20 G. K. Gilbert—The Colorado Plateau Province. 
No less important to the student are the cafions themselves. 
They bear the same relation to a plain that geological cross- 
sections do to a geological map. They introduce in all ca 
gories of observation a third dimension, and enable the con- 
of the eastern seaboard that it does now. It was then, as now, — 
a little colder than the latter, and a great deal drier; and it was 
its dryness which prevented, even at an altitude of some thou- — 
sands of feet, the accumulation of such a deluge of ice as visited — 
the Atlantic seaboard in the same latitude. Only on the high- | 
est mountains was the winter’s precipitation in excess of the 
summer’s melting. 
But while the mantling by glacial drift is inconsiderable, 
xtent. me of 
ose 
exceptional thoroughness. They are indebted to ad climate, 
in ancient and modern times, for the almost entire se 
glacial drift, and for the suppression of vegetation. They are 
indebted to peculiar conditions of drainage for their poverty of 
soil, talus, and loeal drift, and for a system of nat cross-sec- 
tions. Their chief detraction is a mere restriction of their area 
of exposure by overlapping lavas. 
IIL. The material Jor study. 
It remains to consider the nature of the material which is so 
fully exhibited, and examine its claims to attention. It pertains 
chiefly to four departments of geological investigation, viz: 
