[1] Schmidt, Ec. Geol. _Helvelix_, vol. ix., No. 4, p. 590

131

If time be given for the heat to accumulate in the lower depths
of the crushed-up sediments, here is an additional source of
increased temperature. The piled-up masses of the Simplon might
have occasioned a rise due to radioactive heating of one or two
hundred degrees, or even more; and if this be added to the
interior heat, a total of from 800° to 1000° might have prevailed
in the rocks now exposed at the surface of the mountain. Even a
lesser temperature, accompanied by the intense pressure
conditions, might well occasion the appearances of thermal
metamorphism described by Weinschenk, and for which, otherwise,
there is difficulty in accounting.[1]

This increase upon the primarily developed temperature conditions
takes place concurrently with the progress of compression; and
while it cannot be taken into account in estimating the
conditions of initial yielding of the crust, it adds an element
of instability, inasmuch as any progressive thickening by lateral
compression results in an accelerated rise of the goetherms. It
is probable that time sufficient for these effects to develop, if
not to their final, yet to a considerable extent, is often
available. The viscous movements of siliceous materials, and the
out-pouring of igneous rocks which often attend mountain
elevation, would find an explanation in such temperatures.

[1] Weinschenk, _Congrès Géol. Internat._, 1900, i., p. 332.

132

There is no more striking feature of the part here played by
radioactivity than the fact that the rhythmic occurrence of
depression and upheaval succeeding each other after great
intervals of time, and often shifting their position but little
from the first scene of sedimentation, becomes accounted for. The
source of thermal energy, as we have already remarked, is in fact
transported with the sediments—that energy which determines the
place of yielding and upheaval, and ordains that the mountain
ranges shall stand around the continental borders. Sedimentation
from this point of view is a convection of energy.

When the consolidated sediments are by these and by succeeding
movements forced upwards into mountains, they are exposed to
denudative effects greatly exceeding those which affect the
plains. Witness the removal during late Tertiary times of the
vast thickness of rock enveloping the Alps. Such great masses are
hurried away by ice, rivers, and rain. The ocean receives them;
and with infinite patience the world awaits the slow accumulation
of the radioactive energy beginning afresh upon its work. The
time for such events appears to us immense, for millions of years
are required for the sediments to grow in thickness, and the
geotherms to move upwards; but vast as it is, it is but a moment
in the life of the parent radioactive substances, whose atoms,
hardly diminished in numbers, pursue their changes while the
mountains come and go, and the

133

rudiments of life develop into its highest consummations.

To those unacquainted with the results of geological
investigation the history of the mountains as deciphered in the
rocks seems almost incredible.

The recently published sections of the Himalaya, due to H. H.
Hayden and the many distinguished men who have contributed to the
Geological Survey of India, show these great ranges to be
essentially formed of folded sediments penetrated by vast masses
of granite and other eruptives. Their geological history may be
summarised as follows

The Himalayan area in pre-Cambrian times was, in its southwestern
extension, part of the floor of a sea which covered much of what
is now the Indian Peninsula. In the northern shallows of this sea
were laid down beds of conglomerate, shale, sandstone and
limestone, derived from the denudation of Archæan rocks, which,
probably, rose as hills or mountains in parts of Peninsular India
and along the Tibetan edge of the Himalayan region. These beds
constitute the record of the long Purana Era[1] and are probably
coeval with the Algonkian of North America. Even in these early
times volcanic disturbances affected this area and the lower beds
of the Purana deposits were penetrated by volcanic outflows,
covered by sheets of lava, uplifted, denuded and again submerged

[1] See footnote, p. 139.

134

beneath the waters. Two such periods of instability have left
their records in the sediments of the Purana sea.

The succeeding era—the Dravidian Era—opens with Haimanta
(Cambrian) times. A shallow sea now extended over Kumaun, Garwal,
and Spiti, as well as Kashmir and ultimately over the Salt Range
region of the Punjab as is shown by deposits in these areas. This
sea was not, however, connected with the Cambrian sea of Europe.
The fossil faunas left by the two seas are distinct.

After an interval of disturbance during closing Haimanta times,
geographical changes attendant on further land movements
occurred. The central sea of Asia, the Tethys, extended westwards
and now joined with the European Paleozoic sea; and deposits of
Ordovician and Silurian age were laid down:—the Muth deposits.

The succeeding Devonian Period saw the whole Northern Himalayan
area under the waters of the Tethys which, eastward, extended to
Burma and China and, westward, covered Kashmir, the Hindu Kush
and part of Afghanistan. Deposits continued to be formed in this
area till middle Carboniferous times.

Near. the close of the Dravidian Era Kashmir became convulsed by
volcanic disturbance and the Penjal traps were ejected. It was a
time of worldwide disturbance and of redistribution of land and
water. Carboniferous times had begun, and the geographical
changes in

135

the southern limits of the Tethys are regarded as ushering in a
new and last era in Indian geological history the Aryan Bra.

India was now part of Gondwanaland; that vanished continent which
then reached westward to South Africa and eastward to Australia.
A boulder-bed of glacial origin, the Talchir Boulder-bed, occurs
in many surviving parts of this great land. It enters largely
into the Salt Range deposits. There is evidence that extensive
sheets of ice, wearing down the rocks of Rajputana, shoved their
moraines northward into the Salt Range Sea; then, probably, a
southern extension of the Tethys.

