CARBOmFEKOUS SYSTEM. 205 



exceed forty or fifty feet in thickness, but farther southward 

 they reach near a hundred feet in thickness, and produce 

 the obstructions in the Mississippi river known as the Lower 

 Kapids. These are the most important and thickest of any 

 of the silicious beds which separate the limestones of the 

 Sub-carboniferous group. 



The southerly dip of the Iowa rocks carries the Burlington 

 limestone down, so that it is seen for the last time in this 

 State in the valley of Skunk river near the southern 

 boundary of Des Moines county. Fifty miles to the south- 

 ward of this, at Quincy, Illinois, it rises again, unchanged in 

 any of its characters, and is seen well exposed also at Hanni- 

 bal, Missouri, as well as at various other places in both of 

 those States. To the northward of Burlington, its typical 

 locality, it is found frequently exposed along the bluffs that 

 border the Mississippi and Iowa rivers in the counties of Des 

 Moines and Louisa, as well as occasionally upon their smaller 

 tributaries in the same region. The most northerly point 

 at which it has been recognized in its unmistakable characters 

 is in the northern part of Washington county, but it doubt- 

 less extends still further in that direction, although hidden 

 by the overlying drift. Indeed it probably exists as far 

 north as Marshall county, but it has not been recognized 

 there in its unmistakable characters. 



Economic Value. The Burlington limestone affords much 

 valuable material for economic purposes, but which is con- 

 fined, however, entirely to its stone. The lower division 

 being much exposed within the city of Burlington, it has 

 been very largely used there for building purposes and street 

 improvements. It is seldom that it affords suitable ashlars 

 for dressing, but for the purposes of common masonry it is 

 excellent, as it endures exposure to the atmosphere and frost 

 without appreciable change. Good lime may be made from 

 it, but the greater part of the lime used in the region occupied 

 by the formation is made from the upper division, because it 

 usually produces whiter lime, and because that rock is usually 

 nearer to supplies of the necessary fuel. 



