412 Geology of Johnson County. 
ment of a deep vertical fissure. In this same rock are numerous 
other minor potholes of much interest, many of them showing 
unmistakable evidence of having originated in a larger or smaller 
vertical fissure, their upper margins being invariably rounded, or 
more or less strongly flaring, with often a larger or smaller channel 
-cut through to the east by the overflow, thus demonstrating the 
fact that the flow of water which formed them was from the west. 
Many of these potholes are filled (and all showing evidence of 
having been originally) with a hard, stiff, reddish-brown, sometimes 
brownish-black, clay, similar to the underclay of coal seams. Not 
only are these potholes filled by this clay, but also the numerous 
vertical and horizontal fissures and spaces between the rock bed- 
ding filled with the same material. This reddish-brown color, 
however, is probably due to the infiltration from the, in places, 
very ferruginous drift material, which immediately overlies them. 
About thirty years ago, during the geological survey of Iowa by 
‘James Hall, important facts (since mostly obliterated by the exten- 
sive quarrying of the rock) were observed in regard to this phe- 
nomena; and for the sake of greater clearness in the matter, I here 
give a partial compilation of the description of it as found in the 
‘survey :—? 
“ In a cliff of limestone of the Hamilton Group, at Iowa City, 
the following phenomena is’ observed :— 
“Beneath beds of nearly horizontal limestone appears a black band 
extending thirty or forty feet: this consists of black carbonaceous 
mud, the upper part having the character of cannel coal, and the 
lower part a slaty carbonaceous shale. 
“Beneath this, and less extended, a thicker layer of greenish- 
grey clay, of the character of underclay of coal seams, fills the 
upper and broader part of the cavity ; while below this, and occu- 
pying the deepest parts, is a coarse sandstone, which follows, in its 
line of lamination, the curvature of the limestone upon which it 
lies.” 
This description is illustrated by the following diagram :— 
“ Here we have all the phenomena attending a true coal-measure 
seam of coal: the sandstone, the underclay, and the coal seam rest- 
ing upon it; and to complete the analogy, the slaty portion of the 
seam contains fish teeth of carboniferous character. All this 15 
: Hall’s Geological Survey of Iowa, Vol. I., Part 1, pp. 129-133 and 265. 