Subsequent to this ice age the Indian coalfields of the Gondwana
were laid down, with beds rich in the Glossopteris and
Gangamopteris flora. This remarkable carboniferous flora extends
to Southern Kashmir, so that it is to be inferred that this
region was also part of the main Gondwanaland. But its emergence
was but for a brief period. Upper Carboniferous marine deposits
succeeded; and, in fact, there was no important discontinuity in
the deposits in this area from Panjal times till the early
Tertiaries. During the whole of which vast period Kashmir was
covered with the waters of the Tethys.

The closing Dravidian disturbances of the Kashmir region did not,
apparently, extend to the eastern Himalayan area. But the
Carboniferous Period was, in this

136

eastern area, one of instability, culminating, at the close of
the Period, in a steady rise of the land and a northward retreat
of the Tethys. Nearly the entire Himalaya east of Kashmir became
a land surface and remained exposed to denudative forces for so
long a time that in places the whole of the Carboniferous,
Devonian, and a large part of the Silurian and Ordovician
deposits were removed—some thousands of feet in thickness—before
resubmergence in the Tethys occurred.

Towards the end of the Palaeozoic Age the Aryan Tethys receded
westwards, but still covered the Himalaya and was still connected
with the European Palæozoic sea. The Himalayan area (as well as
Kashmir) remained submerged in its waters throughout the entire
Mesozoic Age.

During Cretaceous times the Tethys became greatly extended,
indicating a considerable subsidence of northwestern India,
Afghanistan, Western Asia, and, probably, much of Tibet. The
shallow-water character of the deposits of the Tibetan Himalaya
indicates, however, a coast line near this region. Volcanic
materials, now poured out, foreshadow the incoming of the great
mountain-building epoch of the Tertiary Era. The enormous mass of
the Deccan traps, possessing a volume which has been estimated at
as much as 6,000 cubic miles, was probably extruded over the
Northern Peninsular region during late Cretaceous times. The sea
now began to retreat, and by the close of

137

the Eocene, it had moved westward to Sind and Baluchistan. The
movements of the Earth's crust were attended by intense volcanic
activity, and great volumes of granite were injected into the
sediments, followed by dykes and outflows of basic lavas.

The Tethys vanished to return no more. It survives in the
Mediterranean of today. The mountain-building movements continued
into Pliocene times. The Nummulite beds of the Eocene were, as
the result, ultimately uplifted 18,500 feet over sea level, a
total uplift of not less than 20,000 feet.

Thus with many vicissitudes, involving intervals of volcanic
activity, local uplifting, and extensive local denudation, the
Himalaya, which had originated in the sediments of the ancient
Purana sea, far back in pre-Cambrian times, and which had
developed potentially in a long sequence of deposits collecting
almost continuously throughout the whole of geological time,
finally took their place high in the heavens, where only the
winds—faint at such altitudes—and the lights of heaven can visit
their eternal snows.[1]

In this great history it is significant that the longest
continuous series of sedimentary deposits which the world has
known has become transfigured into the loftiest elevation upon
its surface.

[1] See A Sketch of the _Geography and Geology of the Himalaya
Mountains and Tibet_. By Colonel S. G. Burrard, R.E., F.R.S., and
H. H. Hayden, F.G.S., Part IV. Calcutta, 1908.

138

The diagrammatic sections of the Himalaya accompanying this brief
description arc taken from the monograph of Burrard and Hayden
(loc. cit.) on the Himalaya. Looking at the sections we see that
some of the loftiest summits are sculptured in granite and other
crystalline rocks. The appearance of these materials at the
surface indicates the removal by denudation and the extreme
metamorphism of much sedimentary deposit. The crystalline rocks,
indeed, penetrate some of the oldest rocks in the world. They
appear in contact with Archaean, Algonkian or early Palaeozoic
rocks. A study of the sections reveals not only the severe earth
movements, but also the immense amount of sedimentary deposits
involved in the genesis of these alps. It will be noted that the
vertical scale is not exaggerated relatively to the
horizontal.[1] Although there is no evidence of mountain
building

[1] To those unacquainted with the terminology of Indian geology
the following list of approximate equivalents in time will be of
use

Ngari Khorsum Beds - Pleistocene.
Siwalik Series -     Miocene and Pliocene.
Sirmur Series -      Oligocene.
Kampa System -       Eocene and Cretaceous.
Lilang System -      Triassic.
Kuling System -      Permian.
Gondwana System -    Carboniferous.
Kenawar System -     Carboniferous and Devonian
Muth System -        Silurian.
Haimanta System -    Mid. and Lower Cambrian.
Purana Group -       Algonkian.
Vaikrita System -    Archæan.
Daling Series -      Archæan.

139

on a large scale in the Himalayan area till the Tertiary
upheaval, it is, in the majority of cases, literally correct to
speak of the mountains as having their generations like organic
beings, and passing through all the stages of birth, life, death
and reproduction. The Alps, the Jura, the Pyrenees, the Andes,
have been remade more than once in the course of geological time,
the _débris_ of a worn-out range being again uplifted in succeeding
ages.

