182 
IOWA ACADEMY OF SCIENCE Vol. XXIV, 1917 
The magnitude of the Prairie du Chien-St. Peter uncon- 
formity in Iowa is impressive. It represents a time during 
which many valleys 150 or more feet in depth were cut in 
silicified dolomite. If it be assumed that half as much was re- 
moved over wide areas as was taken from the valleys and that 
the degradation took place at the average rate of a foot in 
6, 000. years, the time involved amounts to almost half a million 
years. This constitutes one of the two greatest physical breaks 
in the Paleozoic of Iowa, the other being between the Missis- 
sippian and Pennsylvanian systems. 
The unconformity is also important taxonomically. The 
Cam bro-Ordovieian line should be drawn at the top of the 
Prairie du Chien where the unconformity is, rather than at the 
base of the Prairie du Chien where conformability with the 
Jordan sandstone is demonstrated by the presence of twenty 
feet or more of transition beds. 
Geological Laboratories, 
State University. 
JgP^i 
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