THE GASES IN ROCKS Bay 
lation of vapor gradually working upward until suddenly able to 
relieve itself, it is fair to suppose that aqueous vapor and the auxiliary 
gases are present in the free state at still greater depths. 
It has been the observation of those who have studied volcanic 
eruptions that water vapor is by far the most abundant of the gaseous 
products of volcanoes. Water is also the principal compound of 
the element hydrogen, which is quantitatively the most important 
gas obtained by heating igneous rocks im vacuo. According to one 
of the common theories of vulcanism, it is water, circulating under- 
ground and necessarily dissolving and absorbing mineral and gaseous 
material, which penetrates to the lavas and gives to them their supply 
of vapor and gases. Water, then, is a critical element in the theories 
of vulcanism, and likely to be a decisive factor upon the basis of 
which many of these theories may’ stand or fall. It is, therefore, of 
great importance to know whether the aqueous vapor, which is so 
copiously exhaled from volcanic vents and plays such a réle in vul- 
canism, is derived originally from the magmas, or is merely under- 
ground water which has been incorporated by the lava in its journey 
upward. A decision of this question will carry with it the solution 
of the allied question concerning the ultimate source of the other 
gases, and also throw much light upon some of the more comprehen- 
sive theories of vulcanism. 
Appealing to the fact that chlorine, in the form of hydrochloric 
acid and volatilized chlorides, is one of the products of volcanoes, one 
of the standard hypotheses attributes the cause of vulcanism to the 
penetration of sea water to the heated interior. If this were so, isolated 
volcanoes far out at sea would be expected to yield much more chlorine 
ihan those on the continents. But the Hawaiian volcanoes exhale 
comparatively little chlorine or sublimed chlorides. It has been 
claimed that rain water, sinking into the cone, would have sufficient 
head to exclude the sea water from the neighborhood of the hot lava. 
Rain, however, falls upon but a small part of the whole cone, whose 
greater portion is under the sea. It would seem that if rain water, 
falling upon a cone built up from the ocean bottom, is able, by means 
of its head, to keep out the sea water which covers the lower slopes, 
the same amount of water precipitated upon a continental volcano 
would be even more efficient in preventing the general underground 