Thus to dwell for a moment on one case only: that of the
Pyrenees. The Pyrenees arose as a range of older Palmozoic rocks
in Devonian times. These early mountains, however, were
sufficiently worn out and depressed by Carboniferous times to
receive the deposits of that age laid down on the up-turned edges
of the older rocks. And to Carboniferous succeeded Permian,
Triassic, Jurassic and Lower Cretaceous sediments all laid down
in conformable sequence. There was then fresh disturbance and
upheaval followed by denudation, and these mountains, in turn,
became worn out and depressed beneath the ocean so that Upper
Greensand rocks were laid down unconforrnably on all beneath. To
these now succeeded Upper Chalk, sediments of Danian age, and so
on, till Eocene times, when the tale was completed and the
existing ranges rose from the sea. Today we find the folded
Nummulitic strata of Eocene age uplifted 11,000 feet, or within
200 feet of the greatest heights of the Pyrenees. And so they
stand awaiting

140

the time when once again they shall "fall into the portion of
outworn faces."[1]

Only mountains can beget mountains. Great accumulations of
sediment are a necessary condition for the localisation of
crust-flexure. The earliest mountains arose as purely igneous or
volcanic elevations, but the generations of the hills soon
originated in the collection of the _débris_, under the law of
gravity, in the hollow places. And if a foundered range is
exposed now to our view encumbered with thousands of feet of
overlying sediments we know that while the one range was sinking,
another, from which the sediments were derived, surely existed.
Through the "windows" in the deep-cut rocks of the Swiss valleys
we see the older Carboniferous Alps looking out, revisiting the
sun light, after scores of millions of years of imprisonment. We
know that just as surely as the Alps of today are founding by
their muddy torrents ranges yet to arise, so other primeval Alps
fed into the ocean the materials of these buried pre-Permian
rocks.

This succession of events only can cease when the rocks have been
sufficiently impoverished of the heat-producing substances, or
the forces of compression shall have died out in the surface
crust of the earth.

It seems impossible to escape the conclusion that in the great
development of ocean-encircling areas of

[1] See Prestwich, _Chemical and Physical Geology_, p. 302.

141

deposition and crustal folding, the heat of radioactivity has
been a determining factor. We recognise in the movements of the
sediments not only an influence localising and accelerating
crustal movements, but one which, in subservience to the primal
distribution of land and water, has determined some of the
greatest geographical features of the globe.

It is no more than a step to show that bound up with the
radioactive energy are most of the earthquake and volcanic
phenomena of the earth. The association of earthquakes with the
great geosynclines is well known. The work of De Montessus showed
that over 94 per cent. of all recorded shocks lie in the
geosynclinal belts. There can be no doubt that these
manifestations of instability are the results of the local
weakness and flexure which originated in the accumulation of
energy denuded from the continents. Similarly we may view in
volcanoes phenomena referable to the same fundamental cause. The
volcano was, in fact, long regarded as more intimately connected
with earthquakes than it, probably, actually is; the association
being regarded in a causative light, whereas the connexion is
more that of possessing a common origin. The girdle of volcanoes
around the Pacific and the earthquake belt coincide. Again, the
ancient and modern volcanoes and earthquakes of Europe are
associated with the geosyncline of the greater Mediterranean, the
Tethys of Mesozoic times. There is no difficulty in understanding
in a

142

general way the nature of the association. The earthquake is the
manifestation of rupture and slip, and, as Suess has shown, the
epicentres shift along that fault line where the crust has
yielded.[1] The volcano marks the spot where the zone of fusion
is brought so high in the fractured crust that the melted
materials are poured out upon the surface.

In a recent work on the subject of earthquakes Professor Hobbs
writes: "One of the most interesting of the generalisations which
De Montessus has reached as a result of his protracted studies,
is that the earthquake districts on the land correspond almost
exactly to those belts upon the globe which were the almost
continuous ocean basins of the long Secondary era of geological
history. Within these belts the sedimentary formations of the
crust were laid down in the greatest thickness, and the
formations follow each other in relatively complete succession.
For almost or quite the whole of this long era it is therefore
clear that the ocean covered these zones. About them the
formations are found interrupted, and the lacuna indicate that
the sea invaded the area only to recede from it, and again at
some later period to transgress upon it. For a long time,
therefore, these earthquake belts were the sea basins—the
geosynclines. They became later the rising mountains of the
Tertiary period, and mountains they

[1] Suess, _The Face of the Earth_, vol. ii., chap. ii.

143

are today. The earthquake belts are hence those portions of the
earth's crust which in recent times have suffered the greatest
movements in a vertical direction—they are the most mobile
portions of the earth's crust."[1] Whether the movements
attending mountain elevation and denudation are a connected and
integral part of those wide geographical changes which result in
submergence and elevation of large continental areas, is an
obscure and complex question. We seem, indeed, according to the
views of some authorities, hardly in a position to affirm with
certainty that such widespread movements of the land have
actually occurred, and that the phenomena are not the outcome of
fluctuations of oceanic level; that our observations go no
further than the recognition of positive and negative movements
of the strand. However this may be, the greater part of
mechanical denudation during geological time has been done on the
mountain ranges. It is, in short, indisputable that the orogenic
movements which uplift the hills have been at the basis of
geological history. To them the great accumulations of sediments
which now form so large a part of continental land are mainly
due. There can be no doubt of the fact that these movements have
swayed the entire history, both inorganic and organic, of the
world in which we live.

[1] Hobbs, _Earthquakes_, p. 58.

