Evening star., 1932-10-18
BY DR.. WILLIAM BOWIE,

Chief, Division of Geodesy, United States
Coast and Geodetic Survey.
AS blocks of wood float in a pond
and icebergs in the sea, so do fragments
of the original crust of the earth float
in CONCEALED sea of plastic rock. The
fragments are continents and islands.
How was the original crust disrupted
and what were the consequences?
These are fundamental geological prob
lens whose solution would greatly as
sist scientists who study the earth's
history and makeup.
Great Advance.
A great advance in our understand
ing of the earth'S diversified surface
and the processes which change its con-
figuration was made by the proof OF
the theory of ''isostasy." Isostasy, ad-
vanced as a hypothesis 10 years ago,
has recently been verified by means OF
geodetic data.
The theory of isostasy assumes that,
on an imaginary level surface some dis
tance below sea level, the pressure OF
the material above is everywhere the
same The depth of this imaginary
surface has been found by geodesistS.
using triangulation, and astronomical
and gravity data, to be about 60 miles.
Weight Differences.
Under the continents. the material
above this surface is lighter than the
average and under the oceans 1t is
heavier. 1t is only by this assumption
that the geodetic data can be harmon
lied. These findings of the geodeslst
have been confirmed by Investigations
of the geochemist. He finds that in
the lava exposed on ocean islands the
heavier elements are present in larger
percentages than in the lava rocks
found in the continents.
Isostasy is true. but there are many
minor details connected with it which
still need explanation. Does the crust
extend to an exactly uniform depth or
does it have roots pressed down into
the plastic subcrustal material under
mountains? How large Is the cross sec
ton Of a block of the earth's crust
Independently in equilibrium? How
rapidly does adjustment take place
after a shift of load?
The origin of the earth is now known,
but a billion and half years ago its
surface must have been about as irreg-
ular as it is today. Sediments were
first laid down approximately that long
ago. and therefore there must have
been running water.
If the earth'S surface at that time
had been practically round, smooth
sphere, the water now in the oceans
would have stood at uniform depth over
the earth, and erosion could not have
occurred.
Satisfactory Theory.
AT present there is only one satisfac-
tory theory to account for the oceans
and continents, and it fits in perfectly
with isostasy. According to that theory.
the earth at one time was covered by
crustal material, such as we now see
under continents and islands, spread as
a shell or blanket around the whole
earth.
A change in the speed Of rotation Of
the earth finally brought 1t into syn
chronization with the tides produced
by the attraction of the sun. thus caus-
ing great distortion of the earth. This
distortion broke the crust into frag-
ments.
Finally a mass, composed largely OF
crustal matter, with some subcruStal
material, was thrown off. This caused
a change in the speed OF rotation. and
the earth again became sphere slightly
dattened at the poles.
Major Fragments.
The continents and large islands are
the major fragments Of the remaining
crust, and the many small islands OF
the ocean and the platforms on which
they rest are the minor ones. The
LIQUID subcrustal material welled up
and took the place of the material re-
moved, but. being more dense, could
not rise SO high.
Is it not possible, as suggested by Sir
George Darwin, that the moon was
formed of the materials disrupted from
the earth? The mass of the moon Is
about the same as that of the missing
part of the crust, and its density is ap-
proximately that of the present crustal
material This idea is speculative, OF
course but it seems more logical than
other theories advanced to account for
oceans and continents.