SHALER: EXPULSION OF GASES. 
105 
lias taken place from many cones. Thus in the case of Etna a 
review of the facts has convinced me that the amount of water 
and rock which has been discharged from that vent is probably 
not less than sixteen hundred cubic miles and may much exceed 
that amount, yet the general level of the country on which the 
volcano rests has not only undergone no subsidence, it has risen 
during the period of ejections by the amount of several hundred 
feet. Substantially the same conditions are observable in many 
other volcanic districts. Unless we are to suppose that the volcanic 
ejections come from an indefinite depth, a view which is clearly 
contradicted by the general presence of water in the lavas, we 
have to suppose that the supply of ejecta is derived from a wide 
field about the pipe through which the explosions take place. 
Evidence from the gas wells of this country shows that, under a 
pressure at the base of the shaft of about a thousand pounds to the 
square inch, gas will travel through tolerably close-textured sand¬ 
stones for a distance of perhaps as much as a mile. As the pressure 
at the base of a volcanic column cannot well be reckoned at less than 
fifty thousand pounds to the square inch, and as the movement 
probably takes place in rocks so far softened by heat that they may 
move along with the gas, the conditions are such that we cannot set 
a limit on the range of the movement. It may well be, as some 
b geologists have believed, that craters as remote as Vesuvius and 
Etna in a measure exchange duties in discharging subterranean 
tensions. 
According to the views which have been advocated in this paper 
it would be a difficult matter for the vapors which lead to volcanic 
eruptions to produce a new opening in a region which was already 
provided with these vents. These channels of escape would tend to 
organize themselves with such a distribution as would provide for the 
relief of the strains. This, in fact, appears to be the case, for although 
peripheral cones may be formed about an ancient volcano, the 
development of entirely new centers of eruption in recent times 
seems not to have occurred. The equations of strain and relief 
appear to be matters of ancient adjustment. In this connection it 
may be well to note that we may eventually be able through the 
distribution of volcanoes to gain a clearer idea as to the changes in 
the forms of continents which have taken place in recent geological 
periods. Thus the eastern coast of North America, and in a way 
that of South America as well, is singularly free from volcanic action. 
