BASAL PART OF THE KASKASKIA. 



73 



group, are underlain by a flne-gfrained ferruginous sandrock. 

 This sandstone is said to be recognizable above the city of 

 Saint Louis, where it is a dozen feet or more in thickness. 

 Southward it rapidly thickens, until in the vicinity of the typi- 

 cal locality it attains a maximum measurement of more than 100 

 feet. 



Fig. 5. An X Vases Sandstone. 



The true significance of this great sandstone separating 

 the Saint Louis and Kaskaskia limestones, does not appear 

 heretofore to have been understood fully, especially when taken 

 in connection with the absence of Kaskaskia rocks north of the 

 Missouri ri ver. Over this latter district is an extension of lime- 

 stone — the St. Louis — which, before the Coal Measures were 

 laid down, was subjected to profound erosion over a large part 

 of its area, and over another adjoining portion, having a great 

 sandstone superimposed. This would seem to indicate that 

 the broad expanse of water which, during the deposition of 

 the Saint Louis beds, reached nearly to the present northern 

 boundaries of Iowa, had retreated more than 400 miles to the 

 southward. Dry land existed over a large part of the area for- 

 merly covered by the Saint Louis waters, and bordering this 

 continental mass, arenaceous deposits were laid down in the 

 shallow littoral waters. 



In all the Carboniferous of the Mississippi basin, therefore, 

 no group of strata appears to form a better defined natural 

 geological unit than those rocks commonly passing under the 

 name of Kaskaskia or Chester. 



The great arenaceous deposit lying at the base of the Kas- 

 kaskia limestone has been termed the "ferruginous sandstone" 

 by Shumard and others. Many observers, however, have con- 

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