Erosion Cycles in Xorfhirestevii Illinois. — Jlershey. 97 
the former are the result of a later C3'^cle of erosion, and that 
the apparent equality in size is due to a ditferenee in resistant 
properties of the rock formations excavated. The hard cherty 
Lower Carboniferous limestones of southern Missouri will re- 
sist the erosive power of the streams several times as well as 
will the rather soft Galena limestone. The habit of the latter 
to stand in perpendicular blutfs is deceptive, and to determine 
the true relation between their resistant properties they should 
be studied in contiguous areas. 
Origin op Drainage Lines. 
The problem of determining the origin of the preglacial 
courses of the streams in northwestern Illinois is tf>o complex 
for the present paper. T will advance, without argument, a 
few propositions in regard to it. 
When northwestern Illinois emerged from the sea near the 
close of the Paleozoic era, the drainage lines were consequent 
on the structure. By the close of cycle No. 1, a complete re- 
adjustment had been brought about, placing the divides in 
the position of the synclinal axe^. During the second erosion 
cycle the streams, revived by an uplift, trenched valleys to 
the new baselevel. and cut awa}^ the divides, almost complet- 
ing a perfect peneplain. At the close of this cycle the divides 
were still over the synclinal troughs, and the drainage systems 
corresponded in a general way with the present (the Missis- 
sippi river being a probable exception). During another re- 
vival of erosive energy, the basins constituting the so-called 
third peneplain were excavated. At the close of this cycle, 
the streams had cut down into the rock-bed of their channels, 
and had trenched new valleys, in some instances in meander- 
ing courses, but in the greater portion of our district compar- 
atively straight. The Pecatonica valley from F'reeport to its 
mouth is a good example of the latter class. This portion of 
the valley is thirty miles king, averages between one and two 
miles in width, and is so straight that a line drawn through 
it would scarcely touch a bluff. Its position seems to be con- 
trolled bj'^ a monoelinal axis, or possibly by a fault. All the 
smaller streams of the district endeavor to follow the axes of 
anticlinal folds as closely as possible. Moreover, as the basins 
persistently follow these same lines, this habit was acquired 
on jjcneplain No. 2 (Tertiary?). 
