32 The Interior of the Earth — Claypole. 
Similar facts may be cited from English geology where the 
Lower Silurian and Cambrian rocks have been forced up from 
depths almost as great as those given above. The Huronian 
series of North America affords another case in point. Some 
of these have been brought to the surface from a depth greatly 
exceeding that which Mr. Davison assigns to the layer of " no 
strain." 
A second point suggests itself as worthy of consideration, 
not as a necessary objection to the theory as a whole but as 
another fact requiring either a change in the depth assigned 
to the neutral layer or some corresponding modification in 
another department of geology, before Mr. Davison's conclu- 
sions can be accepted. The layer of "no strain" must be from 
its very nature, as already remarked, a perfectly qviiescent shell 
in which no movement can possibly occur except the gentle 
secular settlement incident to its condition. No compression 
and no extension being the law of its existence it must form the 
only undisturbed shell in the outer portion of the terrestrial 
sphere. Above it the strata are subject to crushing and below 
it to squeezing. Being without movement it is of course quite 
impossible to find in it the seat of any of those disturbances 
that manifest themselves at the surface. Moreover the shell 
below it exists under such conditions that all movement in it 
must so far as we can see be very gradual and gentle. The 
temperature of its highest layer being about 500° F. and in- 
creasing downward and its mass, at least in the upjDcr part, 
being more or less saturated with water, it must be for the 
most part in a state of aqueo-igneous or even of igneous plastic- 
ity. In such a mass under so constant and enormous a pressure 
no sudden or violent motion can take place. Any slight change 
in the intensity of the pressure will be met and corrected by 
gentle movement or "flow" of the plastic mass. It seems con- 
sequently hopeless to seek here any centre of disturbance or 
commotion. We are therfore driven by exclusion to place all 
such foci in the upper or compressed layer. That is in other 
words, we must seek the seat of the earthquake and of the vol- 
cano within five miles of the surface, in the layer where by 
hypothesis the strata are too cool to suffer much further com- 
pression by cold and are consequently bent and crushed as they^ 
