14 ELEMENTARY GEOLOGY 
part, the ocean ; and a gaseous envelope, the atmosphere. 
Kach of these is important in the study of geology. 
The Atmosphere. — This, which is everywhere pres- 
ent but invisible, surrounds the earth with its life-giving 
substance, and extends to a height of hundreds of miles 
from the surface, though at elevations of three or four 
miles 1t becomes so thin that breathing is difficult. 
The presence of the air is necessary for the existence 
of animal and plant life. It distributes the heat from 
the sun; and in it, clouds are formed, from which rain 
may fall, while by its movement the winds are made 
to blow. The rain and winds, the heat and cold, 
and indeed the very air itself, are among the most 
important of the agents of change with which geology 
deals. 3 
The air is a mixture of gases. Two of these, oxygen 
and nitrogen, are of chief consequence —about 21% of 
the former and 79% of the latter." The really active 
part of the air is oxygen, for everywhere it is found 
entering into chemical combinations with many sub- 
stances. If a piece of coal or wood is burned, the 
oxygen of the air joins with the carbon, and there is 
formed a gas, composed of the two elements, and 
known under the name of carbonic acid gas (carbon 
dioxide, CO,). If a tree dies and decays, a similar 
1 We omit consideration here of the recently discovered argon, which so 
closely resembles nitrogen, and about which, as yet, we know so little. 
