402 
Coal Measures of Central Iowa — Keijes. 
5 d 
Mention has been made inci- 
dentally of the existence of 
clay seams in some portions of 
coals Nos. 1 and 2. A portion 
of these are doubtless due, as 
has been suggested by New- 
berry,* to currents of water 
which corraded through the 
coal beds and associated strata 
— a fine sediment having been 
afterwards deposited; but some 
of these seams present different 
characters, being filled with 
"rubbish" (as called by the mi- 
ners)- — pieces of wood, branches 
and trunks of trees, etc. In 
cases of this kind the chamber 
that is being worked is imme- 
diately closed with solid mason- 
ry to prevent the ingress of 
water. The wood is similar 
to that found in the "forest 
bed" often met with in sinking 
shafts, and it would thus ap- 
pear probable that the corraded 
channels in the carboniferous 
strata might have been filled 
during glacial times. 
The city of Des Moines is sit- 
uated just at the eastern bor- 
der of the middle coal meas- 
ures, the base of which as 
characterized by St. John, and 
as is well shown in different 
sections, is composed of varie- 
gated clays and shales, with 
one or two intercalated bands 
of impure, nodular, limestone. 
These variegated shales have a thickness at Des Moines of thirty 
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*Geol. Ohio, Vol. II, p. ITS. 
