considers only the outer crust, but on the other hand, as we
have pointed out, there are indications in the history of the
earth of periods of relative quiescence followed by periods of
relatively considerable disturbance.

How can these two theories be reconciled in accordance with
what we know of the laws of physics and chemistry and those of
the earth's interior? It seems to me they can by making
suppositions which are perfectly natural regarding the state of
the earth's interior.

We are at liberty to suppose if the facts point that way that
there are the following layers in the earth's masses:--First,
the external, rigid and brittle layer; second, a layer under
such temperature and pressure that it is above its plastic
yield point and may be considered as a viscous fluid. The
pressure must continue to increase toward the center. We do not
know what is the temperature, but it is perfectly possible that
at a greater depth the earth may become rigid once more if the
effect of pressure in promoting solidity and rigidity
continues, as Bridgman tells me he thinks probable. We do not
even have to assume a change in the chemical composition of the
earth's substance, though it is perfectly allowable. This,
then, will be a third layer, once more rigid, perhaps extending
to the center and of very considerable thickness and capable of
accumulating strain from long periods. Blanketed as it would be
by thousands of meters of the first two layers, any change must
be relatively slow.

Kelvin in his computation of the age of the earth from cooling