DEVOKEAlSr SYSTEM. 187 



Geological Age. The full series of formations of Lower Silu- 

 rian age are nearly complete in Iowa, but, as we have shown, 

 only a single formation of Upper Silurian age — the Niagara 

 limestone — is recognized within its limits. Thus, although 

 the formations that immediately underlie and overlie the 

 Niagara limestone in this State are stratigraphically con- 

 formable with it, yet there are several formations recognized 

 in other portions of the country as parts of the full geological 

 series, the serial position of which are in part between it and 

 the Maquoketa shales, and in part between it and the 

 Hamilton limestone and shales above. 



The formation here designated as the Hamilton limestones 

 and shales is, like the Niagara limestone also, serially 

 isolated, although it is quite conformable both with the Upper 

 Silurian strata below, and the Carboniferous above through- 

 out their whole extent in Iowa. This is evidently the case, 

 although the exact junction, neither above nor below, has 

 ever been distinctly seen. The greatest hiatus that exists in 

 our actual observations of the order of succession of Iowa 

 strata, is that between this formation and the Kinderhook 

 beds above, but enough is known to make it certain that no 

 other formation exists between them. Even the "Black 

 Slate," which forms so distinct a horizon at the top of the 

 Devonian series in Ohio, Indiana, Kentucky, Illinois, and 

 Missouri, seems to be entirely wanting in Iowa. 



All the Devonian strata of Iowa evidently belong to a 

 single epoch, undoubtedly referable to the Hamilton period, 

 as recognized by the New York geologists, but when we 

 come to apply a specific name to the formation as it exists in 

 Iowa, it becomes difficult to say with precision to what pre- 

 viously recognized formation of that group it belongs. In 

 its palseontological characters, however, it so much more 

 nearly corresponds with the Hamilton Shales of New York 

 than with any other part of the group, it is referred to that 

 formation; and its name retained as far as the lithological 

 character of the Iowa rocks will admit, rather than to propose 

 a new name, or refer it indefinitely to the whole group as has 

 been previously done. 



