34 Some Geological Problems — Calvin. 
the limestone and sandstone strata had been finished the sea 
retired southward and westward, and Muscatine county became 
a part of the growing continent. The strata of the Kinder- 
hook, Burlington, Keokuk and St. Louis epochs were succes- 
sively deposited in the gradually retreating sea, and at succes- 
cessively greater and greater distances from Muscatine county. 
All this while the agents of erosion were at work in what is 
now the region of Pine creek and Montpelier. There is abso- 
lutely no evidence that the region ever received any Sub-car- 
boniferous deposits. The epochs of the Lower and Upper Coal- 
Measures seem successively to have followed the St. Louis 
epoch in Iowa, and the Iowa coal basin proper, occupies an 
area to the south and west of the region occupied by the St. 
Louis group. While, however, the Carboniferous shales and 
sandstones about Buffalo and Montpelier are of the same age as 
what is known as the Middle or Upper Coal-Measures, I do not 
believe that any very direct connection exists between them 
and our Iowa coal field. The connection seems more direct 
with the Illinois coal field. After a period of subaerial ex- 
posure, represented by the strata of the Sub-carboniferous and 
probably by a considerable portion of the Coal-Measures, the 
region about Pine creek that had been left bare at the close of 
the Devonian, was, by subsidence, carried down beneath a sea 
that gradually encroached upon it from the southeast and caus- 
ed the Illinois coal field along the Mississippi above and below 
Davenport, to overlap eroded strata, not only of the Devonian, 
but of the Upper Silurian age. 
Channels and ravines had been cut in the older strata during 
the long interval they were above the sea level, and in these 
channels and ravines the encroaching sea deposited strata of 
the Carboniferous age. The Carboniferous deposits may have 
overtopped the ridges and highlands, but the relation of their 
upper limit to the present strata cannot be ascertained. Sub- 
sequent erosion, the chief agent being probably the great ice 
sheet of the glacial period, has stripped off the larger part of 
these Carboniferous beds in their northwestern extension, leav- 
ing but fragments of the strata as outlying patches in areas 
that were in some manner peculiarly sheltered. It will be re- 
membered that some of the strata were originally deposited in 
ravines walled in by relatively hard beds of Silurian or Devonian 
