228 



GE1SEEAL GEOLOGY. 



Fig. 12. 



No. 1. Coal-measures and outliers of the same; 2. Sub-carboniferous; 3. 

 Devonian ; 4. Upper Silurian ; 5. Lower Silurian. 



From and after the early part of the epoch of the Lower 

 coal-measures, the former order and direction of the axis of 

 elevation of the strata from the sea as they were successively 

 formed, and the consequent recession of the sea to the south- 

 westward, seems to have been exactly restored, and not to 

 have had the general direction of the dip of subsequently 

 formed strata at all changed, by either of the two elevations 

 and subsidences before mentioned which the land had under- 

 gone. We consequently find the Middle and Upper coal- 

 measures having the same general dip to the southwestward, 

 and their outcrops the same northwesterly and southeasterly 

 trend that the older formations have. It will thus be seen 

 that none of the coal-measure strata lap back; or, in other 

 words, none of them are unconformable, upon each other, 

 although the Lower coal-measures are unconformable upon 

 the older rocks. On the contrary, the lowest strata are 

 found at the borders of the coal-field, and we come to higher 

 and higher ones as we go towards the main body of the 

 field. 



The whole surface of Iowa, as well as that of other parts of 

 of the country adjacent, having been above the sea during 

 the time that the Chester limestone formation was being 

 deposited beneath it to the southward, the surface was 

 subjected to the same kiud of erosion that now produces 

 irregularities upon the earth's surface, such as that of streams 

 and falling rains, and the disintegrating effects of the atmos- 

 phere and frost. It is probable that some of the present 

 streams of northeastern Iowa — that part of the State never 

 having been covered by the sea since its first elevation in early 



