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Caves AND Cave F'orRMATIONS OF THE MrrcHELL LIMESTONE. 
By F. C. Greene. 
THE MITCHELL LIMHSTONE. 
The Mitchell limestone, otherwise known as the St. Louis, barren, or 
‘avernous limestone, is a bluish or grayish, hard, compact, even-grained 
stone, generally having a conchoidal fracture. It is so compact as to make 
it rather impervious. Intercalated layers of blue-gray shale are frequent. 
Large concretions or chert are characteristic of certain horizons. When 
the stone weathers, these masses of chert do not dissolve, but break into 
more or less angular fragments which strew the ground over the Mitchell 
area. In Indiana the formation is also characterized by the common 
presence of a genus of corals known as Lithostrotion or Lonsdaleia. In 
some places, such as western Monroe or southern Crawford County, there 
is a typical white odlite found near the top of the formation. 
Analysis shows the Mitchell to be a very pure calcium carbonate, and 
at Mitchell, Lawrence County, from which place the formation received 
its name, it is extensively quarried for making lime and cement. 
It is found in Harrison, Floyd, Crawford, Washington, Orange, Mar- 
tin, Lawrence, Monroe, Greene, Owen, Morgan, Putnam, Parke, and Mont- 
gomery counties.* It extends south into Kentucky and west into Illinois, 
where it exhibits similar characteristics. 
In the southern part of the State it reaches a thickness of 350 to 400 
feet; in the central part of its area, that is, in Lawrence and Monroe coun- 
ties, the thickness is from 150 to 250 feet, and from here gradually thins 
toward the north.** 
The greater part of the Mitchell lies in the non-glaciated portion of the 
State, thus exposing an erosion topography unaffected by other agencies. 
Several factors enter into the cause of its present topographic aspect. 
During Cretaceous time the area in which the Mitchell is located was 
eroded to base level, forming part of the great Cretaceous peneplain. 
After this event had occurred, a period of elevation began so that erosion 
*Hopkins, T. C., 28th An. Rept. Ind. Dept. Geol., p. 57. 
** Op. cit., p. 58. 
