CAEBONIEEEOUS SYSTEM. 227 



greater unconformability of the coal-measures upon the older 

 rocks, presently to be described. It is worthy of remark, that 

 although these facts leave no doubt that the sea did re-occupy 

 an area of land nearly two hundred miles in width which it 

 had once receded from, we have also the evidence that it was 

 accomplished with very slight disturbance, because we see 

 very little of the effects of the receding and encroaching 

 shore-line; and all flexures of these strata thus far examined 

 can be traced to causes which flexed other and subsequently 

 formed strata also at the same time. 



At the close of the St. Louis limestone epoch the elevation 

 of the land, and the consequent recedence of the shore line to 

 southwestward again, was resumed upon about the same line 

 of elevation that the previously formed strata had. The ele- 

 vation now seems to have taken place more rapidly than 

 before, because the sea border reached much farther to the 

 southward than it had previously done, leaving no trace of the 

 next formation, which might have been deposited during its 

 recedence if it had tarried. The sea-border receded so far to 

 the southward that the northern border of the next formation 

 — the Chester limestone — did not reach within a hundred 

 miles of what is now the southern boundary of Iowa. 



At the close of the epoch of the Chester limestone, the 

 shallow seas in which the Lower coal-measures were formed, 

 again re-occupied the land, extending almost as far north in 

 Iowa as that sea had done in which the Kinderhook beds were 

 deposited. To the northeastward, its deposits originally 

 extended much farther than the border of any of the Sub- 

 carboniferous formations, outliers of which are now found 

 resting not only upon them, but also upon strata of Devonian 

 age ; the remainder- of the formation which originally con- 

 nected these outliers with each other, and with the main 

 body of the coal-field having been removed by subsequent 

 denudation. 



The following diagram, Fig. 12, represents this unconforma- 

 bility of the coal-measures upon the older rocks, and the 

 resting of the outliers upon strata of different ages : 



