292 C. R. KEYES — CARBONIFEROUS STRATA OF CENTRAL IOWA. 



the case of the Bedrock sandstone. The vast sand bed had manifestly been 

 consolidated and elevated above the surface of the sea for a considerable 

 distance; then it was subjected to long-continued denudation, as is shown in 

 the deep gorges and ravines which are still preserved in the hard sandstone. 

 So wide-spread and intense was the action of the erosive agencies that the 

 great sandstone, more than one hundred and fifty feet in thickness, was 

 largely removed ; and at the present day only a few isolated outliers tell of 

 its former great extent. When regional submergence again set in, the old 

 gorges and shore depressions were occupied by coal swamps. 



6. The earliest formed coal seams are far more extensive, both geograph- 

 cally and vertically, than the later ones. On the Avhole, the coal of Iowa 

 may be regarded as distributed in innumerable lenticular basins, sometimes 

 several miles in diameter and six or seven feet in thickness centrally, some- 

 times only a few hundred yards in extent. These occur at many different 

 horizons and interlock with one another, so that a boring may pass through 

 a score or more coal horizons without Meeting more than one or two veins 

 of sufficient thickness for profitable working. 



