Correspondence. 395 
of the Mississippi valley and have filled up fiords at their, mouths of 
unknown depth. The drift has covered the rest of the country higher 
than these hills. 
Spencer has demonstrated from the deep channels at the mouth 
of the St. Lawrence, traces in the gulf, and the continuation of the 
Hudson river in New York bay, and from other data, that the east- 
ern part of the continent once was very greatly elevated, when its 
rivers cut deep canyons and fiords similar to the canyon of the pres- 
ent Colorado. Then a depression and submersion, out of which not 
all the coast was raised. 
The evidences are that the interior of the continent was also ele- 
vated after the Devonian period, and was washed by great denuding 
and erosive floods. The Mississippi valley has survived as the great 
water way of succeeding ages, while others have been filled. This 
is the inference, to some extent, of Prof. Chambcrlin, who says, "the 
course of the Mississippi river is along the shore of the Devonian sea.* 
The present channel of the Mississippi is from 100 to 200 feet above 
its ancient bed. A deep boring at La Crosse showed sand and loose 
material to the depth of 147 feet. Rock river at Janesville is at least 
250 feet above its ancient valley." At Clinton soundings for the 
railroad bridge went through sand and gravel 80, 100 and 150 feet to 
the rock below the water. The ancient channel is the sand plain be- 
yond and has been sounded for more than 200 feet. A buried ravine 
runs through the town plot of Clinton, which has been explored for 
foundations to the depth of 100 feet without reaching the bed rock. 
Prof. Chamberlin says, "the cutting down of the ravines in the drift- 
less area began in the early Devonian agef and was evidently a period 
of elevation. A subsidence took place at the beginning of the Carbon- 
iferous period, and to such an extent as to make a lagoon or placid 
bayou of the river valley up to and beyond Dubuque. In this quiet 
water was deposited clay and vegetation that forms coal. Its depth 
was approximately a hundred feet. A moderate elevation ensued 
which brought the valley to its present level and the coal form<";tion 
was swept away, excepting only the traces left in these fissures and 
pits. 
The most interesting geological history of the Mississippi valley 
is yet to be written. It has only been touched on here and there. 
It was a great river on an elevated plateau on the Niagara in this 
vicinity, at the close of the Devonian period. It fell into the gulf of 
Mexico at the mouth of the Ohio. It cut a broad deep valley varying 
from two to ten miles wide. The river itself is wider where trib- 
utary rivers check the current. The valley grows somewhat narrower 
near Dubuque, above which it has the same depth but is somewhat 
more regular in its outline. It is thence a grand Niagara river gorge 
on a grand scale, to St. Paul. 
The glacial era, at its close, left its marks on many places in the 
*Wi.sconsin reports, vol. i, p. 25,T. 
iGeoIof^y of Wisconsin, vol. i, p. 1.53. 
