100 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
debris of organic life and other importations the ocean floors are, and 
from an early time have been, the seats of a prevailingly rapid sedi¬ 
mentation, we may reasonably assume that the sedimentary strata have 
there in general attained a thickness of one hundred thousand feet or 
more. Assuming this thickness to be at many points as great as one 
hundred and fifty thousand feet, an amount which the known sections 
revealed on the land indicate to be not improbable, we aj^pear to have 
before our minds the essential conditions of volcanic action as we shall 
now set them forth. 
As strata accumulate in successive layers, those which are buried 
acquire the temperature proper to their depth. The admirable dis¬ 
cussion of the rate of increase of temperature downward in the land 
which the late Professor Prestwich has given us, shows that this 
increment is at the rate of about one degree Fahrenheit for each fifty 
feet in depth. It is possible that the rate of increase beneath the sea 
is greater rather than less than it is beneath the land, but on the 
assumption that it is the same it is probable that at the depth of one 
hundred and fifty thousand feet the heat would be about three thousand 
degrees Fahrenheit, which is all that is indicated in volcanic explo¬ 
sions. Noting the fact that as the detritus accumulates in the sedi¬ 
mentary deposits the interstices between the grains are filled with sea 
water, it becomes evident that this construction water contained in 
the lower levels of the section attains a temperature much higher than 
that to which it can well be subjected in our arts, and that it thereby 
acquires a very great explosive tension. As to the amount of this 
water, some studies of the subject have led me to believe that we may 
assume the proportion of the fluid in ordinarily dense sedimentary 
strata at the time of their formation to be about six per cent of their 
mass, so that the water in the lower fifty thousand feet of the buried 
section would be equal to a sheet having a depth of about three 
thousand feet. If these suppositions be true, the total volume of 
water at a temperature of from two to three thousand degrees 
Fahrenheit which can be looked to as a source of volcanic explosions 
is clearly very great, probably quite sufficient to account for all the 
outbreaks that have occurred. 
A verification of this hypothesis is found in the distribution of 
volcanic vents, their absence or extinction in an area of erosion; i.e., 
the lands are accounted for on the fairly warranted supposition that 
in these realms the isogeothermal planes are by secular refrigeration 
descending towards the center of the earth, and thus the tensions of 
