220 GEWEEAi GEOLOGY. 



The sandstone, or middle division, is of little economic value, 

 as the rock is usually too soft for any practical use. It 

 hardens a little, however, upon exposure, and in the absence 

 of better material might serve for cheap structures. It is 

 much like the sandstone of the coal-measures. The lower, 

 or magnesian division, furnishes some excellent material for 

 heavy masonry. On Lick creek in Yan Buren county, near 

 Kilbourne station, four massive layers of this division have 

 furnished some excellent material for abutments and piers for 

 the railroad bridges, and almost any desired quantity may 

 yet be obtained there. Actual tests have shown this to be 

 a very durable and valuable stone. In Des Moines county 

 the magnesian layers of this division are found exposed 

 in the valley of Long creek, about seven miles west of 

 Burlington, and are much used in that city for the necessary 

 dressed stone for the better class of buildings. This is the 

 most easterly extension of the formation in that region, or 

 the exposures there are probably part of an outlier. Wher- 

 ever the magnesian rock of this lower division of the St. 

 Louis limestone has been used it has proved to be a good 

 and durable stone. 



Fossils. None of the palseontological features of this 

 formation stand out with such prominence as some of those 

 of the two preceding formations do, but still they are well 

 marked, and its characteristic fossils are so uniformly present 

 wherever the limestone and its accompaning shales are found 

 that one feels no hesitation as to its identity wherever it 

 appears. 



Yeetebeates are represented only by the remains of fishes. 

 They belong to the two orders, Selachians and Ganoids, prin- 

 cipally to the former, among which the following genera are 

 recognized in Iowa strata, but several other genera are known 

 in the same formation in Illinois : Cladodus, Helodus, Del- 

 todus Chomatodus, CocMiodus, and HomacantTius. 



The Ganoid remains belong to the genus HoloptycJiins or 

 to a closely allied genus. They consist of detached, sub- 

 circular scales from one inch to nearly two inches in diameter. 



