400 
“The Mitchell limestone, otherwise known as the St. Louis, barren, 
or cavernous 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 of 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 pres- 
ence 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 Connty, from which place the formation received its 
name, it is extensively quarried for making lime and cement.? 
“The Mitchell limestone has long been known as the Cavernous lime- 
stone. Both the Wyandotte Cave of Indiana and the Mammoth Cave of 
kentucky occur in its strata. In three counties in the vicinity of Mammoth 
Cave, over five hundred caves are known to exist. These facts lead us to 
investigate the general adaptability of this limestone to cave formation. 
“The reasons of this adaptability are numerous. Besides the bedding 
planes, two sets of vertical joint-planes exist, one set having a general east 
and west direction and the other a north and south direction. Vertical 
joint-planes are probably nore numerous in this than any other of the 
Mississippian iimestones. Owing to the fact that the Mitehell is rather 
impervious and often of a lithographic nature, the down flowing water is 
forced to follow the joint and bedding planes. The underlying Salem lime- 
stone contains joint-planes but is porous enough to become thoroughly sat- 
urated instead of confining the water to joint-planes.” 
The presence of joint-planes, its impermeability and its solubility, are 
the qualities of the Mitchell limestone which make it favorable to the 
development of caves and sinkholes. It is obvious that if a stone is im- 
permeable and has joint-planes, the water will trickle down through these 
joints instead of being absorbed by the rock. If the rock is soluble and the 
?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 counties, the thickness 
is from 150 to 250 feet, and from here gradually thins toward the north.” 
