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BOOK II. 
GHEOGNOSY. 
AN INVESTIGATION OF THE MATERIALS OF THE HARTH’S: 
SUBSTANCE. 
~ Part I.—A GENERAL DESCRIPTION OF THE PARTS OF THE EARTH. 
A piscussion of the geological changes which our planet has under- 
gone, ought to be preceded by a study of the materials of which the 
planet consists. This latter branch of inquiry is termed Geognosy. 
Viewed in a broad way, the earth may be considered as con- 
sisting of (1) two envelopes,—an outer one of gas completely 
surrounding the planet, and an inner one of water covering about 
three-fourths of the globe; and (2) a globe, cool and solid on its ~ 
surface, but possessing a high internal temperature. 
I.—The Envelopes. 
It is certain that the present gaseous and liquid envelopes of the 
planet form only a portion of the original mass of gas and water with 
which the globe was invested. Fully a half of the outer shell or 
crust of the earth consists of oxygen, which there can be no doubt 
once existed in the atmosphere. The extent likewise to which water 
has been abstracted by minerals is almost incredible. It has been 
estimated that already one-third of the whole mass of the ocean has 
been thus absorbed. Eventually the condition of the planet will — 
probably resemble that of the moon—a globe without air or water or 
life of any kind. 
1. The Atmosphere.—The gaseous envelope to which the name 
of atmosphere is given extends at least to a distance of 40 or 45 — 
miles from the earth’s surface, perhaps in a state of extreme tenuity 
to a much greater height. But its thickness must necessarily vary 
with latitude and changes in atmospheric pressure. The layer of air 
lying over the poles is not so deep as that which surrounds the equator. 
Many speculations have been made regarding the chemical 
composition of the atmosphere during former geological periods. 
There can indeed be no doubt that it must originally have differed 
very greatly from its present condition. Besides the abstraction 
of the oxygen which now forms fully a half of the outer crust of 

