Then there would be four layers to the earth like the butternut
of the figure. First, the inner kernel of gas; second, the hard
shell or endocarp; third, a viscous layer like the sarcocarp or
pulp, and outside of all the wrinkled crust of exocarp. If such
is the structure of the earth we may have in the very structure
of the earth itself a reason why from time to time there are
collapses of the middle layer leading to elevations of portions
of the outer rind, and marking off the chapters in geological
history, the lines between geological systems.

There are reasons in facts of observation for believing that
such is the structure of the earth, of which I have as yet said
nothing. We see the interior of a glass marble, I saw the
bubble in the interior of Bridgman's glass marble, how? By
waves, vibrations, which start from the sun or some other
source, and going through it reach my eye. Though the earth is
not penetrated by sunlight it is penetrated by the waves and
vibrations that start from that jar produced by a crack which
we call an earthquake. These vibrations can be received by that
eye of the geologist called a seismograph. The seismologist
tells us there are three kinds of waves sent out in an
earthquake. If you notice the explosion of a blast at a little
too close distance you will notice that you see it first, then
hear it, and then perhaps a little later a few chips of rock
may come flying past your ears. These three things correspond
somewhat to the three kinds of waves which spread forth from an
earthquake. But in the case of the explosion we see the blast
first, then hear later. The waves which produce the sensation
of sight are, we know, lateral disturbances, the waves which