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CHAPTER VII. 
THE CHANGES WHICH. HAVE TAKEN PLACE IN THE VEGE*- 
TATION OF THE GLOBE IN THE COURSE OF GEOLOGICAL 
PERIODS. 
Geoxocy, or the history of the formation of the earth, proves 
that the distribution of land and water on the surface of the 
globe has not always been the same as it is now ; many dis- 
tricts which are now continents were at one time seas, and 
vice versa. ‘The main agent in carrying away masses of land 
from one spot and reconstructing them in another, has been 
water, either slowly by gradual action, or suddenly by mighty 
convulsions. Some idea may be conceived of the extent of 
this agency, from the reflection that the mass of particles in 
a state of suspension and solution which the nver Ganges 
now carries down to the sea every year 1s equal in bulk to 
seventy pyramids as large as the largest of the Egyptian 
pyramids, that of Cheops. Wherever the substances that are 
carried down are deposited at the bottom of the sea or in 
sea-basins, strata are formed which at first fill up the un- 
evennesses of the ground, the uppermost layers then forming 
gradually more and more horizontal deposits. On strata of 
this nature already in existence fresh layers of different sub- 
stances are everywhere deposited. Such a stratification of 
the crust of the globe has not however occurred once only 
or a few times at one particular spot; but at almost every 
place where the surface has been excavated it manifests a 
structure stratified in the greatest variety of ways. 
Nothing is more natural than that in these changes which 
have taken place on the surface of the earth, multitudes of 
plants! should have become enclosed in the soft deposits of 
1 No account is taken in the sequel of the preservation of animal 
remains, 
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