HYVPOTMESTS OF CAUSE {OF GLACIAL PERIODS 559 
included gases be constant for the whole body of the earth, it 
contains potentially a multitude of atmospheres. 
It is a familiar fact that enormous quantities of gases are 
ejected from volcanoes during their active periods. It has been 
very generally assumed that these gases and vapors, among which 
steam vastly preponderates, have a surface origin, and there can 
be no doubt that this is true of some notable part; but, on the 
other hand, there is abundant ground for the belief that another 
notable part is brought from the interior and is a real contribu- 
tion to the earth’s atmosphere and hydrosphere. It may not be 
possible at present to demonstrate this, but inquiry in this 
direction is invited. There seem to be no means of estimating 
from present data even approximately the volume of gas which 
is given forth, but it is certainly large. There are grounds for 
believing that the gases of the interior escape by other than 
volcanic vents. The deep rending and sharp shock of the earth 
in seismic movements, the stresses and fissuring of readjustments, 
the disintegration of crystalline rocks, and the resources of slow 
diffusive penetration are among these. So far as the setting free 
of carbon dioxide by decomposition of the containing rock is 
concerned, it is to be noted that the chemical action which sets 
it free involves a consumption of carbon dioxide very greatly in 
excess of the amount liberated, so that, as computation will 
show, the total effect of the process is one of loss which the 
internal gases only very slightly modify. This, of course, is not 
true of mechanical disintegration. 
b. Exterior sources of gain The meteorites which are con- 
stantly falling to the earth contain included gases, often in great 
volume. They also contain carbonaceous matter which is par- 
tially burned in passing through the air. The nature of the 
included gases is notably similar to those of the crystalline 
rocks, hydrogen and carbon dioxide being the leading constitu- 
ents with nitrogen in very subordinate amount and free oxygen 
essentially absent... Water vapor appears in both meteorites and 
™See numerous papers of A. W. WRIGHT in Am. Jour. Sci., notably Gases con- 
tained in Meteorites, Am. Jour. Sci, 3d series, Vol. XII, No. 69, Sept. 1876. 
