10 
AMERICA THE OLD WORLD. 
tion, though still perceptible, has been partly ob- 
literated, and their substance changed. Such 
effects may often be traced in dikes, which are 
only the cracks in rocks filled by materials poured 
into them at some period of eruption when "lie 
melted masses within the earth were thrown out 
and flowed like water into any inequality or de- 
pression of the surface around. The walls enclos- 
ing such a dike are often found to be completely 
altered by contact with its burning contents, and 
to have assumed a character quite different from 
the rocks of which they make a part ; while the 
mass itself which fills the fissure shows by the 
character of its crystallization that it has cooled 
more quickly on the outside, where it meets the 
walls, than at the centre. 
The first two great classes of rocks, the un- 
stratified and stratified rocks, represent different 
epochs in the world's physical history : the for- 
mer mark its revolutions, while the latter chron- 
icle its periods of rest. All mountains and moun- 
tain-chains have been upheaved by great convul- 
sions of the globe, which rent asunder the surface 
of the earth, destroyed the animals and plants 
living upon it at the time, and were then suc- 
ceeded by long intervals of repose, when all 
things returned to their accustomed order, ocean 
and river deposited fresh beds in uninterrupted 
succession, the accumulation of materials went 
