7he Interior of the Earth — Claypole. 33 
sink to a smaller spherical surface by the contraction of the 
hotter sphere beneath them. Here and here alone according 
to the theory now under consideration, among- these yielding 
and breaking strata, can we seek with any hope of success the 
focus of the earthquake that spreads destruction and ruin at the 
surface, and of those volcanic vents which from time to time 
pour forth their streams of lava. This doctrine has long been 
familiar to seismologists. Here they have been accustomed to 
place the cause of the sudden jar that produces the one and the 
changing conditions of pressure to which in the present state of 
our knowledge we attribute the other. 
But it will cause them not a little difficulty to learn that they 
must descend no deeper than five miles for this puri^ose. They 
will feel a serious objection to being told by the mathematician 
that they must limit their seismic investigations to so thin a layer 
of the crust. Possibly to this they must come at last but much 
readjustment will be necessary for the purpose. Few vulcanol- 
ogists have yet placed their foci so near the surface. The late 
Mr. Mallet inferred from his observations that the shock of the 
great Neapolitan earthquake of 1857 radiated from a centre at 
the depth of seven miles, and the calculations of Capt. Dutton 
and other geologists of the U. S. survey have led them to place 
the seismic focus of the late Charleston earthquake at the depth 
of twelve miles. Probably neither Mr. Mallet nor Capt. Dutton 
would urge these results as anything more than approxima- 
tions. But they and physical geologists generally will scarcely 
be willing to abandon their calculations without Stronger evi- 
dence. The difference caused in the phenomena of an earth- 
quake by the transfer of its focus from a depth of twelve miles 
to one of only five miles from the surface would be so great 
that it could not well be altogether due to errors either of ob- 
servation or of calculation. 
One other remark of Mr. Davison's deserves a moment's 
notice in passing. He makes the statement that " owing to the 
continental wrinkles the amount of stretching under them must 
have been very much less than under the great oceanic areas." 
But it is not easy to see how this can be the case. It seems an 
unavoidable inference from the nature of the layer of " no strain" 
that below it there can be no violent disturbance and that the 
