PRELIMINARY REMARKS. 
3 
ent periods. No two periods are represented by similar groups, 
and their dissimilarity is practically important, furnishing the 
facts by which the periods themselves may be distinguished 
from one another. It is also an interesting feature in a histori¬ 
cal sense, proving by comparison a progression in development 
along the measures of an ascending scale. The periods as they 
approach the present are represented by plants and animals 
more akin to the living, while in the more remote their resem¬ 
blances as a whole are less. Notwithstanding this, the four 
types of the living are represented in the successive periods, 
with the exception of the first. 
§ 5. Prior to the creation of plants and animals there was a 
period very clearly marked by the reign of physical forces; it 
is azoic, and the rocks of this period have no parallel—they 
are all crystalline. Pleat was the predominant and active 
element. Stability and form was given to the earth in this 
period, principally by the escape of heat into space; and con¬ 
densation of aqueous vapor upon a cooling crust, gave origin 
to the surface waters of the globe. Seas and oceans were 
formed in all the basins and great depressions, and tbe culmi¬ 
nating points gave origin to streams which flowed oceanward; 
but the oceans, ere the waters had filled their bosom, began to 
lose their contents by the vaporization of this new element. 
When it had saturated the atmosphere, it fell again to the 
earth. Thus began that vast machinery by which the earth is 
supplied with rain and dews. It has known no suspension. 
The vapors rise upward, and the streams flow onward in per¬ 
petual cadence. At this stage the consolidated crust begins to 
wear, and its debris also begins to be transported and borne 
onward, and its progress is only arrested by the plains and 
depressions, where it accumulates, giving origin to another class 
of rocks whose parentage is indicated by rounded particles and 
masses. The activity of fire diminishes—that of water increases. 
The two forces are antagonistic of each other. One levels the 
surface, the other breaks it up; the first is constant in its action, 
the latter is paroxysmal. The fire slumbers longer, but never 
