16 INTRODUCTION. 



Devonian, Carboniferous, etc., etc. The accompanying ideal 

 vertical section of the earth's crust, copied from Dana's 

 Manual of Geology, presents each of these with their subdi- 

 visions in their recognized order from the lowest or oldest 

 known fossiliferous rocks to the present time. At the com- 

 mencement of the chapter on General Geology is another 

 similar section of all the rocks of Iowa. By comparing the 

 two, one may readily ascertain the place in the general geo- 

 logical scale to which any of our Iowa formations belong. 



None of the strata of our State now lie in a perfectly 

 horizontal position, although that was the position in which 

 they were originally deposited. If they were all now per- 

 fectly horizontal or parallel with the general surface we con Id 

 see only the uppermost one, except in cases where the river 

 valleys had cut through that into others. But by a change 

 in their level, which subsequently took place, or which was 

 progressing slowly during the time of their deposition, they 

 have been so elevated that they are not only much above the 

 level of the sea in which they were deposited, but their edges 

 are slightly upturned, so that they come to the surface suc- 

 cessively, and may be seen in their regular order as one goes 

 from the southwestern to the northeastern corner of the State. 

 In the latter region are found the oldest of these formations, 

 which are geologically the lowest although they occupy a 

 higher actual level than the others thus passed over. It will 

 thus be seen that the position of any geological formation 

 beneath the surface has no necessary relation to, or coinci- 

 dence with the outline of that surface. For example the 

 surface of a region may be very much cut and broken by 

 river valleys, and yet all the underlying strata occupy per- 

 fectly uniform and parallel planes. Again, of the rocks 

 successively exposed along the valley of the Mississippi, 

 those in Allamakee and Clayton counties are the lowest, 

 geologically, although by actual level, the highest; and as 

 one goes down the Mississippi river he goes up in the geo- 

 logical scale. 



Through the kindness of Dr. Mark Ranney, Superintendent 



