CAEBO^IFEROTJS SYSTEM. 



243 



even identity of some of the fossils fonnd in onr Upper 

 coal-measnre strata with those of unquestioned Sub-carbon- 

 iferons age in Europe. They knew that these fossils held a 

 position there beneath the coal-bearing formation of Europe, 

 but they did not know, which is the fact, that in onr Iowa 

 strata the same species of fossils hold a position above onr 

 coal-bearing strata. They had not demonstrated this to be 

 the case as has been done by the writer since the present 

 organization of the Geological Survey of Iowa, by tracing 

 the strata in the field consecutively from the lowest to the 

 highest, and observing them as they dip beneath each other 

 snccessively to the westward; but they erroneously assumed a 

 position for our Upper coal-measure strata beneath our coal- 

 bearing rocks because of a real or supposed identity of the 

 fossils of the former with such as they had found beneath the 

 coal-bearing strata in Europe. In consequence of this they 

 were obliged to infer that the coal-bearing strata of Iowa 

 occupied a broad, shallow depression or basin, and that they 

 thinned out in all directions from its centre, allowing the Sub- 

 carboniferous strata that dip beneath them in central and 

 eastern Iowa to come to the surface again in western Iowa 

 and elsewhere along the valley of the Missouri river. 



The following diagram, Fig. 13, illustrates this fallacious 

 view of the relations of the Sub-carboniferous strata with 

 those of the coal-measures: 



Fig. 13. 



A, represents the coal-bearing strata in the position they were erroneously 

 supposed to occupy, thinning out both to the eastward and westward upon the 

 Sub-carboniferous strata, B., B., B. 



The facts are, that while the coal producing strata do thin 

 out at the surface, upon the sub- carboniferous rocks at the 

 eastern border of the Iowa coal-field, they do not so thin 



