8t. Peter Sandstone in X. W. Illinois. — Hershey. 173 
down for hundreds of miles in every direction. On the con- 
trary, whenever, in passing downward through the St. Peter 
sandstone, the limestone is reached, it is classed as Lower 
Magnesian or CalcifeTOUS limestone, and the sandstone beds 
which are intercalated with it as Calciferous sandstone. I 
shall follow this classification here for the following, in addi- 
tion to the above reasons. The dolomite, shale, chert, and 
breccia here discussed, are an exact imitation of portions of 
the Lower Magnesian of Wisconsin and Missouri. Moreover, 
the same erosion interval appears to be present here as has 
been shown to exist in Wisconsin between the Lower Magne- 
sian limestone and St. Peter sandstone. A slight nonconform- 
ity is doubtless proven b} T the phenomena of the breccia : but 
a greater unconformity is inferred from the following facts : 
Over the highest part of the dolomite exposure the sandstone 
is about 75 feet thick. To the south and west the sandstone 
sometimes reaches 100 feet or more above the creek, and yet 
the creek bed shows sandstone. Also, a short distance east of 
a 6 foot exposure of the dolomite it was only reached in a 
well at 30 feet below the creek level, although both dolomite 
and sandstone are apparently nearly horizontal at this place. 
Furthermore, the St. Peter sandstone is 207 feet thick in 
Stephenson county to the north, and about 200 feet thick at 
the outcrop on Rock river. It is evident that there is consid- 
erable variation in the thickness of the sandstone in this por 
tion of the state. But the upper surface of the sandstone 
appears to be nearly even; hence it must be the lower surface 
whose unevenness produces this variability in thickness. This, 
of course, is caused by a very uneven surface of the Lower 
Magnesian limestone, which was most probably produced by 
subaerial erosion in northwestern Illinois, as in Wisconsin. It 
must he acknowledged that the evidence supporting the 
hypothesis of a ( 'amhro-Si luriaii land surface in Ogle county. 
Illinois, is very weak; but there must have been one small 
spot on the site of tin- presenl Elk Horn valley that was an 
island over which the waves broke jusl previous to the depo- 
sition of the St. Peter sandstone. 
The altitude of the top <»f the Lower Magnesian limestone 
in the Elk Horn valley is about 800 feel above the Bea, mak- 
