268 The American. Geologist. October, 1897 
to uhiiost equal the width of the basin valley, iiearl}' obliter- 
ating it. In Whiteside count}'', Illinois, the upland ridges 
which constitute the dissected Tertiary peneplain, extend to 
the verge of the great canon valley, forming so-called "200- 
foot bluffs." The intervening basin valleys where they are 
transected by the Mississippi valley, form "100-foot bluifs." 
This indicates great widening of the canon valley by a stream 
vastly larger than that which channeled the basin valley. 
Evidence of this class is abundant throughout the stream's 
course between Dubuque and Le Clare. Being quite familiar 
with the characteristics of the valleys eastward from the great 
river, I have never yet come upon its canon valley without 
being impressed with its general appearance of greater youth 
than others of apparently the same system. 
In concluding these remarks on the age of the "larger Mis- 
sissippi" river, I wish to i-epeat, firstly, that I am certain that 
it cannot be earlier than the opening of the Ozarkian or post- 
Lafayette epoch of erosion; secondly, that there is evidence 
to indicate that it did not exist until near the end of the 
Ozarkian epoch; thirdly, that since the large or present 
Mississippi river began to flow in the canon valley on the 
western border of northwestern Illinois, it has widened it 
somewhat, giving it the appearance of greater youth than 
others of the same system ; fourthly, that the relation of this 
newly eroded valley to the Kansas drift below Savanna indi- 
cates that the "larger Mississippi" had come into existence 
before the Kansan glaciation of western Illinois and south- 
eastern Iowa; and lastly, that, while the cause of the creation 
of the "larger Mississippi" may have been an uplift of the 
northern portion of the Mississippi basin, it is just as probable 
that it was due to obstruction of a northern drainage system 
by the accumulating Kansan ice. 
EDITORIAL COMMENT. 
Recent Estimates of Geological Time. 
In an address June 3d at the annual meeting of the Victo- 
ria Institute, in London, Lord Kelvin estimated the age of 
the earth, since it was sufficiently cooled to become the abode 
