26 ELEMENTARY GEOLOGY 
place, and that throughout nature there is a tendency 
to change. Old combinations are breaking up and new 
ones being made. The earth’s crust is, im fact, a great 
chemical laboratory, where mnumerable and important 
reactions are always in progress. 
In the air we find the only common, uncombined ele- 
ments at the surface, One of these, nitrogen, is so 
inert that it does not readily enter mto combination in 
the crust; but the other, oxygen, is always doing so, 
and among the rocks there are vast quantities of this 
element, that have come out of the air to form 
mineral compounds. This change is occurring even 
at the present time. 
Let us briefly examine some of the chief elements 
with especial reference to their importance in geology. 
Oxygen (0). — By far the most abundant and im- 
portant is oxygen, which is the great active element 
of the earth. Not only does it exist in the atmosphere, 
but, carried by spring water, it enters the crevices of 
the rocks and passes down into the crust. 
When decay, either of mineral or organic materials, 
is In progress, oxygen is at hand to aid in the work. 
When conditions favoring combustion are present, it 
is oxygen that is necessary to promote the change. 
So aggressive is this element that it has entered into 
all known parts of the earth; and by the process of 
oxidation it is still engaged in extending its range. 
