Geological Problems in Muscatine County. 17 



subsidence, carried down beneath a sea that gradually en- 

 croached upon it from the southeast and caused 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. 

 Subsequent 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, 

 leaving but fragments of the strata as outlying patches in 

 areas that were in some manner peculiarly sheltered. It will 

 be remembered that some of the strata were originally de- 

 posited in ravines walled in by relatively hard beds of Silurian 

 or Devonian age, and it is in such ravines that the outlying 

 patches chiefly occur. The conditions would be most favor- 

 able for the protection of the soft sandstone strata, at least 

 from the agents that operated during the glacial period, when 

 the ravine occupied by the strata was comparativeh' narrow 

 and had a direction at right angles to the flow of the ice 

 sheet. The largest masses and most extensive area of out- 

 lying coal-measures occur along the Mississippi, between 

 Buffalo and Muscatine. Between these points the river runs 

 from east to west. Was there an old Mississippi occup3ang 

 the same channel practically in pre-carboniferous times. Were 

 these great masses of shales and sandstones laid down in 

 a valley of erosion, and have they been preserved from de- 

 nudation because the valley had a direction at right angles to 

 the ice flow when glacial conditions and glacial erosion were at 

 their culmination ? These questions may be answered 

 affirmatively or negatively by some one who has time and 

 faciHties for working out tlie problem. 1 



I For particulars relating to the distribution of outliers of the Carbonifer- 



