St: Peter Sandstone in X. W. Illinois. — Hershey. 177 
bare. These two secondary axes are so close together that the 
synclinal trough between is almost imperceptible. It is rath- 
er to he described as a flat-topped uplift from one to two 
miles wide, with a slight axis or ridge at either side of it. 
But from the fact that these two bordering ridges diverge and 
become more easily distinguishable to the west, I prefer to 
consider them as two anticlines. They sweep across the dis- 
trict in slightly curved lines, trending in a general east anil 
west direction, and concave to the north. On a line directly 
south from the city of Freeport, the curvature is rather more 
decided than further east or west, the axes there turning from 
a slightly south of east to a slightly north of east direction. 
It has been observed that all the deformations of this portion 
of Illinois which come in by gently curved lines from Iowa 
turn rather abruptly toward the east-northeast on or near this 
same north and south line. From this fact, and because there 
was sometimes a small island and alwa} r s an ascent to an ele- 
vated part of the sea bottom on the site of the present Elk 
Horn valley, it is inferred that this line occupies the position 
of the crest of a southern prolongation of the ancient area of 
uplift which has been frequently denominated the "Isle of 
Wisconsin." This, in the subsequent reelevation and corru- 
gation of the territory, would determine the position of the 
most southern point of the various anticlinals which Chamber- 
lin, McGee, and other geologists have shown to sweep around 
the Isle of Wisconsin in approximately concentric courses. 
Of the two east and west anticlines crossing this area, the 
northern one is the most prominent. The dip on the north 
side is about 100 feet per mile, or 1.7°. This is soon decreased 
to about 40 feet per' mile and so continues six miles to the 
axis of the next synclinal, which lies on the Stephenson and 
Ogle county line. This synclinal is occupied by an upland 
ridge, underlain by Niagara limestone, the edge of which is 
only four miles distant from the outcropping sandstone, and 
less than loo feel higher. The westward prolongation of the 
anticline is occupied by the valley of Carroll creek, where 
Hon. James Shaw, in his "Geology of Northwestern Illinois," 
describes a slight axis. Eastward from this area, the posi- 
tion of the valley of Leaf river was probably largely deter 
mined by this anticline; for it i- the rule in this pari of the 
