234 GENERAL GEOLOGY. 



Such a line, it is true, will leave to the northward and east- 

 ward of it a number of points and outliers which, although 

 composed of true coal-measure strata, are believed to be too 

 slightly developed in almost all cases to give reasonable 

 hope that they will furnish a profitable coal bed. To the 

 southward and westward of such a line t5ie coal-field will be 

 found unbroken save by the occasional cutting of the river 

 valleys down through its strata to the underlying Sub- 

 carboniferous strata. 



In consequence of the recedence to the southwestward of 

 the borders of the Middle and "Upper coal-measures as before 

 described, the Lower coal-measures alone exist to the east- 

 ward and northward of the Des Moines river. It is also a 

 fact that over the whole of this great area, comprising more 

 than three thousand square miles, the formation is very thin, 

 and has been often cut entirely through by the erosion of the 

 valleys by their rivers and creeks. Considering this fact, we 

 can only wonder that it contains coal so abundantly as it does. 

 The Lower coal-measures also occupy a large area westward 

 and southward of that river, but their southerly dip there 

 passes them beneath the Middle coal-measures at no great 

 distance from the river. The western and southern border 

 the surface area occupied by the Lower coal-measures is 

 somewhat irregular, and in some parts indefinite, but the 

 area on that side of the Des Moines is more than half as 

 great as it is upon the other. 



Westward from the Des Moines river, in Webster county, 

 nothing more is seen of the coal-measure strata except a 

 small exposure on Lizard creek, about four miles west of 

 Fort Dodge. The coal-measure strata of this part of the 

 State are all of the Lower formation, and beyond it to the 

 westward; these as well as all other stratified rocks, are lost 

 to view beneath the heavy Drift Deposit which helps to 

 constitute the broad ridge of the Great Watershed, until we 

 reach the Missouri river, at Sioux City. Here we find strata 

 of Cretaceous age. These last named strata belong in the 

 geological series, above all others in the State, and if the 



