This increase upon the primarily developed temperature conditionstakes place concurrently with the progress of compression; andwhile it cannot be taken into account in estimating theconditions of initial yielding of the crust, it adds an elementof instability, inasmuch as any progressive thickening by lateralcompression results in an accelerated rise of the goetherms. Itis probable that time sufficient for these effects to develop, ifnot to their final, yet to a considerable extent, is oftenavailable. The viscous movements of siliceous materials, and theout-pouring of igneous rocks which often attend mountainelevation, would find an explanation in such temperatures.
[1] Weinschenk, _Congrès Géol. Internat._, 1900, i., p. 332.
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There is no more striking feature of the part here played byradioactivity than the fact that the rhythmic occurrence ofdepression and upheaval succeeding each other after greatintervals of time, and often shifting their position but littlefrom the first scene of sedimentation, becomes accounted for. Thesource of thermal energy, as we have already remarked, is in facttransported with the sediments—that energy which determines theplace of yielding and upheaval, and ordains that the mountainranges shall stand around the continental borders. Sedimentationfrom this point of view is a convection of energy.
When the consolidated sediments are by these and by succeedingmovements forced upwards into mountains, they are exposed todenudative effects greatly exceeding those which affect theplains. Witness the removal during late Tertiary times of thevast thickness of rock enveloping the Alps. Such great masses arehurried away by ice, rivers, and rain. The ocean receives them;and with infinite patience the world awaits the slow accumulationof the radioactive energy beginning afresh upon its work. Thetime for such events appears to us immense, for millions of yearsare required for the sediments to grow in thickness, and thegeotherms to move upwards; but vast as it is, it is but a momentin the life of the parent radioactive substances, whose atoms,hardly diminished in numbers, pursue their changes while themountains come and go, and the
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rudiments of life develop into its highest consummations.
To those unacquainted with the results of geologicalinvestigation the history of the mountains as deciphered in therocks seems almost incredible.
The recently published sections of the Himalaya, due to H. H.Hayden and the many distinguished men who have contributed to theGeological Survey of India, show these great ranges to beessentially formed of folded sediments penetrated by vast massesof granite and other eruptives. Their geological history may besummarised as follows
The Himalayan area in pre-Cambrian times was, in its southwesternextension, part of the floor of a sea which covered much of whatis now the Indian Peninsula. In the northern shallows of this seawere laid down beds of conglomerate, shale, sandstone andlimestone, derived from the denudation of Archæan rocks, which,probably, rose as hills or mountains in parts of Peninsular Indiaand along the Tibetan edge of the Himalayan region. These bedsconstitute the record of the long Purana Era[1] and are probablycoeval with the Algonkian of North America. Even in these earlytimes volcanic disturbances affected this area and the lower bedsof the Purana deposits were penetrated by volcanic outflows,covered by sheets of lava, uplifted, denuded and again submerged
[1] See footnote, p. 139.
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beneath the waters. Two such periods of instability have lefttheir 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 Rangeregion of the Punjab as is shown by deposits in these areas. Thissea 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 movementsoccurred. The central sea of Asia, the Tethys, extended westwardsand now joined with the European Paleozoic sea; and deposits ofOrdovician and Silurian age were laid down:—the Muth deposits.
The succeeding Devonian Period saw the whole Northern Himalayanarea under the waters of the Tethys which, eastward, extended toBurma and China and, westward, covered Kashmir, the Hindu Kushand part of Afghanistan. Deposits continued to be formed in thisarea till middle Carboniferous times.
Near. the close of the Dravidian Era Kashmir became convulsed byvolcanic disturbance and the Penjal traps were ejected. It was atime of worldwide disturbance and of redistribution of land andwater. Carboniferous times had begun, and the geographicalchanges in
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the southern limits of the Tethys are regarded as ushering in anew and last era in Indian geological history the Aryan Bra.
India was now part of Gondwanaland; that vanished continent whichthen reached westward to South Africa and eastward to Australia.A boulder-bed of glacial origin, the Talchir Boulder-bed, occursin many surviving parts of this great land. It enters largelyinto the Salt Range deposits. There is evidence that extensivesheets of ice, wearing down the rocks of Rajputana, shoved theirmoraines northward into the Salt Range Sea; then, probably, asouthern extension of the Tethys.
