lilver ]'a//c//s of fin- (Jzarl,- PUtfeaii. — Hcrshcii. 345 
streams in Missouri, namely, the Osage river on tlie northern 
slope of the Ozark uplift, and the White and James rivers and 
Flat creek on the southeastern slope of the same highland 
region. 
The ancient or Tertiary valley of the Rock river, whicli is 
still occupied by this stream to the mouth of the Kishwaukee 
river, averages three miles in width and 150 feet in depth. 
The upper trough of the White river in Missouri averages 
perhaps three to five miles in width and 200 feet in depth. It 
will thus be seen that they are approximately e(|ual in size ; 
the amount of strata excavated by the White river is some- 
what greater than bv Rock river, but at the Sfime time the 
drainage area of the former above the point of comparison is 
greater than of the latter. I am not so well acquainted witli 
the valley of the Osage, but know its upper trough to be sim- 
ilar in size to the Tertiary valley of Rock river. 
The Tertiary or preglacial valley of the Pecatonica river in 
northwestern Illinois — a stream comparable in size to the 
James river in southwestern Missouri — is from one to two 
miles wide and averages 150 feet in depth. 'JMie upper trough 
of the James river varies from a half mile to one mile in 
width, with an average depth of 200 feet. The Pecatonica 
vallej^ is thus somewhat larger than the upper valley of the 
James; but the former is excavated in softer strata than the 
latter, and apparentl}^ required about the same length of time 
for the performance of the work of erosion. 
Again, tiic ])reglacial valley of Yellow creek in northwest- 
ern Illinois — a stream com])arable in size to Flat creek in 
southwestern Missouri — is ;},000 feet in width and 150 feet in 
depth. The upper gorge or valley of Flat creek varies from 
one-fourth to three-fourths of a mile in width, and averages 
200 feet in depth. So tliey, likewise, are ai)proximately 
equal in size. 
Other valleys in northwestern Illinois and southwestern 
Wisconsin could be compared with valleys in the Ozark 
region with a like result. It is evident that as the valleys in 
the former region are about equal in size to the upper gorges 
or valleys in the Ozark region, when all the conditions of 
areas of drainage basins, drainage gradient, and rock strata 
are similar, they required about the same length of time for 
