Coal Measures of Central Iowa — Keyes. 401 
disintegrate into a fine black clay. For this reason the roof 
of coal No. 3. is not very substantial, and soon after the coal 
has been removed, unless properly protected, large masses are 
continually becoming detached. This gradual destruction of 
the roof is often attended by still more serious consequences es- 
pecially just east of the city of Des Moines, on account of the 
presence above the rOof shales of much sand and water. Al- 
though here the coal is much thicker than elsewhere in the 
county it is almost impossible often to obtain it for reasons just 
stated, and lately several mines within this area, have been 
flooded and abandoned. 
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The observations here presented, supplemented by those made 
in various other portions of the Iowa coal field, appear 
to be indicative of the infeasibility of correlating the dif- 
erent coal horizons. In the region under consideration the 
numerous sections point to an attenuation, in all directions 
from the city, of the Des Moines coal fields. Other portions 
of the state afford similar phenomena in corroboration of the 
suggestions already presented, proving conclusively that the in- 
dividual coal beds of Iowa are not as continuous nor as extensive 
as has hitherto been supposed. It is not to be inferred, how- 
ever, that the aggregate amount of coal in the state is actually 
less than has usually been considered, but rather that the various 
seams of coal instead of being continuous over wide areas as is 
popularly regarded, are disposed in a series of broadly lenticular 
layers; or in separate basins, having a maximum thickness in 
the direction of the approximate centre, and becoming greatly 
attenuated toward the borders. The areas occupied by the var- 
ious coal basins might thus encroach upon one another; and a 
boring in one locality would thus disclose coal horizons which 
only a few miles away would be entirely unrepresented. The 
above diagram illustrates this supposed attenuation of three coal 
beds at different horizons. 
