296 C. R. KEYES — THE PRINCIPAL MISSISSIPPIAN SECTION. 



Louis, where it is a dozen feet or more in thickness ; southward it rapidly 

 thickens until in the vicinity of the typical locality it attains a maximum 

 measurement of more than 100 feet. 



The true significance of this great sandstone separating the St. Louis 

 ami Kaskaskia limestones does not appear heretofore to have been under- 

 stood fully, especially when taken in connection with the absence of 

 Kaskaskia rocks north of the Missouri river. Here is an extension of 

 limestone — the St. Louis — that before the Coal Measures were laid down 

 was greatly eroded over a large part of its area, and over another adjoin- 

 ing portion having a great sandstone superimposed. This would seem 

 to indicate that the broad expanse of waters which, during the deposition 

 of the St. Louis beds, reached nearly to the present northern boundaries 

 of Iowa had retreated more than 403 miles to the southward. Dry land 

 existed over a large part of the area formerly covered by the St. Louis 

 waters, and bordering this continental nia^ arenaceous deposits were 

 laid down in the shallow littoral waters. 



In all the Carboniferous of the Mississippi basin no group of strata 

 appears to form a better defined natural geological unit than those beds 

 commonly passing under the name of Kaskaskia or Chester. 



The great arenaceous deposit lying at the l>a<e of the Kaskaskia, lime- 

 stone has been termed the '' ferruginous sandstone " by Shumard and 

 others. Many observers, however, have confounded it with a lithologi- 

 cally similar sandrock situated at the base of the Coal Measures and 

 consequently located on, instead of under, the Kaskaskia. For conveni- 

 ence in reference and in order to avoid further confusion this great sand- 

 stone will be called here the Aux Fases sandstone, from the river of that 

 name in Ste. Genevieve county. Missouri, on which the rock is exposed. 

 Of course in northern Missouri and Iowa, where the superior member of 

 the Mississippian series is wanting, the basal sandrock of the Coal Meas- 

 ures occupies the same stratigraphical position as the lower Kaskaskia 

 sandstone — that is, superimposed upon the St. Louis. 



Kaskaskia Limestone and Shales. — Everywhere over that part of the upper 

 Mississippi valley in which the Kaskaskia is absent the St. Louis rocks. 

 as already stated, are weathered and deeply channeled, many gorges 

 passing downward even to the Keokuk, thus showing pretty conclusively 

 that these portions of the territory were actually above sea level during a 

 part of the Kaskaskia deposition. That the northern shore-line con- 

 tinued to move southward after the Kaskaskia epoch had begun, and 

 perhaps even until the latter half of the interval had set in, is shown by 

 the successive attenuation of the several beds and by the deeply excavated 

 ravines, where soon afterward were laid down the local sandstones and 

 shales of the Coal Measures. In a number of cases, at least, these hardened 



