438 GEOLOGY. 



Areas Where the Devonian Comes to the Surface. 



While the Devonian system is widely distributed it does not appear 

 at the surface over large areas. Obviously in the beginning the 

 Devonian formations were as wide-spread as the Devonian seas; but 

 since most of the area of North America covered by the Devonian 

 seas was also covered by the seas of later periods, the Devonian system 

 was mostly buried. Subsequent withdrawals of the sea and warpings 

 of the surface have allowed the overlying formations to be worn away, 

 exposing the Devonian at many points, though in the aggregate the 

 areas of exposed Devonian constitute but a small fraction of the area 

 where the system is buried. It probably underlies much of the area 

 in the eastern half of the United States where the Carboniferous rocks 

 constitute the uppermost formations, and considerable areas in the 

 west where younger formations conceal it. Strata of equivalent age 

 must underlie much of the sea. 



The absence of the Devonian strata in certain situations is hardly 

 less significant than their presence at others. Thus it is to be noticed 



that between Iowa and Indiana, 

 Devonian formations do not appear 

 at the surface between the Silurian 

 on the north and the Carboniferous 



tzj 



Sn 



Fig. 201.— Figure illustrating the oc- ° n the south. The absence of' De- 

 currence of remnants of Devonian V onian beds here might indicate 



material in fissures in Niagara lime- 



stone, near Elmhurst (Cook Co)., either that the deposits of the Car- 

 Illmois. boniferous period extended farther 



north than those of the Devonian, concealing the latter, or that Devo- 

 nian beds, once deposited north of the present border of the Carbon- 

 iferous system, were worn away before the deposition of the latter. 

 Of these alternatives, the former was, until recently, regarded the 

 more probable; but a recent find near Chicago (at Elmhurst 1 ) shows 

 that the Devonian system, or some part of it, once covered a por- 

 tion of the area where the Silurian beds now appear at the surface. 

 Evidence of the former extension of a formation over an area where 

 it does not now occur is usually found in the form of outliers (see Fig. 184 

 and p. 393) ; but in this case, the Devonian remnant does not constitute 



1 Weller, Jour. Geol., Vol. VII, pp. 483-8. 



