AN intelligent observer of the geological changes progressing in
southern Europe in Eocene times would have seen little to inspire
him with a premonition of the events then developing. The
Nummulitic limestones were being laid down in that enlarged
Mediterranean which at this period, save for a few islands,
covered most of south Europe. Of these stratified remains, as
well as of the great beds of Cretaceous, Jurassic, Triassic, and
Permian sediments beneath, our hypothetical observer would
probably have been regardless; just as today we observe, with an
indifference born of our transitoriness, the deposits rapidly
gathering wherever river discharge is distributing the sediments
over the sea-floor, or the lime-secreting organisms are actively
at work. And yet it took but a few millions of years to uplift
the deposits of the ancient Tethys; pile high its sediments in
fold upon fold in the Alps, the Carpathians, and the Himalayas;
and--exposing them to the rigours of denudation at altitudes where
glaciation, landslip, and torrent prevail--inaugurate a new epoch
of sedimentation and upheaval.
146
In the case of the Alps, to which we wish now specially to refer,
the chief upheaval appears to have been in Oligocene times,
although movement continued to the close of the Pliocene. There
was thus a period of some millions of years within which the
entire phenomena were comprised. Availing ourselves of Sollas'
computations,[1] we may sum the maximum depths of sedimentary
deposits of the geological periods concerned as follows:--
Pliocene - - - - - 3,950 m.
Miocene - - - - - 4,250 m.
Oligocene - - - - 3,660 m.
Eocene - - - - - - 6,100 m.
and assuming that the orogenic forces began their work in the
last quarter of the Eocene period, we have a total of 13,400 m.
as some measure of the time which elapsed. At the rate of io
centimetres in a century these deposits could not have collected
in less than 13.4 millions of years. It would appear that not
less than some ten millions of years were consumed in the genesis
of the Alps before constructive movements finally ceased.
The progress of the earth-movements was attended by the usual
volcanic phenomena. The Oligocene and Miocene volcanoes extended
in a band marked by the Auvergne, the Eiffel, the Bohemian, and
the eastern Carpathian eruptions; and, later, towards the close
of the movements in Pliocene times, the south border
[1] Sollas, Anniversary Address, Geol. Soc., London, 1909.
147
regions of the Alps became the scene of eruptions such as those
of Etna, Santorin, Somma (Vesuvius), etc.
We have referred to these well-known episodes with two objects in
view: to recall to mind the time-interval involved, and the
evidence of intense crustal disturbance, both dynamic and
thermal. According to views explained in a previous essay, the
energetic effects of radium in the sediments and upper crust were
a principal factor in localising and bringing about these
results. We propose now to inquire if, also, in the more intimate
structure of the Alps, the radioactive energy may not have borne
a part.
What we see today in the Alps is but a residue spared by
denudation. It is certain that vast thicknesses of material have
disappeared. Even while constructive effects were still in
progress, denudative forces were not idle. Of this fact the
shingle accumulations of the Molasse, where, on the northern
borders of the Alps, they stand piled into mountains, bear
eloquent testimony. In the sub-Apennine series of Italy, the
great beds of clays, marls, and limestones 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-
148
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° C. if the
gradients were such as obtain in mountain regions generally; and
600° 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. _Congrès Géol._, 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 active in advancing the new views were at first
opposed to them. In the change of conviction of these eminent
geologists we have the strongest proof of the convincing nature
of the observations and the reality of the tectonic features upon
which the recent views are founded.
The lesser mountains which stand along the northern border of the
great limestone Alps, those known as the Préalpes, present the
strange characteristic of resting upon materials younger than
themselves. Such mountains as the remarkable-looking Mythen, near
Schwyz, for instance, are weathered from masses of Triassic and
Jurassic rock, and repose on the much more recent Flysch. In
sharp contrast to the Flysch scenery, they stand as abrupt and
gigantic erratics, which have been transported from the central
zone of the Alps lying far to the south. They are strangers
petrologically,
150
stratigraphically, and geographically,[1] to the locality in
which they now occur. The exotic materials may be dolomites,
limestones, schists, sandstones, or rocks of igneous origin. They
show in every case traces of the severe dynamic actions to which
they have been subjected in transit. The igneous, like the
sedimentary, klippen, can be traced to distant sources; to the
massif of Belladonne, to Mont Blanc, Lugano, and the Tyrol. The
Préalpes are, in fact, mountains without local roots.
