356 21ie American Geoloyim. DcciiuIkt, \mr> 
Jongar time of increasc-'d precipitation during- tlie Kaiisan than 
during the lowan and later glacial stages, I should say that 
the ratio of i)ost-Lafayette or Quaternary time to post-Co- 
himbia time, as recorded in the Ozarks, is approximate!}' as (i 
to 1. In other words, about ;)()(),()()() years may have elapsed 
«ince the Qiiaternar}'^ valleys of the Ozark region began to be 
excavated. A comparison of the size of the Quaternary troughs 
of the region with various valleys and parts of valleys in the 
drift-covered area, whose age has been calculated from various 
phenomena connected with them, also brings a probable mean 
of 1^00,000 years for the age of the Quaternary valleys of Mis- 
souri and Arkansas. 
The most reliable estimates of the length of the Quaternary 
era give it 200,000 years as a minimum and 800,000 years as a 
maximum. The amount of material removed from the upper 
troughs or Tertiary valleys of the Ozarks, being on an average 
twelve times as great as from the Quaternary" valleys, would 
requii-e, under similar condition of erosion, from 2,400,000 to 
3,600,000, or, in round numbers, somewhere between two and 
four millions of years. This agrees well with estimates of the 
length of the Tertiary era derived from studies of the changes 
of its raolluscan faunas, whereby the ])rol>ability of the cor- 
rectness of the figures is confirmed. 
Any estimates which might be made of the length of time 
occupied in forming the Jura-Cretaceous peneplain over what 
is now the Ozark plateau or mountain region would, owing to 
a want of knowledge of some of the factors, be of no value 
whatever, and I will not attempt it. The erosion doubtless 
amounted to a removal of at least 100 feet, or perhaps several 
hundreds of feet of strata, mostly sandstone and shale, with 
a little limestone, from the entire Ozark region. 
C'ONCLUSION. 
We have now endeavored to trace the changes in the topog- 
raphy of the Ozark region, and the history of the erosion of 
its valleys. We have found no new formation, no earth move- 
ment separate in time and (juality from those which have 
atl'ected all the eastern portion of the continent. The Ozarks 
particii)ated in the movements of elevation and subsidence of 
the contigiu>us areas. The best marked epochs or ])cri(>tls of 
■erosinn ai'c as well dctincd here as (-Isewiiere: and the jirinci- 
