300 
CEMENT MATERIALS AND INDUSTRY 
[BULL 
The underlying formation, the Opeche, also being soft, the limestone nearly cverj 
where presents an escarpment, and the many canyons which are cut through it hav , 
vertical walls of the limestone. 
The Minnekahta limestone presents more details of structure than any other foj 
mat ion of the Black Hills. Normally it dips outward away from the central area! 
from 5° to 30°, but there are frequent variations in the amount and direction. Thes 
variations are due to the fact that the formation is a relatively hard bed of homogt 
neous rock lying between masses of soft, red shales, so that it was free to flex whei 
ever pressure was exerted, the plasticity of the inclosing beds favoring local flexiniL 
and warping. Its beds are sometimes traversed by small faults and minute crunj L 
plings, but considering the large amount of deformation to which the formation half 
been subjected the flexures are but little broken. The formation is uniform iW 
character throughout, being a thin-bedded, light-colored limestone containing ma§ 
nesia and more or less clay as an impurity. Its thin bedding is a characteristi 
feature, although the thin layers are so cemented together that the formation prri 
sents a massive appearance. On weathering and through the action of frost it break- 
into slabs usually 2 to 3 inches in thickness. On the western side of the Black Hilltl i 
notably in the region from east of Clifton northward, its coloring is slightly darkei, 
varying from dove color to lead gray, and some of the beds present a seminodulaj: 
structure. An increased admixture of clay is also observed in some layers. 1 
general appearance of the formation is always slightly pinkish, with a tinge 
purple, from which the term "purple limestone" originated. The thickness of th 
formation was measured at many points; a few representative measurements are 
follows: 
Thickness of Minnekahta limestone. 
Fee 
Spring Creek 
Battle Creek ■. 
Hot Springs 
Stockade Beaver Creek 28-; 
Cambria well 
This relatively uniform thickness indicates very uniform conditions of depositio 
during the accumulation of the red bed deposits, the Opeche formation below, aj I 
the Spearfish formation above. An analysis of a typical sample of the Minnekah 
limestone is as follows: 
Analysis of Minnekahta limestone. 
Constituent. 
Per cent. 
Lime . 
31.51 
19.85 
.36 
1.25 
44. 66 
.07 
1.12 
None. 
Magnesia 
Alumina, iron, etc 
Water 
Carbonic acid 
Sulphuric acid (SO s ) 
Silica 
Manganese, soda, and potash 
