32 
AMERICAN GEOLOGY. 
How do we know that the present valleys and mountains 
were not coeval with the foundations of the earth, or that many 
of them have been formed since it was inhabited? This fact is 
determined, like all other facts, by observation. Though we 
do not witness their formation, still our observations are not 
the less certain and true. We learn first what rocks compose 
the mountain and its valley—the arrangements of their strata, 
and the relative position of the principal and subordinate 
masses. We examine its cliffs, its fractures, and veins. With 
equal care we examine the valley, and compare its formations 
with those of the mountain. We find they agree. If a traveler 
should find by the roadside the parts of a broken walking- 
stick, how would he know that they were parts of one stick? 
He would find that the pieces were the same kind of wood, that 
they were colored and polished alike, and that the ends of the 
fractured parts fitted each other. So, in the same way, the 
strata of the mountain are the same as the valley, and the frac¬ 
tured ends, if brought together, would fit each other. But the 
cliffs are a thousand feet above the valley, and their present 
position is incompatible with the mode of formation of all sedi¬ 
mentary rocks. They are not in the position required by 
sediments, hence a part has been broken from the mass and 
elevated, and now forms the mountain mass, while parts of it 
still form the valley below; and the rocks themselves still 
retain the marks of the operating force in their curved and con¬ 
torted beds. They too are the repositories of fossil remains of 
the same kinds. Long since the time they formed the ocean 
bed, they were raised from the depth of the Rea, and their frac¬ 
tures and dislocations attest the action of forces which elevated 
them to the positions they now occupy. 
§ 29. River systems. The machinery by which the earth is 
watered is extremely simple. The atmosphere, set in motion 
by heat, is the carrier, and mountains and hills the condensers 
of moisture. The rivers receive their supply of water from an 
infinitude of streams flowing from the sides of mountain chains. 
The simple process of condensation of the moisture of clouds 
