'6 The American GeotoyisL Anfjust, 1896 
■•'mounds" alone. Now it is* evident that, when tlie baselevel 
passed behnv the summit plane of the "mounds," the drainage 
systems were not in the synclinal troughs, but had been read- 
justed during some previous cycle so as to occupy the posi- 
tion of the anticlinal axes. This does not necessarily imply 
the jjresence of a peneplain, but. as studies in other portions 
of the continent have shown thjit such a readjustment of 
drainage systems is generally accompanied by the formation 
of a peneplain, we may presume that such was the casein our 
district also. It remains to determine, however, whether the 
''mounds" reach the level of the peneplain or are merely the 
worn down remnants of higher elevations. 
The Niagara limestone in Iowa, adjoining the district occu- 
pied by the "mounds," reaches a maximum thickness of 350 
feet. It thins slightly in all directions, but at least 200 feet 
of it is due over these elevations. Now in the central portion 
of Jo Daviess county, where there are plateaus exceeding one 
mile in width, less than half of this thickness of the Niagara 
formation remains. If the summit of the "nu)nnds" is not an 
eroded peneplain, but due to the presence of the resistant Ni- 
agara limestone, we should expect their hight to be dependent 
largely on the width. For instance, a ridge of unequal width 
should have a serrated crest line. It is also ditiicult to under- 
stand the conditions under which subaerial erosion, acting on 
the summit of a ridge, could remove the larger part of a ho- 
mogeneous hard limestone formation and finally leave it as a 
flat-topped plateau exceeding one mile in w^idth. Indeed, the 
evidence is almost conclusive that the removal of the greater 
portion of the limestone was elfected during a cycle in which 
the baselevel of erosion approximately coincided with the 
summit plane of the "mounds," The preservation of these 
remnants of the peneplain, to the present day, is due to the 
fortuitous circumstance of the baselevel bearing such a rela- 
tion to the structural planes as to preserve strips of the Niag- 
ara terrane over the synclinal troughs, while the anticlinals 
carried the surface of the soft Cincinnati shales above the 
]ieneplain, constituting them lines of weakness readily seized 
upon by the streams. 
It is not asserted that all the "mounds" of this district 
reach the level of tlie [jeneplain. Tlie suniniit of man}' of the 
