88 G. K. Gilbert—The Colorado Plateau Province. 
all points of the compass, so as to learn its changes, not merel 
along a simple line, but throughout an extended area. Wit 
such exposures, unconformity cannot escape detection, and the 
history of a system of sediments can be made out with a com: 
pleteness that surely cannot be excelled elsewhere. 
Part IL. Eroston. 
It remains to indicate the scope of the material bearing upon 
the subject of erosion, and with that intent I will discuss cer 
tain problems which the region has propounded. The first 
may be called 
The Problem of the Cations. 
duced only under conditions equally extreme; and natural 
laws are often best tested and exemplified by the consideration 
of their operation under exceptional circumstances. Already 
ess important factors, and in undertaking it I shall take the 
ntegration and transportation. 
nsportation is chiefly peformed by running water. 