In this last-named essential feature, the Préalpes do not differ
from the still greater limestone Alps which succeed them to the
south. These giants, _e.g._ the Jungfrau, Wetterhorn, Eiger, etc.,
are also without local foundations. They have been formed from
the overthrown and drawn-out anticlines of great crust-folds,
whose synclines or roots are traceable to the south side of the
Rhone Valley. The Bernese Oberland originated in the piling-up of
four great sheets or recumbent folds, one of which is continued
into the Préalpes. With Lugeon[2] we may see in the phenomenon of
the formation of the Préalpes a detail; regarding it as a normal
expression of that mechanism which has created the Swiss Alps.
For these limestone masses of the Oberland are not indications of
a merely local shift of the sedimentary covering of the Alps.
Almost the whole covering has
[1] De Lapparent, _Traité de Géologie_, p. 1,785.
[2] Lugeon, _Bulletin Soc. Géol. de France_, 1901, p. 772.
151
been pushed over and piled up to the north. Lugeon[l] concludes
that, before denudation had done its work and cut off the
Préalpes from their roots, there would have been found sheets, to
the number of eight, superimposed and extending between the Mont
Blanc massif and the massif of the Finsteraarhorn: these sheets
being the overthrown folds of the wrinkled sedimentary covering.
The general nature of the alpine structure
{Fig. 8}
will be understood from the presentation of it diagrammatically
after Schmidt of Basel (Fig. 8).[2] The section extends from
north to south, and brings out the relations of the several
recumbent folds. We must imagine almost the whole of these
superimposed folds now removed from the central regions of the
Alps by denudation,
[1] Lugeon, _loc. cit._
[2] Schmidt, _Ec. Geol. Helvetiae_, vol. ix., No. 4.
152
and leaving the underlying gneisses rising through the remains of
Permian, Triassic, and Jurassic sediments; while to the north the
great limestone mountains and further north still, the Préalpes,
carved from the remains of the recumbent folds, now stand with
almost as little resemblance to the vanished mountains as the
memories of the past have to its former intense reality.
These views as to the origin of the Alps, which are shared at the
present day by so many distinguished geologists, had their origin
in the labours of many now gone; dating back to Studer; finding
their inspiration in the work of Heim, Suess, and Marcel
Bertrand; and their consummation in that of Lugeon, Schardt,
Rothpletz, Schmidt, and many others. Nor must it be forgotten
that nearer home, somewhat similar phenomena, necessarily on a
smaller scale, were recognised by Lapworth, twenty-six years ago,
in his work on the structure of the Scottish Highlands.
An important tectonic principle underlies the development of the
phenomena we have just been reviewing. The uppermost of the
superimposed recumbent folds is more extended in its development
than those which lie beneath. Passing downwards from the highest
of the folds, they are found to be less and less extended both in
the northerly and in the southerly direction, speaking of the
special case--the Alps--now before us. This feature might be
described somewhat differently. We might say that those folds
which had their roots farther
153
to the south were the most drawn-out towards the north: or again
we might say that the synclinal or deep-seated part of the fold
has lagged behind the anticlinal or what was originally the
highest part of the fold, in the advance of the latter to the
north. The anticline has advanced relatively to the syncline. To
this law one exception only is observed in the Swiss Alps; the
sheet of the Brèche (_Byecciendecke_) falls short, in its northerly
extension, of the underlying fold, which extends to form the
Préalpes.
