es afford evidence of
these destructive processes continued into Pliocene times. We
have already referred to Schmidt's estimate that the sedimentary
covering must have in places amounted to from 15,000 to 20,000
metres. The evidence for this is mainly tectonic or structural;
but is partly forthcoming in the changes which the materials now
open to our inspection plainly reveal. Thus it is impos-

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sible to suppose that gneissic rocks can become so far plastic as
to flow in and around the calcareous sediments, or be penetrated
by the latter--as we see in the Jungfrau and elsewhere--unless
great pressures and high temperatures prevailed. And, according
to some writers, the temperatures revealed by the intimate
structural changes of rock-forming minerals must have amounted to
those of fusion. The existence of such conditions is supported by
the observation that where the.crystallisation is now the most
perfect, the phenomena of folding and injection are best
developed.[1] These high temperatures would appear to be
unaccountable without the intervention of radiothermal effects;
and, indeed, have been regarded as enigmatic by observers of the
phenomena in question. A covering of 20,000 metres in thickness
would not occasion an earth-temperature exceeding 500 deg. C. if the
gradients were such as obtain in mountain regions generally; and
600 deg. is about the limit we could ascribe to the purely passive
effects of such a layer in elevating the geotherms.

Those who are still unacquainted with the recently published
observations on the structure of the Alps may find it difficult
to enter into what has now to be stated; for the facts are,
indeed, very different from the generally preconceived ideas of
mountain formation. Nor can we wonder that many geologists for
long held

[1] Weinschenk, C. R. _Congres Geol._, 1900, p. 321, et seq.

149

back from admitting views which appeared so extreme. Receptivity
is the first virtue of the scientific mind; but, with every
desire to lay aside prejudice, many felt unequal to the
acceptance of structural features involving a folding of the
earth-crust in laps which lay for scores of miles from country to
country, and the carriage of mountainous materials from the south
of the Alps to the north, leaving them finally as Alpine ranges
of ancient sediments reposing on foundations of more recent date.
The historian of the subject will have to relate how some who
finally were most a