130 IOWA ACADEMY OF SCIENCES. 



converted into mixed clays and soils, that there is small 

 chance for the shales to crop out. On the whole, the 

 different formations are remarkably well outlined on the 

 surface of the ground, and the stratigraphical bearings of 

 any particular locality are readily made out with ease and 

 confidence. 



The layers of the area occupied by. the coal measures 

 are, with some minor exceptions, tilted toward the west, 

 and are now beveled. Deformation has not yet been suffi- 

 ciently marked to change this general arrangement, except 

 perhaps at the extreme southern extremity of the great 

 coal fields, where the Ouachita mountains cross. 



Nowhere else is the lenticular character of the strata 

 and terranes better shown than in the coal measures. In- 

 appreciation of this fact has led to great over-estimations 

 of the actual thickness of the coal measures as a whole, 

 and of its several parts. This element of error will be 

 largely overcome when it is more carefully considered that 

 the various formations form a series of limited, interlocking 

 lenses, instead of continuous sheets of nearly uniform 

 thickness over the entire district occupied by the coal- 

 beariug terranes. The slightly tilted and beveled beds, as 

 we find them in the region under consideration, present 

 phenomena comparable to the shingled roof of a house. If, 

 along the surface, the thickness of the various outcropping 

 strata were measured successively and then added together, 

 a very different result would be obtained regarding the 

 thickness than if the measurements were made in a boring. 

 In the case of both the shingles and the tilted strata there 

 would be enormous over-estimates of values. That this is 

 really so in regard to strata was recently shown in central 

 Iowa, where a test cross section was made under very favor- 

 able conditions. The added surface measurements gave a 

 figure three times as great as the actual borings. 



There are in the so-called coal measures, composing 

 what the geologists of the region now^ term the Arkansan, 

 Des Moines and Missourian series, fifteen distinctive shale 

 formations, separated in the upper part of the section by 

 extensive limestones. All of these terranes carry coal to 



