156 
IOWA ACADEMY OP SCIENCE 
The blue shales below the Grassy shales and above the Cedar limestone show 
in deep-well sections a thickness of at least 125 feet. They are without doubt 
a continuation of the Lime Creek shales. Along the Mississippi river they be- 
come attenuated towards the northeast and some little distance south of Musca- 
ine and to the south of Hannibal they fail altogether. Fifty miles southwest 
of the last mentioned place, near Pulton, they appear to be fully represented 
by the 50 feet of Snyder shales which immediately overlie the Callaway lime- 
stone. Prom Burlington to the northwest they are recognizp,ble as far as Mar- 
shall county and characteristic Lime Creek fossils have been taken from well- 
drillings in this district. Prom Marshall the belt swerves to the east some- 
what and in Ployd county the Kinderhook blue shales directly cover them. 
In the delimitation of geologic formations I place far more weight on the 
stratigraphic evidence of a well-marked unconformity than on the occurrence 
of a fauna of Devonic aspects high up in the thick shale succession. To me 
unconformity means more than any other classificatory or correlative criterion*. 
* American Geologist, Vol. XVIII, p. 289, 1896. 