Subsequent to this ice age the Indian coalfields of the Gondwanawere laid down, with beds rich in the Glossopteris andGangamopteris flora. This remarkable carboniferous flora extendsto Southern Kashmir, so that it is to be inferred that thisregion was also part of the main Gondwanaland. But its emergencewas but for a brief period. Upper Carboniferous marine depositssucceeded; and, in fact, there was no important discontinuity inthe deposits in this area from Panjal times till the earlyTertiaries. During the whole of which vast period Kashmir wascovered with the waters of the Tethys.
The closing Dravidian disturbances of the Kashmir region did not,apparently, extend to the eastern Himalayan area. But theCarboniferous Period was, in this
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eastern area, one of instability, culminating, at the close ofthe Period, in a steady rise of the land and a northward retreatof the Tethys. Nearly the entire Himalaya east of Kashmir becamea land surface and remained exposed to denudative forces for solong a time that in places the whole of the Carboniferous,Devonian, and a large part of the Silurian and Ordoviciandeposits were removed—some thousands of feet in thickness—beforeresubmergence in the Tethys occurred.
Towards the end of the Palaeozoic Age the Aryan Tethys recededwestwards, but still covered the Himalaya and was still connectedwith the European Palæozoic sea. The Himalayan area (as well asKashmir) remained submerged in its waters throughout the entireMesozoic Age.
During Cretaceous times the Tethys became greatly extended,indicating a considerable subsidence of northwestern India,Afghanistan, Western Asia, and, probably, much of Tibet. Theshallow-water character of the deposits of the Tibetan Himalayaindicates, however, a coast line near this region. Volcanicmaterials, now poured out, foreshadow the incoming of the greatmountain-building epoch of the Tertiary Era. The enormous mass ofthe Deccan traps, possessing a volume which has been estimated atas much as 6,000 cubic miles, was probably extruded over theNorthern Peninsular region during late Cretaceous times. The seanow began to retreat, and by the close of
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the Eocene, it had moved westward to Sind and Baluchistan. Themovements of the Earth's crust were attended by intense volcanicactivity, and great volumes of granite were injected into thesediments, followed by dykes and outflows of basic lavas.
The Tethys vanished to return no more. It survives in theMediterranean of today. The mountain-building movements continuedinto Pliocene times. The Nummulite beds of the Eocene were, asthe result, ultimately uplifted 18,500 feet over sea level, atotal uplift of not less than 20,000 feet.
Thus with many vicissitudes, involving intervals of volcanicactivity, local uplifting, and extensive local denudation, theHimalaya, which had originated in the sediments of the ancientPurana sea, far back in pre-Cambrian times, and which haddeveloped potentially in a long sequence of deposits collectingalmost continuously throughout the whole of geological time,finally took their place high in the heavens, where only thewinds—faint at such altitudes—and the lights of heaven can visittheir eternal snows.[1]
In this great history it is significant that the longestcontinuous series of sedimentary deposits which the world hasknown has become transfigured into the loftiest elevation uponits surface.
[1] See A Sketch of the _Geography and Geology of the HimalayaMountains and Tibet_. By Colonel S. G. Burrard, R.E., F.R.S., andH. H. Hayden, F.G.S., Part IV. Calcutta, 1908.
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The diagrammatic sections of the Himalaya accompanying this briefdescription arc taken from the monograph of Burrard and Hayden(loc. cit.) on the Himalaya. Looking at the sections we see thatsome of the loftiest summits are sculptured in granite and othercrystalline rocks. The appearance of these materials at thesurface indicates the removal by denudation and the extrememetamorphism of much sedimentary deposit. The crystalline rocks,indeed, penetrate some of the oldest rocks in the world. Theyappear in contact with Archaean, Algonkian or early Palaeozoicrocks. A study of the sections reveals not only the severe earthmovements, but also the immense amount of sedimentary depositsinvolved in the genesis of these alps. It will be noted that thevertical scale is not exaggerated relatively to thehorizontal.[1] Although there is no evidence of mountainbuilding
[1] To those unacquainted with the terminology of Indian geologythe following list of approximate equivalents in time will be ofuse
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 DevonianMuth System - Silurian.Haimanta System - Mid. and Lower Cambrian.Purana Group - Algonkian.Vaikrita System - Archæan.Daling Series - Archæan.