Contemplating such a generalised section as Professor Schmidt's,
or, indeed, more particular sections, such as those in the Mont
Blanc Massif by Marcel Bertrand,[1] of the Dent de Morcles,
Diablerets, Wildhorn, and Massif de la Brèche by Lugeon,[2] or
finally Termier's section of the Pelvoux Massif,[3] one is
reminded of the breaking of waves on a sloping beach. The wave,
retarded at its base, is carried forward above by its momentum,
and finally spreads far up on the strand; and if it could there
remain, the succeeding wave must necessarily find itself
superimposed upon the first. But no effects of inertia, no
kinetic effects, may be called to our aid in explaining the
formation of mountains. Some geologists have accordingly supposed
that in order to account for
[1] Marcel Bertrand, _Cong. Géol. Internat._, 1900, Guide Géol.,
xiii. a, p. 41.
[2] Lugeon, _loc. cit._, p. 773.
[3] De Lapparent, _Traite de Géol._, p. 1,773.
154
the recumbent folds and the peculiar phenomena of increasing
overlap, or _déferlement_, an obstacle, fixed and deep-seated, must
have arrested the roots or synclines of the folds, and held them
against translational motion, while a movement of the upper crust
drew out and carried forward the anticlines. Others have
contented themselves by recording the facts without advancing any
explanatory hypothesis beyond that embodied in the incontestable
statement that such phenomena must be referred to the effects of
tangential forces acting in the Earth's crust.
It would appear that the explanation of the phenomena of
recumbent folds and their _déferlement_ is to be obtained directly
from the temperature conditions prevailing throughout the
stressed pile of rocks; and here the subject of mountain
tectonics touches that with which we were elsewhere specially
concerned--the geological influence of accumulated radioactive
energy.
As already shown[1], a rise of temperature due to this source of
several hundred degrees might be added to such temperatures as
would arise from the mere blanketing of the Earth, and the
consequent upward movement of the geotherms. The time element is
here the most important consideration. The whole sequence of
events from the first orogenic movements to the final upheaval in
Pliocene times must probably have occupied not less than ten
million years.
[1] _Mountain Genesis_, p. 129, et seq.
155
Unfortunately the full investigation of the distribution of
temperature after any given time is beset with difficulties; the
conditions being extremely complex. If the radioactive heating
was strictly adiabatic--that is, if all the heat was conserved and
none entered from without--the time required for the attainment of
the equilibrium radioactive temperature would be just about six
million years. The conditions are not, indeed, adiabatic; but, on
the other hand, the rocks upraised by lateral pressure were by no
means at 0° C. to start with. They must be assumed to have
possessed such temperatures as the prior radiothermal effects,
and the conducted heat from the Earth's interior, may have
established.
It would from this appear probable that if a duration of ten
million years was involved, the equilibrium radioactive
temperatures must nearly have been attained. The effects of heat
conducted from the underlying earthcrust have to be added,
leading to a further rise in temperature of not less than 500° or
600° . In such considerations the observed indications of high
temperatures in materials now laid bare by denudation, probably
find their explanation (P1. XIX).
The first fact that we infer from the former existence of such a
temperature distribution is the improbability, indeed the
impossibility, that anything resembling a rigid obstacle, or
deep-seated "horst," can have existed beneath the present
surface-level, and opposed the northerly movement of the
deep-lying synclines. For
156
such a horst can only have been constituted of some siliceous
rock-material such as we find everywhere rising through the
worn-down sediments of the Alps; and the idea that this could
retain rigidity under the prevailing temperature conditions, must
be dismissed. There is no need to labour this question; the horst
cannot have existed. To what, then, is the retardation of the
lower parts of the folds, their overthrow, above, to the north,
and their _déferlement_, to be ascribed?
A little consideration shows that the very conditions of high
temperature and viscosity, which render untenable the hypothesis
of a rigid obstacle, suffice to afford a full explanation of the
retardation of the roots of the folds. For directed translatory
movements cannot be transmitted through a fluid, pressure in
which is necessarily hydrostatic, and must be exerted equally in
every direction. And this applies, not only to a fluid, but to a
body which will yield viscously to an impressed force. There will
be a gradation, according as viscosity gives place to rigidity,
between the states in which the applied force resolves itself
into a purely hydrostatic pressure, and in which it is
transmitted through the material as a directed thrust. The nature
of the force, in the most general case, of course, has to be
considered; whether it is suddenly applied and of brief duration,
or steady and long-continued. The latter conditions alone apply
to the present case.