The Earth's Shifting Crust by Charles Hapgood - wire of information

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EARTH'S SHIFTING CRUST
evennesses, corresponding to those at the surface, and that
the crust varies considerably in thickness from place to place.
They think, for example, that the crust is thicker under the
continental surfaces, and thinner under the oceans, and that
it is thickest of all under mountain ranges and high plateaus.
Continents and mountain ranges not only stick up higher
but they also stick down deeper. That is because they are
composed, as an average, of lighter rock. The analogy is to
an iceberg. An iceberg floats with one tenth of its mass above
sea level, and nine tenths of it submerged. It is lighter than
water per unit volume, and floats in the water displacing its
own weight, and leaving its own excess volume above the
surface. Continents and mountain chains, composed on
the average of lighter rock, stand in the same sort of hydro-
static, gravitational balance, and their downward projections
are thought to be much greater than their upward, visible
projections. The downward projections of mountain chains
are called "mountain roots."
The underside of the crust, then, has a sort of negative
geography. The features of the upper surface are repeated in
reverse on the undersurface, although, naturally, the details
are missing. The effects are rather smoothed out. We should
expect that the Rocky Mountains would make a sizable
bump on the underside of the crust, but we couldn't expect
to find any small, sharp bump just under Pikes Peak. The
tensile strength of the crust, though limited,
is sufficient to
smooth out the minor features.
As we attempt to envisage the situation at the bottom of
the crust, we must remember that the rocks are subjected to
increasing pressure with depth, and probably to increasing
heat, and as a consequence they must tend to lose their rigidity
and strength. We don't just come suddenly to the bound-
ary of the crust at a given depth. On the contrary, the crust
just fades away. The rocks of the lowest part of the crust
must be very weak indeed, so that a very slight lateral pressure
may suffice to displace them.