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on a large scale in the Himalayan area till the Tertiaryupheaval, it is, in the majority of cases, literally correct tospeak of the mountains as having their generations like organicbeings, and passing through all the stages of birth, life, deathand 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 succeedingages.
Thus to dwell for a moment on one case only: that of thePyrenees. The Pyrenees arose as a range of older Palmozoic rocksin Devonian times. These early mountains, however, weresufficiently worn out and depressed by Carboniferous times toreceive the deposits of that age laid down on the up-turned edgesof the older rocks. And to Carboniferous succeeded Permian,Triassic, Jurassic and Lower Cretaceous sediments all laid downin conformable sequence. There was then fresh disturbance andupheaval followed by denudation, and these mountains, in turn,became worn out and depressed beneath the ocean so that UpperGreensand rocks were laid down unconforrnably on all beneath. Tothese now succeeded Upper Chalk, sediments of Danian age, and soon, till Eocene times, when the tale was completed and theexisting ranges rose from the sea. Today we find the foldedNummulitic strata of Eocene age uplifted 11,000 feet, or within200 feet of the greatest heights of the Pyrenees. And so theystand awaiting
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the time when once again they shall "fall into the portion ofoutworn faces."[1]
Only mountains can beget mountains. Great accumulations ofsediment are a necessary condition for the localisation ofcrust-flexure. The earliest mountains arose as purely igneous orvolcanic elevations, but the generations of the hills soonoriginated in the collection of the _débris_, under the law ofgravity, in the hollow places. And if a foundered range isexposed now to our view encumbered with thousands of feet ofoverlying 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 valleyswe see the older Carboniferous Alps looking out, revisiting thesun light, after scores of millions of years of imprisonment. Weknow that just as surely as the Alps of today are founding bytheir muddy torrents ranges yet to arise, so other primeval Alpsfed into the ocean the materials of these buried pre-Permianrocks.
This succession of events only can cease when the rocks have beensufficiently impoverished of the heat-producing substances, orthe forces of compression shall have died out in the surfacecrust of the earth.
It seems impossible to escape the conclusion that in the greatdevelopment of ocean-encircling areas of
[1] See Prestwich, _Chemical and Physical Geology_, p. 302.
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deposition and crustal folding, the heat of radioactivity hasbeen a determining factor. We recognise in the movements of thesediments not only an influence localising and acceleratingcrustal movements, but one which, in subservience to the primaldistribution of land and water, has determined some of thegreatest geographical features of the globe.
It is no more than a step to show that bound up with theradioactive energy are most of the earthquake and volcanicphenomena of the earth. The association of earthquakes with thegreat geosynclines is well known. The work of De Montessus showedthat over 94 per cent. of all recorded shocks lie in thegeosynclinal belts. There can be no doubt that thesemanifestations of instability are the results of the localweakness and flexure which originated in the accumulation ofenergy denuded from the continents. Similarly we may view involcanoes phenomena referable to the same fundamental cause. Thevolcano was, in fact, long regarded as more intimately connectedwith earthquakes than it, probably, actually is; the associationbeing regarded in a causative light, whereas the connexion ismore that of possessing a common origin. The girdle of volcanoesaround the Pacific and the earthquake belt coincide. Again, theancient and modern volcanoes and earthquakes of Europe areassociated with the geosyncline of the greater Mediterranean, theTethys of Mesozoic times. There is no difficulty in understandingin a
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general way the nature of the association. The earthquake is themanifestation of rupture and slip, and, as Suess has shown, theepicentres shift along that fault line where the crust hasyielded.[1] The volcano marks the spot where the zone of fusionis brought so high in the fractured crust that the meltedmaterials are poured out upon the surface.
In a recent work on the subject of earthquakes Professor Hobbswrites: "One of the most interesting of the generalisations whichDe Montessus has reached as a result of his protracted studies,is that the earthquake districts on the land correspond almostexactly to those belts upon the globe which were the almostcontinuous ocean basins of the long Secondary era of geologicalhistory. Within these belts the sedimentary formations of thecrust were laid down in the greatest thickness, and theformations follow each other in relatively complete succession.For almost or quite the whole of this long era it is thereforeclear that the ocean covered these zones. About them theformations are found interrupted, and the lacuna indicate thatthe sea invaded the area only to recede from it, and again atsome later period to transgress upon it. For a long time,therefore, these earthquake belts were the sea basins—thegeosynclines. They became later the rising mountains of theTertiary period, and mountains they
[1] Suess, _The Face of the Earth_, vol. ii., chap. ii.
