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

160 EARTH'S SHIFTING CRUST
are based on the assumption that the whole mass of the icecap
is uncompensated. They are only approximations, and
must be modified as soon as more information as to the de-
gree of the isostatic adjustment of the icecap
is available.
On the other hand, it does not necessarily follow that the
gravity measurements to be taken in Antarctica during the
coming year will be able to provide a definite answer to
the question. It is obvious that the theory of isostasy is at pres-
ent under considerable attack, and that differences of opinion
exist as to the validity of the various methods of interpreting
the findings about the gravitational balance of the crust.
Daly himself remarked that probably none of the present
methods for determining the degree of isostatic adjustment
of a crustal sector was capable of getting very close to the
truth. A far more serious attack on the theory was made some
time ago by Hubbert and Melton. They pointed
out that
all methods of reducing gravity data to measure isostatic
the dis-
compensation depend upon assumptions regarding
tribution of materials in the earth, which must be regarded
as essentially unknown (226:688). They conclude:
The fields providing data on the subject of isostasy are geodesy
[measurement of the shape of the earth], seismology [study of patterns
of earthquake waves], and geology. The data of the first, which until
recently have provided the main support of isostatic theory, have
been shown by Hopfner to be invalid. The data of the second have
only an indirect bearing on the question. The data of the last are
more often than not contrary to isostatic expectations. Hence, the
theory of isostasy must for the present be regarded as resting upon a
none too secure foundation, and it is hardly trustworthy for use as a
major premise in present discussions of earth problems (226:695).
In the following pages I shall proceed on the assumption
that the isostatic principle is generally correct, even though
present methods of measurement are unsatisfactory. I shall
assume that the materials of the earth's surface are under
constant gravitational pressure to conform to the ellipsoidal
shape of the earth, and that this applies to the icecap, and
will bring about a tendency of the crust under the icecap