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1976, H. Jeffreys clings to this explanation of orogeny basing
the cooling on the gradual decline of radioactivity.
The Alps were from early times the classical ground. Here
the shortening of the cross section of the geosyncline during
the orogenic phase was repeatedly calculated by turning the
folds mentally into flat layers. The figure was first around
500 km, then gradually shortening down to about 250 km,
when it was realized that the folding was essentially due to
gravitational gliding during the geosynclinal stage, and was
a questionable measure of an “approachment of the strong
thrusting borderlands”.
Still, the thrust seen in the germanotype block faulting of
the consolidated Hercynian rocks on the north side of the Alps
was obvious enough. But that is no proof of a thrust from
Africa right across the weak geosynclinal filling. Of course,
these hard rocks would form a mold for the weak folding
rocks when, due to gravitational flow, these weak masses met
harder rocks. Here, and in connection with the later uplift of
the more or less consolidated mountain chain, there was quite
enough cause for a stress field, in the Hercynian foreland to
cause block faulting.
After the discovery of radioactivity, shrinking of the earth
lost general interest, and Wegener’s hypothesis of continental
drift was generally abandoned between 1930 and 1940.
But one thing remained firmly rooted from the old times:
If the cooling of the earth did not do the trick, then some
other internál process must help. There must be currents in
the Upper Mantle which, first formed the mysterious globally
encircling and swaying narrow geosynclines and, second, thrust
them together when something triggered that process. The
famous deep trenches could be such newly formed geosynclines,
some hoped.
With this faith in the Upper Mantle possibilities, the Geo-
physical Monograph 13 (of the Amer. Geophys. Union, 1969)
was edited under the title The Earth’s Crust and Upper Mantle.
This valuable collection of material had the aim to pave the