St. Peter Sandstone in JV. W. Illinois. — Hershey. 169 
under great heat and pressure, upon the limestone blocks, and 
it is, therefore, a marked instance of contact metamorphism. 
Chondrodite is in fact a mineral resulting from the contact 
action of igneous magmas on magnesian rocks, and, far from 
being a function of their geological age, it is most certainly 
indicative of the close proximity of intruded igneous rocks. 
In conclusion, from the establishment of the continuity of 
the white and the blue fossiliferous limestones in New Jer- 
sey ; their conformability with the fossiliferous Cambrian 
quartzite; their unconformability with the underlying 
gneisses; their identity of chemical composition; their dif- 
ferentiation only through igneous action ; and their similar- 
ity of dip and strike — the writer has proved their Cambrian 
age and that their reference to the Arclnean is no longer 
tenable. 
THE ELK HORN CREEK AREA OF ST. PETER 
SANDSTONE IN NORTHWESTERN 
ILLINOIS. 
By Oscar H. Hershey, Freeport, 111. 
In the various reports of the Illinois Geological Survey we 
find mentioned and described only three areas of outcrop of 
the St. Peter sandstone in the state, namely, along Hock river 
from Grand de Tour to Oregon, along the Illinois river in La 
Salle county, and at the Cap an Gres bluff in Calhoun county. 
Nor have I been able to learn from any other published source 
of the surface exposure of this characteristic formation in 
any but the above tracts. So when, some months ago, I acci- 
dentally discovered a considerable outcrop of the sandstone 
in a valley in the extreme western part of Ogle county I made 
a somewhat detailed study of it, which study lias resulted in 
bringing to light several perhaps not unimportant facts. 
As indicated by the accompanying map, the outcrop under 
discussion occupies the valley of the Elk Horn creek and 
tributary streams, in Ogle county, about one to seven miles 
southwest of the village of Fores ton. It reaches the Carroll 
county line at several points, at the most northern of which it 
extends into that county about half a mile. 
The topography of the tract is essentially that of the drift- 