144

To sum the contents of this essay in the most general terms, we
find that in the conception of denudation as producing the
convection and accumulation of radiothermal energy the surface
features of the globe receive a new significance. The heat of the
earth is not internal only, but rather a heat-source exists at
the surface, which, as we have seen, cannot prevail to the same
degree within; and when the conditions become favourable for the
aggregation of the energy, the crust, heated both from beneath
and from above, assumes properties more akin to those of its
earlier stages of development, the secular heat-loss being
restored in the radioactive supplies. These causes of local
mobility have been in operation, shifting somewhat from place to
place, and defined geographically by the continental masses
undergoing denudation, since the earliest times.

145

ALPINE STRUCTURE

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.

It follows from this that, although a tangential force

157

or pressure be engendered by a crustal movement occurring to the
south, and the resultant effects be transmitted northwards, these
stresses can only mechanically affect the rigid parts of the
crust into which they are carried. That is to say, they may
result in folding and crushing, or horizontally transporting, the
upper layers of the Earth's crust; but in the deeper-lying
viscous materials they must be resolved into hydrostatic pressure
which may act to upheave the overlying covering, but must refuse
to transmit the horizontal translatory movements affecting the
rigid materials above.

Between the regions in which these two opposing conditions
prevail there will be no hard and fast line; but with the
downward increase of fluidity there will be a gradual failure of
the mechanical conditions and an increase of the hydrostatic.
Thus while the uppermost layers of the crust may be transported
to the full amount of the crustal displacement acting from the
south (speaking still of the Alps) deeper down there will be a
lesser horizontal movement, and still deeper there is no
influence to urge the viscous rock-materials in a northerly
direction. The consequences of these conditions must be the
recumbence of the folds formed under the crust-stress, and their
_déferlement_ towards the north. To see this, we must follow the
several stages of development.

The earliest movements, we may suppose, result in flexures of the
Jura-Mountain type—that is, in a

158

succession of undulations more or less symmetrical. As the
orogenic force continues and develops, these undulations give
place to folds, the limbs of which are approximately vertical,
and the synclinal parts of which become ever more and more
depressed into the deeper, and necessarily hotter, underlying
materials; the anticlines being probably correspondingly
elevated. These events are slowly developed, and the temperature
beneath is steadily rising in consequence of the conducted
interior heat, and the steady accumulation of radioactive energy
in the sedimentary rocks and in the buried radioactive layer of
the Earth. The work expended on the crushed and sheared rock also
contributes to the developing temperature. Thus the geotherms
must move upwards, and the viscous conditions extend from below;
continually diminishing the downward range of the translatory
movements progressing in the higher parts. While above the folded
sediments are being carried northward, beneath they are becoming
anchored in the growing viscosity of the medium. The anticlines
will bend over, and the most southerly of the folds will
gradually become pushed or bent over those lying to the north.
Finally, the whole upper part of the sheaf will become
horizontally recumbent; and as the uppermost folds will be those
experiencing the greatest effects of the continued displacement,
the _déferlement_ or overlap must necessarily arise.

We may follow these stages of mountain evolution

159

in a diagram (Fig. 9) in which we eliminate intermediate
conditions, and regard the early and final stages of development
only. In the upper sketch we suppose the lateral compression much
developed and the upward movement of the geotherms in progress.
The dotted line may be assumed to be a geotherm having a
temperature of viscosity. If the conditions here shown persist

{Fig. 9}

indefinitely, there is no doubt that the only further
developments possible are the continued crushing of the sediments
and the bodily displacement of the whole mass to the north. The
second figure is intended to show in what manner these results
are evaded. The geotherm of viscosity has risen. All above it is
affected mechanically by the continuing stress, and borne
northwards in varying

160

degree depending upon the rigidity. The folds have been
overthrown and drawn out; those which lay originally most to the
south have become the uppermost; and, experiencing the maximum
amount of displacement, overlap those lying beneath. There has
also been a certain amount of upthrow owing to the hydrostatic
pressure. This last-mentioned element of the phenomena is of
highly indeterminate character, for we know not the limits to
which the hydrostatic pressure may be transmitted, and where it
may most readily find relief. While, according to some of the
published sections, the uplifting force would seem to have
influenced the final results of the orogenic movements, a
discussion of its effects would not be profitable.

161

OTHER MINDS THAN OURS?

IN the year 1610 Galileo, looking through his telescope then
newly perfected by his own hands, discovered that the planet
Jupiter was attended by a train of tiny stars which went round
and round him just as the moon goes round the Earth.

It was a revelation too great to be credited by mankind. It was
opposed to the doctrine of the centrality of the Earth, for it
suggested that other worlds constituted like ours might exist in
the heavens.

Some said it was a mere optic illusion; others that he who looked
through such a tube did it at the peril of his soul—it was but a
delusion of Satan. Galileo converted a few of the unbelievers who
had the courage to look through his telescope. To the others he
said, he hoped they would see those moons on their way to heaven.
Old as this story is it has never lost its pathos or its
teaching.

The spirit which assailed Galileo's discoveries and which finally
was potent to overshadow his declining years, closed in former
days the mouths of those who asked the question written at the
head of this lecture: "Are we to believe that there are other
minds than ours?"

