THE GROWTH OF CONTINENTS. 
127 
tlie reasons above stated, that the term which is 
indeed often, though not invariably, applied to 
them, be exclusively adopted, — that of the Ages 
of Nature. 
But these Ages are themselves susceptible of 
subdivisions, which should also be accurately de- 
fined. What is the nature of these subdivisions ? 
They are all connected with sudden physical 
changes in the earth’s surface, more or less lim- 
ited in their action, these changes being them- 
selves related to important alterations in the 
organic world. Although I have stated that one 
general character prevailed during each of the 
Ages, yet there was nevertheless a constant pro- 
gressive action running through them all, and at 
various intervals both the organic and the physi- 
cal world received a sudden impulse in conse- 
quence of marked and violent changes in the 
earth-crust, bringing up new elevations, while at 
the same time the existing animal creation was 
brought to a close, and a new set of beings was 
introduced. These changes are not yet accu- 
rately defined in America, because the age of her 
mountains is not known with sufficient exactness , 
but their limits have been very extensively traced 
in Europe, and this coincidence of the various 
upheavals with the introduction of a new popu- 
lation differing entirely from the preceding one, 
has been demonstrated so clearly, that it may be 
