132 CARBONIFEROUS LIMESTONES 
On approaching the Missouri, the hills bordering its extensive bottoms, known 
as Council Bluffs, attract particular attention, not only from their contour, but 
from their geological formation. Where vegetation has been removed from their 
slopes, they are seen to be composed chiefly of a fine, ash-coloured, siliceous marl, 
or loam, effervescing with acids. In favourable situations, many species of ter- 
restrial and fluviatile shells were discovered, of the same species as are found in 
similar deposits in the Wabash Valley, which are considered contemporaneous with 
the Loess of the Rhine. 
The most abundant species are, Helix thyroideus, H. alternata, H. monodon, 
Helicina occulina, Succinea campestris (?), and Pupa armifera. The base of the hills 
under this marl is gravel and drift; the whole resting on carboniferous strata, which 
show themselves at intervals near the bed of the Missouri, and in a few places at 
the base of the bluffs. 
The bottoms of the Missouri at Council Bluffs vary from eight to twelve, or even 
fifteen miles in width. Towards the narrows of the Nishnabotna, these bluffs con- 
tract in width, until, below the mouth of Nodoway River, they are only two or 
three miles apart. The highlands throughout this distance present great uniformity 
of outline, appearance, and composition, proving that within these limits, embracing 
‘ / 
i! ; t I (ivi 
My AUS 
HILLS OF SILICEOUS MARL, COUNCIL BLUFFS. 
nearly two degrees of latitude, the waters of the Missouri, many hundred miles from 
their embouchure, have been pent up into vast lake-like expansions, at the bottom 