162

Today we consider the question in a very different spirit. Few
would regard it as either foolish or improper. Its intense
interest would be admitted by all, and but for the limitations
closing our way on every side it would, doubtless, attract the
most earnest investigation. Even on the mere balance of judgment
between the probable and the improbable, we have little to go on.
We know nothing definitely as to the conditions under which life
may originate: whether these are such as to be rare almost to
impossibility, or common almost to certainty. Only within narrow
limits of temperature and in presence of certain of the elements,
can life like ours exist, and outside these conditions life, if
such there be, must be different from ours. Once originated it is
so constituted as to assail the energies around it and to advance
from less to greater. Do we know more than these vague facts?
Yes, we have in our experience one other fact and one involving
much.

We know that our world is very old; that life has been for many
millions of years upon it; and that Man as a thinking being is
but of yesterday. Here is then a condition to be fulfilled. To
every world is physically assigned a limit to the period during
which it is habitable according to our knowledge of life and its
necessities. This limit passed and rationality missed, the chance
for that world is gone for ever, and other minds than ours
assuredly will not from it contemplate the universe. Looking at
our own world we see that the tree of life has,

163

indeed, branched, leaved and, possibly, budded many times; it
never bloomed but once.

All difficulties dissolve and speculations become needless under
one condition only: that in which rationality may be inferred
directly or indirectly by our observations on some sister world
in space, This is just the evidence which in recent years has
been claimed as derived from a study of the surface of Mars. To
that planet our hope of such evidence is restricted. Our survey
in all other directions is barred by insurmountable difficulties.
Unless some meteoric record reached our Earth, revelationary of
intelligence on a perished world, our only hope of obtaining such
evidence rests on the observation of Mars' surface features. To
this subject we confine our attention in what follows.

The observations made during recent years upon the surface
features of Mars have, excusably enough, given rise to
sensational reports. We must consider under what circumstances
these observations have been made.

Mars comes into particularly favourable conditions for
observation every fifteen years. It is true that every two years
and two months we overtake him in his orbit and he is then in
"opposition." That is, the Earth is between him and the sun: he
is therefore in the opposite part of the heavens to the sun. Now
Mars' orbit is very excentric, sometimes he is 139 million miles
from the sun, and sometimes he as as much as 154 million miles
from the sun. The Earth's orbit is, by comparison, almost

164

a circle. Evidently if we pass him when he is nearest to the sun
we see him at his best; not only because he is then nearest to
us, but because he is then also most brightly lit. In such
favourable oppositions we are within 35 million miles of him; if
Mars was in aphelion we would pass him at a distance of 61
million miles. Opposition occurs under the most favourable
circumstances every fifteen years. There was one in 1862, another
in 1877, one in 1892, and so on.

When Mars is 35 million miles off and we apply a telescope
magnifying 1,000 diameters, we see him as if placed 35,000 miles
off. This would be seven times nearer than we see the moon with
the naked eye. As Mars has a diameter about twice as great as
that of the moon, at such a distance he would look fourteen times
the diameter of the moon. Granting favourable conditions of
atmosphere much should be seen.

But these are just the conditions of atmosphere of which most of
the European observatories cannot boast. It is to the honour of
Schiaparelli, of Milan, that under comparatively unfavourable
conditions and with a small instrument, he so far outstripped his
contemporaries in the observation of the features of Mars that
those contemporaries received much of his early discoveries with
scepticism. Light and dark outlines and patches on the planet's
surface had indeed been mapped by others, and even a couple of
the canals sighted; but at the opposition of 1877 Schiaparelli
first mapped any considerable

165

number of the celebrated "canals" and showed that these
constituted an extraordinary and characteristic feature of the
planet's geography. He called them "canali," meaning thereby
"channels." It is remarkable indeed that a mistranslation appears
really responsible for the initiation of the idea that these
features are canals.

In 1882 Schiaparelli startled the astronomical world by declaring
that he saw some of the canals double—that is appearing as two
parallel lines. As these lines span the planet's surface for
distances of many thousands of miles the announcement naturally
gave rise to much surprise and, as I have said, to much
scepticism. But he resolutely stuck to his statement. Here is his
map of 1882. It is sufficiently startling.

In 1892 he drew a new map. It adds a little to the former map,
but the doubling was not so well seen. It is just the strangest
feature about this doubling that at times it is conspicuous, at
times invisible. A line which is distinctly seen as a single line
at one time, a few weeks later will appear distinctly to consist
of two parallel lines; like railway tracks, but tracks perhaps
200 miles apart and up to 3,000 or even 4,000 miles in length.

Many speculations were, of course, made to account for the origin
of such features. No known surface peculiarity on the Earth or
moon at all resembles these features. The moon's surface as you
know is cracked and

166

streaked. But the cracks are what we generally find cracks to
be—either aimless, wandering lines, or, if radiating from a
centre, then lines which contract in width as they leave the
point of rupture. Where will we find cracks accurately parallel
to one another sweeping round a planet's face with steady
curvature for, 4,000 miles, and crossing each other as if quite
unhampered by one another's presence? If the phenomenon on Mars
be due to cracks they imply a uniformity in thickness and
strength of crust, a homogeneity, quite beyond all anticipation.
We will afterwards see that the course of the lines is itself
further opposed to the theory that haphazard cracking of the
crust of the planet is responsible for the lines. It was also
suggested that the surface of the planet was covered with ice and
that these were cracks in the ice. This theory has even greater
difficulties than the last to contend with. Rivers have been
suggested. A glance at our own maps at once disposes of this
hypothesis. Rivers wander just as cracks do and parallel rivers
like parallel cracks are unknown.

