80 The American Geologist. Au{,Mist, i>(96 
at a greater hight than is attained b}' any of tlie ridges there. 
In short, we have abundant evidence that penei)lain No. 2 has 
been entirely destroyed throughout the countrj'^ near the 
ancient Rock river, and that the upland there found is a later 
and lower eroded peneplnin. 
The present altitude of penei)lain No. 2 in northwestern 
Illinois reaches a maximum, near Warren in Jo Daviess county, 
of 1,015 feet above the sea. This is apparently on a north and 
south axis of deformation of the peneplain, for both eastward 
and westward from it there is a gentle descending slope of 
the upland surface. Near the Mississippi river it slightly 
exceeds 900 feet above the sea, and in the Rock river valley 
its altitude is about the same. Indeed, on the Wisconsin line, 
the peneplain for a score of miles both east and west from 
Warren keeps well up to. the 1,000 foot level. There is a gentle 
descent southward at the average rate of three feet per mile. 
Furthermore, there is an east and west slope from the above 
mentioned axis of deformation, which trends northwest to 
southeast nearly in the position of the axis of the Grand de 
Tour-LaSalle uplift. Along this strvictural ridge, the pene- 
plain, after crossing the state line near Warren at 1,015 feet, 
still maintains an altitude exceeding 900 feet where it crosses 
the present Rock river between Oregon and Grand de Tour. 
At Freeport the peneplain is about 925 feet above the sea; 
and at Rockford, 28 miles east, though none of the ridges in 
the vicinity quite reach its level, it may be placed approxi- 
mately at 850 feet. There is thus a very gentle inclination of 
the peneplain on the northeast slope of the structural ridge 
above mentioned; but from its summit it slopes more rapidly 
toward the sovithwest. I have not the data for definitely de- 
termining its altitude in the southwestern portion of the 
district, but should locate it at Savanna as about 900 feet 
above the sea, and at Rock Island as 800 feet. 
At the close of cycle No. 2 northwestern Illinois was a 
nearly perfect plain, sloping very gently toward the nearest 
sea coast and surmounted by about a score of slight eleva- 
tions, some isolated and others grouped into clusters of hills. 
The altitude of this peneplain depended on the distance to 
the sea. As its age has not yet been determined, we cannot 
say where the coast stood during this cycle. We will assume 
