know 
only the beginning and the end. The intermediate stages are discerned 
only by the imagination. Yet I am tempted here to view this period in a 
way which may be in some measure speculative, though not wholly so. 
Some deductions may be made from established principles governing 
erosion which may fairly claim to be something more than mere specula- 
tion. 
At the close of the Miocene, or thereabout, the greater part of the 
denudation of the Mesozoic should have been accomplished, and it is worth 
while to inquire in what manner this work may have been done. In the 
fourth chapter of this book I have spoken of the general fact that tlie 
attack of erosion is directed chiefly against the edges of the strata and the 
steeper slopes, and operates but feebly upon flat surfaces. The entering 
cuts are made by the corrading streams. The whole region had, during 
the long interval of Eocene and Miocene time, undergone a great amount 
of uplifting, and this progressive movement itself constitutes a condition 
highly favorable to corrasion; for the higher the country rises- the greater 
become the declivities of the streams, and of those factors which determine 
a stream to corrade the most potent by far is declivity. While the coun- 
try rises, therefore, the sti’eams are making the reliefs greater — are creating 
larger surfaces of edgewise exposure and longer and steeper slopes. Thus, 
every advantage is given the agents of erosion. 
The area thus exposed to rapid denudation was a very large one, and 
the corrasion of streams apparently went on over its entire expanse, without 
any very great local variations of amount, except perhaps near the borders 
of the watershed. While the normal method of decay is expressed in the 
recession of cliffs, we must not suppose that single and comparatively 
straight lines of cliffs stretched across the whole region and slowly wasted 
