SHALER: EXPULSION OF GASES. 
101 
the vapor are continually diminishing. In this case the ability of the 
superincumbent strata to hold in the vapor is continually augmenting. 
Beneath the sea, the deposition of strata, by the consequent rise in 
the isogeothermal jdanes which it induces, leads to a progressive 
increase in the subterranean tensions and to the consequent explosions. 
In a general way the liability to explosion will depend on a compli¬ 
cated equation in which the rate of deposition, the amount of water 
contained in the sediments, and the conductivity of the crust at the 
point in question will be the principal factors. Of these the rate of 
sedimentation is probably in all cases where the process is long con¬ 
tinued the matter of most importance. In case the sedimentation 
rate is so slow that it no more than arrests the downward movement 
of the isogeothermal planes, there would be no increase in the tensions ; 
something like this condition may well exist on certain portions of 
the sea floors when the rate of deposition is slow. 
As the sea bottoms near the sites of lar^e craters receive laro;e 
amounts of sediments from these vents, we perceive that the volcanoes 
may by their contributions in the way of detritus provide a means 
for the perpetuation of their activities. It may be that in a vol¬ 
canic center there is a constant process of working over the strata, 
the materials from the base being returned to the top there to be laid 
down with much water in the interstices which by subsequent accumu¬ 
lations is brought to the zone of high temperatures. As the rate of 
increase in the temperature in going downward in a volcanic district 
is likelv to account for the fact that the laro;e stores of lava which 
are near the surface are much greater than in the average, it may 
be that the heat required to bring about explosions is found at a 
relatively slight depth. In this manner the eruptions occurring in 
a volcanic center such as that of the Java district may, as above 
remarked, help to maintain the conditions which originally led to 
the development of explosions. 
It seems evident that the general conditions of volcanic ejections 
closely resemble those which bring about the formations of the shock 
fountains above described : in both there is the development of gases 
at a high tension at a depth below the surface ; in both the forma¬ 
tions of the columns of ascent take place in the sliaft-like manner ; in 
both the ejections appear to be due to the fact that the gases manage 
in some way to select a particular path for their ascent; in both the 
ejections of the solid and the fluid matter depend upon the existence of 
vapors at high tension. It is of some importance to determine whether 
