St. Peter Sandstone in N", W. Illinois. — Hershey. 171 
sandstone presents the singular phenomenon of resisting ero- 
sion well en masse, but yielding very readily on angular 
projections. This is characteristic of the St. Peter sandstone 
throughout its extent, and it has been commented on by many 
writers. 
It is here proposed to give a short description of the differ- 
ent formations observed in the area covered by the map. But 
it is presumed that the reader is already well acquainted with 
their general characteristics as exposed in other outcrops, so 
that only those features which seem peculiar to this vicinity 
will be mentioned. 
Description ok the Rock Formations. 
Lower Magnesian limestone. Extending for several hun- 
dred yards along the south bank of Elk Horn creek, where it 
crosses the Freeport and Sterling road two and a half miles 
southwest of Foreston, there is exposed a formation which is 
essentially different from the overlying St. Peter sandstone. 
It reaches a hight of 10 feet above the creek level and pre- 
sents the following section, in descending order : 
Feet. 
1. Brecciated siliceous dolomite apparently broken and 
re-cemented on a seashore 1 
2. Greenish and yellow-brown, argillaceous dolomite, and 
thin laminated siliceous shales :i 
3. Thick, light-buff or yellow stratum of subcrystalline 
dolomite 1 
4. Very thin-bedded siliceous dolomite and cherl -I 
5. Heavy-bedded dolomite, like No. :>. exposed to the 
creek level, about 1 
The brecciated limestone at the surface of the formation is 
perhaps the most interesting of all. A stratum of dolomite 
which was originally laid down and lithified to n compact 
condition, has been broken up into angular fragments of vari- 
ous sizes reaching as much as one foot or more in length, 
which have been thrown together in a confused ina>s and re- 
cemented, apparently by the same dolomitic substance. The 
breccia as here exposed averages one foot in thickness, hut it 
reaches as much as several feet in some other places. The 
contact with the overlying sandstone is Dot well exposed, hut 
it is presumed to be unconformable. Now of the three chief 
processes by which limestone breccias may be produced, 
namely, (1) by pressure, chiefly lateral, as in the Devonian 
