110 



CARBONIFEROUS 



LIMESTONES 



Creeks, in Jefferson County. Three beds were there examined by Dr. Shumard, 

 who describes them as corresponding with beds numbered 2, 3, and 4, in Section 

 9, D. 



In the ravines immediately northeast of Keosauqua, ten feet of sandstone is 

 visible, with five or six feet of magnesian limestone superimposed. This latter 

 becomes concretionary, as it passes upwards into a white, compact, purer limestone, 

 containing a small Terebratula and Spirifer. (See Section No. 14, D.) In digging 

 wells at this village, limestone is struck after penetrating the subsoil ; and water 

 is obtained on reaching the underlying sandstone. 



In reviewing the modifications of the different members of the Upper Carboni- 

 ferous Limestone Series, as they present themselves at various points between the 

 Mississippi and the great bend of the Des Moines, in which Keosauqua is situated, 

 though there be a certain degree of persistence in the members, still one can trace, 

 even in short distances, considerable modifications, not only in the thickness, but 

 also in the composition of the rock. 



An example of this occurs on the south half of Section 26, Township 68 north, 

 Range 8 west. There, eight feet of sandstone (e') can be seen under the upper 

 concretionary limestone (/'), while on the north half of the same Section, the bed 

 has shrunk to a thickness of but two feet and a half. 



Again : on Section 29, Township 69 north, Range 9 west, twenty feet of grit- 

 stone (e') intervene between the Upper and Lower Concretionary Limestones (d f 

 and /') ; while on Section 32 of the same township and range, the layer of gritstone 

 attains a thickness of some two or three feet only ; and that, too, so blended with 

 the limestone above and below, as to be hardly recognisable as a distinct member. 



So, also, on Section 32, Township 66 north, and Range 6 west, as well as on 

 Sections 12 and 13, Township 67 north, Range 7 west, the magnesian building- 

 stone V occurs in a solid mass, eight, ten, even twelve or fifteen feet thick ; while 

 on Sections 33 and 34, Township 68 north, and Range 8 west, it becomes so 

 concretionary and so mixed with sandstone, that it is with difficulty it can be 

 recognised. 



Yet again : the pebbly gritstone, which, in one of the quarries on the bluffs 

 of Nassau Slue, is eight feet thick without a seam, diminishes on Sections 27 

 and 34, Township 65, Range 5, to four feet in thickness; while on the Des 

 Moines, in Township 68, and Range 8, and some other localities, it seems to be 

 wholly deficient. 



These variations in the thickness, structure, and composition of the different 

 members of the Upper Carboniferous Series, ought to be closely noted by persons 

 selecting materials for construction, in the district where this series prevails. 



On Indian Creek, two miles and a half from the State line, probably in Section 

 2, Township 67 north, and Range 9 west, ten to twelve feet of brown sandstone 

 reposes on sixteen to eighteen feet of concretionary limestone (d') . 



Passing from the Des Moines west towards the waters of the Fox and Chariton, 

 rocks are much less frequently seen exposed on the surface than on the Des Moines 

 and its tributaries ; and the inhabitants of that portion of Iowa have often difficulty 

 in procuring water, even at considerable depths. This may probably be attributable 



