26 



INTRODUCTION. 



merit of uniformity, and, so far at least as the Palaeozoic 

 rocks of Iowa are concerned, it seems to answer the purpose 

 well. 



The terms nsed in this arrangement may be referred to 

 two categories, one applicable to geological objects and the 

 other applicable to geological time, thus; 



time: 

 Epoclis constitute Periods, periods constitute Ages. 



objects: 

 Formations constitute Groups, groups constitute Systems. 



The changes made in accordance with this arrangement in 

 the classification of the rocks of Iowa hitherto used, will be 

 pointed out in the course of the descriptions contained in the 

 chapter on General Geology; and may also be seen in the 

 following table which has been constructed to show at a 

 glance the classification proposed: 



A TABLE 



the Classification of Iowa Bodes. 









Approxi- 



SYSTEMS. 



GROUPS. 



FORMATIONS. 



mate 



{Ages.) 



{Periods.) 



(JEpochs.) 



thickness 









in feet. 



Cretaceous 



Carboniferous. 



Devonian 

 Upper 



Lower Silurian . 



Azoic 



Post-Tertiary 



Lower Cretaceous 



Coal Measures . . 

 Sub-carboniferous. 



Hamilton, . 

 Niagara. . . 

 Cincinnati . 



Trenton .'. 



Primordial . . 

 Huronian {?). 



Drift 



Inoceramus beds 



Woodbury sandstone and shales 



Nishnabotany sandstone 



Upper coal measures . . 



Middle coal measures . . , . 



Lower coal measures 



St. Louis limestone., 



Keokuk limestone 



Burlington limestone 



Kinclerhook beds .• , 



Hamilton shales and limestone 



Niagara limestone 



Maquoketa shales 



Galena limestone 



Trenton limestone 



i St. Peter's sandstone 



! Lower Magnesian limestone . . 



[Potsdam sandstone 



I Sioux quartzite 



Estimated total thickness of all Iowa strata 

 *This is the thickness exposed in Iowa, only. 



50 

 130 



100 

 200 

 200 

 200 



75 



90 

 190 

 175 

 200 

 350 



80 

 250 

 200 



80 

 250 

 300 



50* 



3170 



