GRASSY BLACK SHALE 
309 
measurement westward. Before disappearing below river-level 
in the Keokuk syncline, it reaches a thickness of 30 feet. In sec¬ 
tions at Keokuk it is not definitely recognized or separated from 
the associated shales. At Morning Sun, north of Burlington, it 
is distinctly present in a number of deep-well sections. It is 
traced farther north to beyond Muscatine, where Udden gives 
it-the name of Sweetland formation. There it is 45 feet thick, 
rests in notable unconformity upon the Devonic limestones, and 
has resting upon it unconformably the Des Moines coal measures. 
The Grassy shale is of exceptional paleontological interest at 
this time. Despite its associated faunal affinities it doubtless rep¬ 
resents the nethermost member of the Carbonic section in the 
Upper Mississippi Valley. At Louisiana this shale reclines di¬ 
rectly upon Siluric limestone. A few miles away it lies immedi¬ 
ately upon the Callaway (Devonic) limestone. Farther on, what 
appears to be the Lime Creek shales are found next beneath. 
At its base, therefore, a marked unconformity exists, which is 
also well displayed at the north, above Muscatine. 
As recently noted the settlement of the Grassy black shale 
with the Carbonic section seems to set at rest several moot ques¬ 
tions. It doubtless represents a part of the Chattanooga black 
shales which, in the South, appear, according to the best authori¬ 
ties, to constitute the base of the Mississippian series. It is not 
to be regarded as Devonic in age as suggested by Udden.^® It is 
not a local development of uncertain affinities, as stated by Cal¬ 
vin;^® nor does it underlie the Lime Creek shales as indicated 
in his general section of Iowa. Thus it appears that Owen 
and Norwood,^^ in drawing the line of separation between the 
Devonic and Carbonic strata in the Mississippi valley at the Black 
shale, displayed phenomenally keen insight into the real geologic 
succession of the region. 
As also recently observed the Burlington Kinderhook section 
remains for further explanation. The disposition of the terranes 
admit of another interpretation for that presented.^^ When the 
Devonian Interval in Missouri was under discussion the inclina- 
16 Iowa Geol. Surv., Vol. IX, p. 289, 1899. 
17 Am. Jour. Sci., (4), Vol. XXXVI, p. 162, 1913. 
18 Iowa Geol. Surv., Vol. IX, p. 301, 1899. 
19 Journal of Geology, Vol. XIV, p. 572, 1906. 
20 Iowa Geol. Surv., Vol. XVII, p. 192, 1907. 
21 Researches on Protozoic and Carboniferous Rocks of Central Kentucky, 1847. 
22 Am. Jour. Sci., (4), Vol. XXXVI, p. 163, 1913. 
23 Bull. Geol. Soc. America, Vol. XIII, p. 69, 1902. 