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are today. The earthquake belts are hence those portions of theearth's crust which in recent times have suffered the greatestmovements in a vertical direction—they are the most mobileportions of the earth's crust."[1] Whether the movementsattending mountain elevation and denudation are a connected andintegral part of those wide geographical changes which result insubmergence and elevation of large continental areas, is anobscure and complex question. We seem, indeed, according to theviews of some authorities, hardly in a position to affirm withcertainty that such widespread movements of the land haveactually occurred, and that the phenomena are not the outcome offluctuations of oceanic level; that our observations go nofurther than the recognition of positive and negative movementsof the strand. However this may be, the greater part ofmechanical denudation during geological time has been done on themountain ranges. It is, in short, indisputable that the orogenicmovements which uplift the hills have been at the basis ofgeological history. To them the great accumulations of sedimentswhich now form so large a part of continental land are mainlydue. There can be no doubt of the fact that these movements haveswayed the entire history, both inorganic and organic, of theworld in which we live.
[1] Hobbs, _Earthquakes_, p. 58.
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To sum the contents of this essay in the most general terms, wefind that in the conception of denudation as producing theconvection and accumulation of radiothermal energy the surfacefeatures of the globe receive a new significance. The heat of theearth is not internal only, but rather a heat-source exists atthe surface, which, as we have seen, cannot prevail to the samedegree within; and when the conditions become favourable for theaggregation of the energy, the crust, heated both from beneathand from above, assumes properties more akin to those of itsearlier stages of development, the secular heat-loss beingrestored in the radioactive supplies. These causes of localmobility have been in operation, shifting somewhat from place toplace, and defined geographically by the continental massesundergoing denudation, since the earliest times.
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ALPINE STRUCTURE
AN intelligent observer of the geological changes progressing insouthern Europe in Eocene times would have seen little to inspirehim with a premonition of the events then developing. TheNummulitic limestones were being laid down in that enlargedMediterranean which at this period, save for a few islands,covered most of south Europe. Of these stratified remains, aswell as of the great beds of Cretaceous, Jurassic, Triassic, andPermian sediments beneath, our hypothetical observer wouldprobably have been regardless; just as today we observe, with anindifference born of our transitoriness, the deposits rapidlygathering wherever river discharge is distributing the sedimentsover the sea-floor, or the lime-secreting organisms are activelyat work. And yet it took but a few millions of years to upliftthe deposits of the ancient Tethys; pile high its sediments infold upon fold in the Alps, the Carpathians, and the Himalayas;and—exposing them to the rigours of denudation at altitudes whereglaciation, landslip, and torrent prevail—inaugurate a new epochof sedimentation and upheaval.
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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. Therewas thus a period of some millions of years within which theentire phenomena were comprised. Availing ourselves of Sollas'computations,[1] we may sum the maximum depths of sedimentarydeposits 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 thelast 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 iocentimetres in a century these deposits could not have collectedin less than 13.4 millions of years. It would appear that notless than some ten millions of years were consumed in the genesisof the Alps before constructive movements finally ceased.
The progress of the earth-movements was attended by the usualvolcanic phenomena. The Oligocene and Miocene volcanoes extendedin a band marked by the Auvergne, the Eiffel, the Bohemian, andthe eastern Carpathian eruptions; and, later, towards the closeof the movements in Pliocene times, the south border
[1] Sollas, Anniversary Address, Geol. Soc., London, 1909.
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regions of the Alps became the scene of eruptions such as thoseof Etna, Santorin, Somma (Vesuvius), etc.
We have referred to these well-known episodes with two objects inview: to recall to mind the time-interval involved, and theevidence of intense crustal disturbance, both dynamic andthermal. According to views explained in a previous essay, theenergetic effects of radium in the sediments and upper crust werea principal factor in localising and bringing about theseresults. We propose now to inquire if, also, in the more intimatestructure of the Alps, the radioactive energy may not have bornea part.
