78 TJiC ^iniericdii (rtoloi/inf. August, 1896 
anticlinal axes or at least with the main divides in the posi- 
tion of the structural troughs. The altitude of the land at 
that period depended f)n the position of the sea coast, which 
Avas probabl}'' not far distant: and consequent!}^ we may con- 
sider the plain to have been i)ractically at sea-level. The 
cycle was terminated by a slight ujilift of the district, estab- 
lishing a new baselevel of erosion, rejuvenating the streams, 
causing them to excavate new valleys, widening them until 
the divides had nearly disappeared as prominent topographic 
forms (except the ''mounds"), and finally completing the new 
peneplain which we shall next consider. 
Peneiilain Xo. 2. While some slight doubt may remain 
concerning the presence of a peneplain passing through the 
summit of the " mounds," none whatever can be entertained 
as to the nature of the plain at their base. It is the most 
characteristic topographic feature of this district and is a 
true basal plain of erosion. Although it corresponds through- 
out a large portion of the district witii the surface of the 
Galena limestone terrane, it is unaffected by the structural 
folds of the indurated formations. In Stephenson county it 
passes within six miles from the surface of the Galena lime- 
stone, through 100 feet of Cincinnati shales and well into the 
lower portion of the Niagara formation. Continuing south on 
the same line into Ogle county, in six miles farther it has 
again beveled the edges of the gently inclined Cincinnati 
strata and cut away half of the Galena limestone. Fifteen 
miles southeastward it coincides with the surface of the St, 
Peter sandstone. In the vicinity of the Mississippi river it 
passes through the gently inclined strata of the Galena. Cin- 
cinnati, and Niagara formations, and always maintains a 
nearly even surface. 
At the foot of the ''mf)unds " this peneplain is usually well 
preserved as a level plain: but in approaching the main lines 
of drainage it has been deeply eroded and remains only as 
widely separated, gently sloping ridges. One of these ridges 
terminates in the high rock ridge on the western side of the 
city of Freeport. Northward from it the country rock is the 
Galena limestone, and beyond the Pecatonica valley tlie first 
remnant of this peneplain is a higher upland ridge about 
three miles distant. But southward from it. where the soft 
