320 GQVENIMILIDS, 1s KIB VATS, 
One the other hand, all evidence goes to show that the 
Louisiana limestone gradually becomes thinner as the distance 
increases from the type locality northward, until at Keokuk it is 
less than one fifth of its original thickness. Everything indi- 
cates that it has faded out completely long before the city of 
Burlington is reached. If the Hannibal shales have retained any- 
thing of their normal thickness, as they have in the long distance 
from Louisiana to Keokuk, the horizon of the Louisiana lime- 
. stone would be expected to be not far above the river level at 
Burlington. No bed at or near this horizon has been found that 
would correspond in lithological or any other of the characters 
of the Louisiana formation. The only layer of the whole Bur- 
lington section below the base of the Burlington limestone, that 
at all resembles the Louisiana is the Productal limestone ( No. 
3 of Keyes, No. 4 of Weller ), with a coralline zone at the base 
( No. 3 of Weller ), and overlying the Chonopectus sandstone. 
The lithologial characters of the two are only remotedly related. 
There are strong stratigraphic reasons, however, for connecting 
this stratum, as well as those above it, up to the Burlington 
limestone, with the Kinderhook limestones still farther north at 
LeGrand, in Marshall county, lowa. Still other grounds exist 
for believing the Productal zone at Burlington to be the atten- 
uated margin of LeGrand beds.* 
All the stratigraphic evidence, as disclosed by the Missis- 
sippi River cross-section, the deep wells along the course, and the 
general geological features of the region appear to indicate, 
beyond much doubt, that the Louisiana limestone actually does 
become attenuated northward from the type locality, and that 
the underlying Grassy Creek shales and the overlying Hannibal 
shales merge north of Keokuk. If this be the correct interpre- 
tation, the section at Burlington, below the top of the Chono- 
pectus sandstone, including over 100 feet of shales beneath the 
river level, represents considerably more than the Hannibal 
shales of Missouri. 
™There is, therefore, apparently little possibility of the Productal limestone 
representing anything other than the Chouteau beds as exposed farther south. 
