96 PHILOSOPHICAL SOCIETY OF WASHINGTON. 
The principal tributaries entering the rivers frorh the right simi- 
larly conform to two antagonistic laws in their relation to topog- 
raphy: 1. Most of them flow throughout their courses in directions 
coincident with local and general slopes, and avoid elevations in 
their vicinity ; and, 2. Many of them originate with directions ap- 
proaching those normal to their localities, but curve more and more 
to the left toward their mouths, until they flow directly against the 
general slope, and enter the rivers at large angles; and all such 
streams have high north banks which they closely hug, and low 
south banks which they avoid. 
So the drainage system of eastern Iowa is essentially independent 
of the more general topographic features, though affected by local 
topography ; and the relations of the waterways to local topography 
are largely anomalous, and without parallel elsewhere. 
Though essentially continuous stratigraphically, and of unques- 
tionable genetic unity, the loess of eastern Iowa is variable in many 
characters, and may be separated into three geographic divisions; 
viz: 1, the Driftless Region division; 2, the Riparian division ; 
and, 3, the Southern division. That of the first division forms the 
surface throughout the Driftless Region, as it exists in Iowa, and 
everywhere overlaps the eastern border of the drift; it is generally 
rather coarse, heterogeneous, and non-calcareous, and yields depau- 
perate fossils of characteristic species; it reposes upon or gradu- 
ates into a thin stratum of water-worn erratic materials, which, in 
turn, rests upon either the residuary clays of the Driftless Region 
or the margin of the drift-sheet ; its western border is exceedingly 
sinuous, affects the greatest altitudes, and invariably overlooks the 
contiguous drift-plain; and, in capping the elevated Niagara escarp- 
ment, it forms the highest land within hundreds of miles, except in 
northerly directions. The loess of the Riparian division occurs 
chiefly in the elongated ridgesso common and so intimately asso- 
ciated with the waterways in eastern-central Iowa; it is often fos- 
siliferous, and its characters are generally typical; it usually grad- 
uates downward into stratified sands or gravels, which may or may 
not merge into drift; and it invariably seeks the highest summits in 
the region;—for the ridges in which the rivers have carved their 
cafions are always loess-topped; wherever streams avoid low-lying 
valleys for high-lying plateaus, the plateaus are of loess exteriorly; 
and the high northern banks of the aberrant tributaries are gen- 
erally loess-capped. The loess of the Southern division prevails 
