267 
No. IV.—Drab, fine-grained lithographic limestone. Wesley Chapel 
Gulf, three miles east of Orangeville. Fifty feet above the Lost River chert 
near the middle of the Paoli limestone. 
Stilton. MNOS ees Go SONS oe abou cee cod Oa chee acne eee eae 1.520 
MON eh ePIC gO Xl GE yer ianccvereusieie) Seusrars ee ed ae) Sites aye «sous = 278 
GREATEST I eeebtcces torch Seat eae cho-iG eo ceo oe an ne ae miele 
PAUIUITTLLTN Aerator eeRe ter ctene oa Rn eae ehenc et aict's oy hain hehe ph wie de, Brea ns y5)5) 
GuilcimecHEbonateren Aa ae sec hace beet ne ae Cee > 95.001 
Wraterrexp clled ates Oe © soaps cette, cites ois ot a clekeiel asics ois ets .630 
WndetermimederanG al OSS accrue ca acinice eae esi OF 
100.000 
One of the first things to be noted in the Mitchell limestone analyses 
is the persistent presence of a much larger per cent. of silica than is com- 
mon to an otherwise pure limestone; and it is at least singular that the 
quantity of silica should be reduced one-half in the weathered specimen 
from the same horizon. To test whether the less percentage of silica in 
specimen No. 3 might not be due to a difference in the chemical composition 
of the unweathered stone from which it was taken, the soft, gray, broken- 
down surface of No. 2 was tested, and found to contain but 4.82 per cent. 
of silica as against the 10.67 per cent. of the unweathered mass. The 
silica from all the analyses was disengaged as an impalpable powder, and 
it is singular that the insoluble silica should be the first one of the salts 
to disappear in the process of dissolution. Another fact of note is the 
constant presence of alumina and a small quantity of magnesia. The low 
percentage of magnesia doubtless explains why the Mitchell limestones are 
so readily disintegrated by carbon dioxide in solution. 
SUMMARY. 
The caves of the St. Louis group in Indiana may be divided into three 
classes: The wet caves of the lower and more impure Mitchell limestone; 
the subterranean channels, caves and sinkholes of the middle and upper 
Mitchell limestone, and those of the upper Mitchell and Paoli limestone. 
And as to origin: Those in which mechanical forces were dominant; those 
in which the mechanical and chemical action was nearly equal, and those 
in which chemical dissolution was the principal factor. 
