Coal Measures and Limestone in Iowa. — Keyes. 99 
danger, and perhaps one reason for their supercession by the 
later and lighter ganoid and teleosts may be that the greater 
activity of the latter enabled thera to attack in the only vul- 
nerable place. Be this as it may. the fact remains that these 
ponderous creatures died out. Their load of bony plates sank 
to the bottom and there lay sometimes almost undisturbed in 
the muddy shale only to be resurrected by man after millions 
of years had passed away. 
THE UNCONFORMITY OF THE COAL MEASURES 
AND THE ST. LOUIS LIMESTONE IN IOWA. 
By Charles Rollin Keyes. 
At Keokuk, in the extreme southeastern corner of Iowa, 
outliers of Coal Measure deposits are found in a number of 
places near the summits of the bluffs bordering the Missis- 
sippi and Des Moines rivers. The bluffs rise to a hight of 
150 to 200 feet above the river level. The} T are made up al- 
most entirety of massive limestones of Lower Carboniferous 
age. The uppermost member of the calcareous series is the 
St. Louis Limestone. As represented in this locality it is 
partly a compact, coarse-grained limerock, regularly bedded, 
partly a brecciated limestone. The St. Louis in Iowa is of 
particular interest as it is the great floor upon which were 
deposited unconformably the Coal Measures of the region.' 
The unconformity is quite marked at numerous places. The line 
of contact is displa} T ed distinctly in a ravine north of Rand 
park, Keokuk, as shown in the accompan3 T ing plate taken 
from photograph (plate iv). The lower light portion is the 
brecciated St. Louis limestone; the upper dark part is the 
basal sandstone of the Coal Measures. The latter is a rather 
soft, friable rock, light buff or brownish in color and fills the 
uneven, channeled surface of the St. Louis limestone. 
As early as 1857, in a paper published in the American 
Journal of Science, James Hall wrote: "I have ascertained in 
the most satisfactory manner that the coal fields of Iowa, 
Missouri and Illinois rest uncomformably upon the strata be- 
neath, whether these strata be Carboniferous limestones, De- 
vonian, Upper Silurian or Lower Silurian Rocks."* Although 
*Ani. Jour. Sci., (2), vol. xxvii, pp. 197, New Haven, 1857. 