In time the many suggestions were put aside. One only remained.
That the lines are actually the work of intelligence; actually
are canals, artificially made, constructed for irrigation
purposes on a scale of which we can hardly form any conception
based on our own earthly engineering structures.

During the opposition of 1894, Percival Lowell, along with A. E.
Douglass, and W. H. Pickering,

167

observed the planet from the summit of a mountain in Arizona,
using an 18-inch refracting telescope and every resource of
delicate measurement and spectroscopy. So superb a climate
favoured them that for ten months the planet was kept under
continual observation. Over 900 drawings were made and not only
were Schiaparelli's channels confirmed, but they added 116 to his
79, on that portion of the planet visible at that opposition.
They made the further important discovery that the lines do not
stop short at the dark regions of the planet's surface, as
hitherto believed, but go right on in many cases; the curvature
of the lines being unaltered.

Lowell is an uncompromising advocate of the "canal" theory. If
his arguments are correct we have at once an answer to our
question, "Are there other minds than ours?"

We must consider a moment Lowell's arguments; not that it is my
intention to combat them. You must form your own conclusions. I
shall lay before you another and, as I venture to think, more
adequate hypothesis in explanation of the channels of
Schiaparelli. We learn, however, much from Lowell's book—it is
full of interest.[1]

Lowell lays a deep foundation. He begins by showing that Mars has
an atmosphere. This must be granted him till some counter
observations are made.

[1] _Mars_, by Percival Lowell (Longmans, Green & Co.), 1896,

168

It is generally accepted. What that atmosphere is, is another
matter. He certainly has made out a good case for the presence of
water as one of its constituents,

It was long known that Mars possessed white regions at his poles,
just as our Earth does. The waning of these polar snows—if indeed
they are such—with the advance of the Martian summer, had often
been observed. Lowell plots day by day this waning. It is evident
from his observations that the snowfall must be light indeed. We
see in his map the south pole turned towards us. Mars in
perihelion always turns his south pole towards the sun and
therefore towards the Earth. We see that between the dates June
3rd to August 3rd—or in two months—the polar snow had almost
completely vanished. This denotes a very scanty covering. It must
be remembered that Mars even when nearest to the sun receives but
half our supply of solar heat and light.

But other evidence exists to show that Mars probably possesses
but little water upon his surface. The dark places are not
water-covered, although they have been named as if they were,
indeed, seas and lakes. Various phenomena show this. The canals
show it. It would never do to imagine canals crossing the seas.
No great rivers are visible. There is a striking absence of
clouds. The atmosphere of Mars seems as serene as that of Venus
appears to be cloudy. Mists and clouds, however, sometime appear
to veil his face and add to the difficulty of

169

making observations near the limb of the planet. Lowell concludes
it must be a calm and serene atmosphere; probably only
one-seventh of our own in density. The normal height of the
barometer in Mars would then be but four and a half inches. This
is a pressure far less than exists on the top of the highest
terrestrial mountain. A mountain here must have an altitude of
about ten miles to possess so low a pressure on its summit. Drops
of water big enough to form rain can hardly collect in such a
rarefied atmosphere. Moisture will fall as dew or frost upon the
ground. The days will be hot owing to the unimpeded solar
radiation; the nights bitterly cold owing to the free radiation
into space.

We may add that in such a climate the frost will descend
principally upon the high ground at night time and in the
advancing day it will melt. The freer radiation brings about this
phenomenon among our own mountains in clear and calm weather.

With the progressive melting of the snow upon the pole Lowell
connected many phenomena upon the planet's surface of much
interest. The dark spaces appear to grow darker and more
greenish. The canals begin to show themselves and reveal their
double nature. All this suggests that the moisture liberated by
the melting of the polar snow with the advancing year, is
carrying vitality and springtime over the surface of the planet.
But how is the water conveyed?

Lowell believes principally by the canals. These are

170

constructed triangulating the surface of the planet in all
directions. What we see, according to Lowell, is not the canal
itself, but the broad band of vegetation which springs up on the
arrival of the water. This band is perhaps thirty or forty miles
wide, but perhaps much less, for Lowell reports that the better
the conditions of observation the finer the lines appeared, so
that they may be as narrow, possibly, as fifteen miles. It is to
be remarked that a just visible dot on the surface of Mars must
possess a diameter of 30 miles. But a chain of much smaller dots
will be visible, just as we can see such fine objects as spiders'
webs. The widening of the canals is then accounted for, according
to Lowell, by the growth of a band of vegetation, similar to that
which springs into existence when the floods of the Nile irrigate
the plains of Egypt.

If no other explanation of the lines is forthcoming than that
they are the work of intelligence, all this must be remembered.
If all other theories fail us, much must be granted Lowell. We
must not reason like fishes—as Lowell puts it—and deny that
intelligent beings can thrive in an atmospheric pressure of four
and half inches of mercury. Zurbriggen has recently got to the
top of Aconcagua, a height of 24,000 feet. On the summit of such
a mountain the barometer must stand at about ten inches. Why
should not beings be developed by evolution with a lung capacity
capable of living at two and a half times this altitude. Those
steadily

171

curved parallel lines are, indeed, very unlike anything we have
experience of. It would be rather to be expected that another
civilisation than our own would present many wide differences in
its development.

