DEVONIAN SYSTEM. 185 



Upper Silurian age in the State. This area comprises a wide 

 strip of country, the general trend of which is northwestward 

 and southeastward. It is nearly two hundred miles long and 

 from forty to fifty miles broad; the details of its outlines 

 may be seen upon the geological map. 



All the limestones hitherto described are nearly pure dolo- 

 mites except the Trenton, which is also in part magnesian. 

 After the close of the Upper Silurian age the proportion of 

 magnesia in the limestones of Iowa are gradually diminished. 

 Thus the limestones of Devonian age are composed in part of 

 magnesian strata, and in part of common limestone. These 

 magnesian strata do not seem to be confined to any particular 

 part of the vertical thickness of the formation, and their 

 texture is also variable. In some parts of it, mainly towards 

 its base, these strata are quite uniformly bedded, com- 

 pact and uniform in texture, and afford some excellent 

 material for solid masonry. In other parts, they are soft, 

 sandy, and worthless ; and in still other parts they are com- 

 paratively soft, fine grained, slightly silicious — as for 

 example, at Waverly — and afford material for hydraulic 

 lime. A large part of the formation is composed of calca- 

 reous shales and shaly limestone. At Rockford, in Floyd 

 county, the shales assume the character of a marly clay 

 which is very fossiliferous. 



So far as has yet been ascertained, none of these litho- 

 logical variations characterize any particular horizon of this 

 formation over any large portion of the area occupied by it, 

 but the concretionary, or partially brecciated, bluish-grey, 

 common limestone is its prevailing lithological characteristic. 

 This portion is inclined to become fragmentary upon exposure 

 to atmosphere and frost. 



Owing to this lithological variation, and the fact that no 



very deep exposures of its strata are found at any one 



locality, because the streams upon it run in the direction 



of the line of strike, the full thickness of the formation 



has not been accurately ascertained, but it is estimated at 



about one hundred and fifty feet. 

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