94 PHILOSOPHICAL SOCIETY OF WASHINGTON. 
in direction of drainage in these is fortuitous: 1. The Wisconsin 
Driftless Region so far extends into the northeastern corner of Iowa 
as to include all of the triangular area bounded on the southwest 
by the elevated Niagara escarpment extending from the extreme 
eastward projection of the state northwestwardly to the Minnesota 
line, fifty miles west of the Mississippi. Within this tract, the drain- 
age was originally determined by general surface slope and by rock- 
structure, and the present topography, which is varied and pictu- 
resque, was developed by sub-zrial erosion. 2. Within the far more 
extensive tract formed by the glacial drift and its derivatives, the 
surface is a gently undulating plain, over which the general relief 
is inconspicuous, and the local topography faintly defined though 
singularly uniform and symmetric in character ; and here the par- 
allelism in drainage is prevalent and characteristic. There are, in- 
deed, both local and general exceptions to this parallelism, which 
exemplify a variety of types of aberrant behavior of the streams; 
but while these impair the geographic symmetry of the drainage 
system, they add much more largely to its geologic significance: 
Putting together the instances of accordant, and neglecting the in- 
stances of aberrant extension of water-lines, a normal direction of 
drainage for the whole of the drift-formed tract might be empiric- 
ally determined; which normal direction is represented by a sym- 
metric series of slightly divergent and slightly curved lines, concave 
to the northeastward, radiating from a point north of the state in 
a general southeasterly direction, toward the Mississippi. Probably 
nowhere else on the surface of the globe does so symmetric a normal 
drainage system exist, and assuredly nowhere else does the sum of 
directions of stream-flow over so considerable an area present so few 
examples of departure from the normal. 
The broader topographic features of eastern Iowa are dependant 
upon geologic structure. The dip of the rocks is to the southwest, 
and the outcrops of the several formations represented form suc- 
cessive approximately parallel zones (trending northwest and south. 
east), of which those of the Niagara and Hamilton are widest. Now 
the Niagara rocks resisted well the planation of the pre-quaternary 
eons, and their eastern margin is accordingly defined by a promi- 
nent escarpment varying from 1,000 to 1,350 feet in“altitude, from 
which there is a steep northeasterly slope to the Mississippi, and a 
gentle inclination, corresponding to the dip of the strata, in the op- 
posite direction. The Hamilton rocks, on the other hand, have so 
