246 POPULAR SCIENCE MONTHLY. 



source of the water wliich, converted into steam, takes sucli a con- 

 spicuous part in volcanic eruption is open to several objections. 



In almost all land areas the rocks below tlie surface are satu- 

 rated with water, the source of which is mainly rain. Excepting 

 that the pressure of the sea on its floor tends to force water into 

 the rocks beneath, there does not seem any good reason for con- 

 cluding that the earth's crust where covered by the sea is more 

 highly charged with water than the portions beneath land areas. 



Another argument for the presence of sea water in volcanoes 

 is that after eruption the country about a volcano is sometimes 

 whitened for many miles with salt, and also that some of the 

 vapors arising from volcanic vents are such as might be expected 

 to occur if the substances contained in sea water were sufficiently 

 heated. It is to be remembered, however, that large bodies of 

 salt derived in some instances from the evaporation of sea water 

 occur among stratified rocks, and also that many sedimentary de- 

 posits are saturated with saline water. It thus becomes evident 

 that communication between the conduit of a volcano and the sea 

 is not the only means by which saline water can come in contact 

 with molten rocks. 



It is well known that volcanoes as a rule are located near the 

 borders of continents, or on the floor of the sea. This fact is more 

 in harmony, however, with the idea proposed by Dana, that the 

 margins of continents are determined by the location of weak 

 belts in the earth's crust, along which maximum movement takes 

 place, than that the presence of surface water bodies is essential 

 to the existence of volcanoes. In support of this conclusion it 

 may be pointed out that volcanoes of recent date occur in the 

 Great Basin, hundreds of miles distant from the Pacific. The 

 Great Basin is a region of faults, and as much a belt of weak- 

 ness in the earth's crust as if it had chanced to be situated near 

 the sea. 



Owing to the increase of pressure with depth, it is evident that 

 cavities in rocks in which any considerable bodies of water can be 

 stored must become less and less frequent as the distance below 

 the surface increases. As has been shown by Van Hise, at a depth 

 in excess of about thirty thousand feet what may be termed ap- 

 preciable cavities can not exist. Rocks under pressure become 

 compact, so that deeply seated rocks must be less porous than 

 similar material near the surface. These considerations lead to 

 the conclusion that water- charged portions of the earth's crust are 

 superficial. Hence the water given off by volcanoes in the form 

 of steam, and probably also the gases produced by the dissocia- 

 tion of the elements composing water and the vaporization of the 

 various salts it contains, must reach volcanic conduits in their 

 upper portions. These considerations add strength to the conclu- 



