70 THE GASES IN ROCKS. 



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 erup- 

 tions that water-vapor is by far the most abundant of the gaseous products 

 of volcanoes. Water is also the principal compound of the element hydro- 

 gen, which is quantitatively the most important gas obtained by heating 

 igneous rocks in vacuo. According to one of the common theories of 

 vulcanism, it is water, circulating underground 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 role in 

 vulcanism, is derived originally from the magmas, or is merely underground 

 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 comprehensive 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 than 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 water from coming in contact with the lava in the conduit. 

 Whatever may be the reason for the small amount of chlorine given off 

 by the volcanoes of Hawaii, sea-water does not reach the heated lavas in 

 sufficient quantities to affect them appreciably. 



On account of the pressure exceeding the crushing strength of the 

 rock, pores and crevices can not exist at depths greater than 30,000 feet 

 according to the most generous estimate, 1 and it is probable that continu- 

 ous cracks cease much short of this. Beyond this extreme figure, meteoric 

 waters can not be regarded as of any quantitative importance, on account 

 of the extreme slowness of diffusion through solid bodies not containing 

 minute fractures. Liquid carbon dioxide still existing under great pres- 

 sure in sand grains of Pre-Cambrian age is a concrete example of this 

 slowness. While, theoretically, water may extend downward to the limit 

 of the zone of fracture, the testimony of deep mining appears to show that 



1 Hoskins, 16th Ann. Rept. U. S. Geol. Surv., p. 853. 



