20 INTRODUCTION. 



Tims far each of the great systems in the geological scale 

 are represented in the model by only a single sheet, bnt for 

 the purpose of illustrating the relations of the coal-bearing 

 formations to those which immediately underlie and overlie 

 them, bnt which belong to the same system with them, (the 

 Carboniferous) each of the other sheets is made to represent 

 subdivisions of systems. Thus the one which rests upon the 

 Devonian represents the sub-carboniferous group of forma- 

 tions, which in Iowa are four in number. This sub-carbo- 

 niferous group of formations, although it belongs to the 

 Carboniferous System, contains no coal, that being nearly all 

 contained in the Lower and Middle coal-measures, both of 

 which are represented by the single sheet which rests upon 

 that which represents the sub-carboniferous. This sheet is 

 seen to pass beneath another to the southwestward, which 

 represents the Upper or barren coal measures. This last 

 named formation is often styled the unproductive coal mea- 

 sures, and generally with great propriety; yet in Iowa the 

 formation contains one rather unimportant bed of coal which 

 appears at various points along the valley of the Nodaway 

 river. 



This arrangement of sheets in the model explains very 

 clearly why we may expect to find coal by deep mining in 

 southwestern Iowa, and why it is useless to look for it in the 

 northwestern and other parts of the State. Finally the last 

 sheet of the series represents all there is of the Cretaceous 

 foimations to be found in Iowa. These are the latest formed 

 strata in the State, (the surface deposits of course not being 

 included in this statement), but it will be seen by reference to 

 the general section before referred to, that even they are very 

 ancient compared with other foimations found elsewhere in 

 the world, and even farther west in our own country. 



Following these explanations of the general principles of 

 geology and of the geological stiucture of our State, it seems 

 proper also to make further explanation of matters, concern- 

 ing which many citizens have questioned every person 

 connected with the survey. 



