154 
IOWA ACADEMY OP SCIENCE 
localities were not yet carefully studied*. Comparisons of the Iowa and Missouri 
sections are made in the report on the geology of Des Moines countyf. At one 
time$ it seemed that upon faunal grounds the Kinderhook shales as exposed at 
Louisiana could just as well be included in the Devonic section, but this old 
view long since gave way to the stratigraphic evidence. 
The Chouteau limestone is quite thin on the Mississippi river, but rapidly 
becomes thicker to the westward. At Louisiana the Hannibal shales are 75 
feet thick; at Keokuk, 65 feet; at Burlington about 50 feet of the blue shales 
in the base of the river-bluffs are assignable here. The Louisiana limestone 
which is 50 feet thick at the type-locality, becomes gradually thinner north- 
ward, until at Keokuk it is only 10 feet in thickness, and soon vanishes alto- 
gether, as shown by well-sections. This permits the overlying and under- 
lying shales of Missouri to come together in Iowa and form one continuous 
shale-section. 
Immediately beneath the Louisiana limestone at the original locality are two 
feet of blue shales. This apparently insignificant layer is usually included with 
the Grassy black shales below§. It now seems to have much greater importance. 
Northward from Louisiana these shales rapidly become thicker. At Hannibal 
they measure 50 feet in thickness; at Keokuk probably not less than 50 feet; 
beyond, they merge with the Hannibal shales. 
The Grassy black shales || are only four feet thick at Louisiana. They attain 
a greater vertical measurement northward. Before disappearing below river- 
level in the Keokuk syncline, they reach a thickness of 30 feet. In well-sections, 
at Keokuk they have not been definitely recognized or separated from the as- 
sociated shales. At Morning Sun, north of Burlington, they are distinctly 
present in a number of deep-well sections. They have been traced further 
north to beyond Muscatine, where Udden** has given them the title. of Sweet- 
land beds. Here they are 45 feet thick; rest in notable unconformity upon 
the Cedar limestones; and have resting upon them unconformably the Des 
Moines coal measures. 
Below the black shales there are still other blue shales. They are not exposed 
above river-level at either end of the syncline; but as shown in deep-well sec- 
tions, at Keokuk, there are at least 125 feet referable to them; at Burlington 
about 100 feet; and at Morning Sun, 50 feet. When the lowaff and Missouri$t 
reports were printed it was surmised that this part of the great shale-section 
at Burlington rested directly upon or was an integral portion of the shales 
called farther north the Lime Creek formation. Since that time this view has 
proved to be really correct. The shales in question actually continue in full 
development to the Minnesota boundary. They rest on the Callaway limestone 
in Missouri, which appears to be the exact equivalent of the Cedar limestone in 
Iowa. 
The Grassy shales are of exceptional interest since, in spite of their associated 
faunal asperities, they probably represent the basal member of the Carbonic 
*American Geologist, Vol. X, p. 384, 1892. 
Howa Geol. Surv., Vol. Ill, p. 436, 1894. 
iTrans. St. Louis Acad. Sci., Vol. VII, p. 369, 1897. 
§Proc. Iowa Acad. Sci., Vol. V, p. 66, 1898. 
llProc. Iowa Acad. Sci., Vol. V, p. 60, 1898. 
**Iowa Geol. Surv., Vol. IX, p. 289, 1899. 
ttlowa Geol. Surv., Vol. I, p. 55, 1893. 
JJMissouri Geol. Surv., Vol. IV, p. 56, 1894. 