What then is the picture we have before us according to Lowell?
It is a sufficiently dramatic one.

Mars is a world whose water supply, never probably very abundant,
has through countless years been drying up, sinking into his
surface. But the inhabitants are making a brave fight for it,
They have constructed canals right round their world so that the
water, which otherwise would run to waste over the vast deserts,
is led from oasis to oasis. Here the great centres of
civilisation are placed: their Londons, Viennas, New Yorks. These
gigantic works are the works of despair. A great and civilised
world finds death staring it in the face. They have had to triple
their canals so that when the central canal has done its work the
water is turned into the side canals, in order to utilise it as
far as possible. Through their splendid telescopes they must view
our seas and ample rivers; and must die like travellers in the
desert seeing in a mirage the cool waters of a distant lake.

Perhaps that lonely signal reported to have been seen in the
twilight limb of Mars was the outcome of pride in their splendid
and perishing civilisation. They would leave some memory of it:
they would have us witness how great was that civilisation before
they perish!

I close this dramatic picture with the poor comfort

172

that several philanthropic people have suggested signalling to
them as a mark of sympathy. It is said that a fortune was
bequeathed to the French Academy for the purpose of communicating
with the Martians. It has been suggested that we could flash
signals to them by means of gigantic mirrors reflecting the light
of our Sun. Or, again, that we might light bonfires on a
sufficiently large scale. They would have to be about ten miles
in diameter! A writer in the Pall Mall Gazette suggested that
there need really be no difficulty in the matter. With the kind
cooperation of the London Gas Companies (this was before the days
of electric lighting) a signal might be sent without any
additional expense if the gas companies would consent to
simultaneously turn off the gas at intervals of five minutes over
the whole of London, a signal which would be visible to the
astronomers in Mars would result. He adds, naively: "If only
tried for an hour each night some results might be obtained."

II

We have reviewed the theory of the artificial construction of the
Martian lines. The amount of consideration we are disposed to
give to the supposition that there are upon Mars other minds than
ours will—as I have stated—necessarily depend upon whether or not
we can assign a probable explanation of the lines upon purely
physical grounds. If it is apparent that such

173

lines would be formed with great probability under certain
conditions, which conditions are themselves probable, then the
argument by exclusion for the existence of civilisation on Mars,
at once breaks down.

{Fig. 10}

As a romance writer is sometimes under the necessity of
transporting his readers to other scenes, so I must now ask you
to consent to be transported some millions

174

of miles into the region of the heavens which lies outside Mars'
orbit.

Between Mars and Jupiter is a chasm of 341 millions of miles.
This gap in the sequence of planets was long known to be quite
out of keeping with the orderly succession of worlds outward from
the Sun. A society was formed at the close of the last century
for the detection of the missing world. On the first day of the
last century, Piazzi—who, by the way, was not a member of the
society—discovered a tiny world in the vacant gap. Although
eagerly welcomed, as better than nothing, it was a disappointing
find. The new world was a mere rock. A speck of about 160 miles
in diameter. It was obviously never intended that such a body
should have all this space to itself. And, sure enough, shortly
after, another small world was discovered. Then another was
found, and another, and so on; and now more than 400 of these
strange little worlds are known.

But whence came such bodies? The generally accepted belief is
that these really represent a misbegotten world. When the Sun was
younger he shed off the several worlds of our system as so many
rings. Each ring then coalesced into a world. Neptune being the
first born; Mercury the youngest born.

After Jupiter was thrown off, and the Sun had shrunk away inwards
some 20o million miles, he shed off another ring. Meaning that
this offspring of his should grow up like the rest, develop into
a stable world with the

175

potentiality even, it may be, of becoming the abode of rational
beings. But something went wrong. It broke up into a ring of
little bodies, circulating around him.

It is probable on this hypothesis that the number we are
acquainted with does not nearly represent the actual number of
past and present asteroids. It would take 125,000 of the biggest
of them to make up a globe as big as our world. They, so far as
they are known, vary in size from 10 miles to 160 miles in
diameter. It is probable then—on the assumption that this failure
of a world was intended to be about the mass of our Earth—that
they numbered, and possibly number, many hundreds of thousands.

Some of these little bodies are very peculiar in respect to the
orbits they move in. This peculiarity is sometimes in the
eccentricity of their orbits, sometimes in the manner in which
their orbits are tilted to the general plane of the ecliptic, in
which all the other planets move.

The eccentricity, according to Proctor, in some cases may attain
such extremes as to bring the little world inside Mars' mean
distance from the sun. This, as you will remember, is very much
less than his greatest distance from the sun. The entire belt of
asteroids—as known—lie much nearer to Mars than to Jupiter.

As regards the tilt of their orbits, some are actually as much as
34 degrees inclined to the ecliptic, so that in fact they are
seen from the Earth among our polar constellations.

176

From all this you see that Mars occupies a rather hot comer in
the solar system. Is it not possible that more than once in the
remote past Mars may have encountered one of these wanderers? If
he came within a certain distance of the small body his great
mass would sway it from its orbit, and under certain conditions
he would pick up a satellite in this manner. That his present
satellites were actually so acquired is the suggestion of Newton,
of Yale College.

Mars' satellites are indeed suspiciously and most abnormally
small. I have not time to prove this to you by comparison with
the other worlds of the solar system. In fact, they were not
discovered till 1877—although they were predicted in a most
curious manner, with the most uncannily accurate details, by
Swift.

One of these bodies is about 36 miles in diameter. This is
Phobos. Phobos is only 3.700 miles from the surface of Mars. The
other is smaller and further off. He is named Deimos, and his
diameter is only 10 miles. He is 12,500 miles from Mars' surface.
With the exception of Phobos the next smallest satellite known in
the solar system is one of Saturn's—Hyperion; almost 800 miles in
diameter. The inner one goes all round Mars in 7½ hours. This is
Phobos' month. Mars turns on his axis in 24 hours and 40 minutes,
so that people in Mars would see the rise of Phobos twice in the
course of a day and night; lie would apparently cross the sky

177

going against the other satellite; that is, he would move
apparently from west to east.

We may at least assume as probable that other satellites have
been gathered by Mars in the past from the army of asteroids.

Some of the satellites so picked up would be direct: that is,
would move round the planet in the direction of his axial
rotation. Others, on the chances, would be retrograde: that is,
would move against his axial rotation. They would describe orbits
making the same various angles with the ecliptic as do the
asteroids; and we may be sure they would be of the same varying
dimensions.

We go on to inquire what would be the consequence to Mars of such
captures.

A satellite captured in this manner is very likely to be pulled
into the Planet. This is a probable end of a satellite in any
case. It will probably be the end of our satellite too. The
satellite Phobos is indeed believed to be about to take this very
plunge into his planet. But in the case when the satellite picked
up happens to be rotating round the planet in the opposite
direction to the axial rotation of the planet, it is pretty
certain that its career as a satellite will be a brief one. The
reasons for this I cannot now give. If, then, Mars picked up
satellites he is very sure to have absorbed them sooner or later.
Sooner if they happened to be retrograde satellites, later if
direct satellites. His present satellites are recent additions.
They are direct.

178

The path of an expiring satellite will be a slow spiral described
round the planet. The spiral will at last, after many years,
bring the satellite down upon the surface of the primary. Its
final approach will be accelerated if the planet possesses an
atmosphere, as Mars probably does. A satellite of the dimensions
of Phobos—that is 36 miles in diameter—would hardly survive more
than 30 to 60 years within seventy miles of Mars' surface. It
will then be rotating round Mars in an hour and forty minutes,
moving, in fact, at the rate of 2.2 miles per second. In the
course of this 30 or 60 years it will, therefore, get round
perhaps 200,000 times, before it finally crashes down upon the
Martians. During this closing history of the satellite there is
reason to believe, however, that it would by no means pursue
continually the same path over the surface of the planet. There
are many disturbing factors to be considered. Being so small any
large surface features of Mars would probably act to perturb the
orbit of the satellite.

The explanation of Mars' lines which I suggest, is that they were
formed by the approach of such satellites in former times. I do
not mean that they are lines cut into his surface by the actual
infall of a satellite. The final end of the satellite would be
too rapid for this, I think. But I hope to be able to show you
that there is reason to believe that the mere passage of the
satellite, say at 70 miles above the surface of the planet, will,
in itself, give rise to effects on the crust of the planet
capable

179

of accounting for just such single or parallel lines as we see.

In the first place we have to consider the stability of the
satellite. Even in the case of a small satellite we cannot
overlook the fact that the half of the satellite near the planet
is pulled towards the planet by a gravitational force greater
than that attracting the outer half, and that the centrifugal
force is less on the inner than on the outer hemisphere. Hence
there exists a force tending to tear the satellite asunder on the
equatorial section tangential

{Fig. 11}

to the planet's surface. If in a fluid or plastic state, Phobos,
for instance, could not possibly exist near the planet's surface.
The forces referred to would decide its fate. It may be shown by
calculation, however, that if Phobos has the strength of basalt
or glass there would remain a considerable coefficient of safety
in favour of the satellite's stability; even when the surfaces of
planet and satellite were separated by only five miles.

We have now to consider some things which we expect will happen
before the satellite takes its final plunge into the planet.

180

This diagram (Fig. 11) shows you the satellite travelling above
the surface of the planet. The satellite is advancing towards, or
away from, the spectator. The planet is supposed to show its
solid crust in cross section, which may be a few miles in
thickness. Below this is such a hot plastic magma as we have
reason to believe underlies much of the solid crust of our own
Earth. Now there is an attraction between the satellite and the
crust of the planet; the same gravitational attraction which
exists between every particle of matter in the universe. Let us
consider how this attraction will affect the planet's crust. I
have drawn little arrows to show how we may consider the
attraction of the satellite pulling the crust of the planet not
only upwards, but also pulling it inwards beneath the satellite.
I have made these arrows longer where calculation shows the
stress is greater. You see that the greatest lifting stress is
just beneath the satellite, whereas the greatest stress pulling
the crust in under the satellite is at a point which lies out
from under the satellite, at a considerable distance. At each
side of the satellite there is a point where the stress pulling
on the crust is the greatest. Of the two stresses the lifting
stress will tend to raise the crust a little; the pulling stress
may in certain cases actually tear the crust across; as at A and
B.