What we see today in the Alps is but a residue spared bydenudation. It is certain that vast thicknesses of material havedisappeared. Even while constructive effects were still inprogress, denudative forces were not idle. Of this fact theshingle accumulations of the Molasse, where, on the northernborders of the Alps, they stand piled into mountains, beareloquent testimony. In the sub-Apennine series of Italy, thegreat beds of clays, marls, and limestones afford evidence ofthese destructive processes continued into Pliocene times. Wehave already referred to Schmidt's estimate that the sedimentarycovering must have in places amounted to from 15,000 to 20,000metres. The evidence for this is mainly tectonic or structural;but is partly forthcoming in the changes which the materials nowopen to our inspection plainly reveal. Thus it is impos-
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sible to suppose that gneissic rocks can become so far plastic asto flow in and around the calcareous sediments, or be penetratedby the latter—as we see in the Jungfrau and elsewhere—unlessgreat pressures and high temperatures prevailed. And, accordingto some writers, the temperatures revealed by the intimatestructural changes of rock-forming minerals must have amounted tothose of fusion. The existence of such conditions is supported bythe observation that where the.crystallisation is now the mostperfect, the phenomena of folding and injection are bestdeveloped.[1] These high temperatures would appear to beunaccountable without the intervention of radiothermal effects;and, indeed, have been regarded as enigmatic by observers of thephenomena in question. A covering of 20,000 metres in thicknesswould not occasion an earth-temperature exceeding 500° C. if thegradients were such as obtain in mountain regions generally; and600° is about the limit we could ascribe to the purely passiveeffects of such a layer in elevating the geotherms.
Those who are still unacquainted with the recently publishedobservations on the structure of the Alps may find it difficultto enter into what has now to be stated; for the facts are,indeed, very different from the generally preconceived ideas ofmountain formation. Nor can we wonder that many geologists forlong held
[1] Weinschenk, C. R. _Congrès Géol._, 1900, p. 321, et seq.
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back from admitting views which appeared so extreme. Receptivityis the first virtue of the scientific mind; but, with everydesire to lay aside prejudice, many felt unequal to theacceptance of structural features involving a folding of theearth-crust in laps which lay for scores of miles from country tocountry, and the carriage of mountainous materials from the southof the Alps to the north, leaving them finally as Alpine rangesof ancient sediments reposing on foundations of more recent date.The historian of the subject will have to relate how some whofinally were most active in advancing the new views were at firstopposed to them. In the change of conviction of these eminentgeologists we have the strongest proof of the convincing natureof the observations and the reality of the tectonic features uponwhich the recent views are founded.
The lesser mountains which stand along the northern border of thegreat limestone Alps, those known as the Préalpes, present thestrange characteristic of resting upon materials younger thanthemselves. Such mountains as the remarkable-looking Mythen, nearSchwyz, for instance, are weathered from masses of Triassic andJurassic rock, and repose on the much more recent Flysch. Insharp contrast to the Flysch scenery, they stand as abrupt andgigantic erratics, which have been transported from the centralzone of the Alps lying far to the south. They are strangerspetrologically,
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stratigraphically, and geographically,[1] to the locality inwhich they now occur. The exotic materials may be dolomites,limestones, schists, sandstones, or rocks of igneous origin. Theyshow in every case traces of the severe dynamic actions to whichthey have been subjected in transit. The igneous, like thesedimentary, klippen, can be traced to distant sources; to themassif of Belladonne, to Mont Blanc, Lugano, and the Tyrol. ThePréalpes are, in fact, mountains without local roots.
In this last-named essential feature, the Préalpes do not differfrom the still greater limestone Alps which succeed them to thesouth. These giants, _e.g._ the Jungfrau, Wetterhorn, Eiger, etc.,are also without local foundations. They have been formed fromthe overthrown and drawn-out anticlines of great crust-folds,whose synclines or roots are traceable to the south side of theRhone Valley. The Bernese Oberland originated in the piling-up offour great sheets or recumbent folds, one of which is continuedinto the Préalpes. With Lugeon[2] we may see in the phenomenon ofthe formation of the Préalpes a detail; regarding it as a normalexpression of that mechanism which has created the Swiss Alps.For these limestone masses of the Oberland are not indications ofa 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.
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been pushed over and piled up to the north. Lugeon[l] concludesthat, before denudation had done its work and cut off thePréalpes from their roots, there would have been found sheets, tothe number of eight, superimposed and extending between the MontBlanc massif and the massif of the Finsteraarhorn: these sheetsbeing 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 diagrammaticallyafter Schmidt of Basel (Fig. 8).[2] The section extends fromnorth to south, and brings out the relations of the severalrecumbent folds. We must imagine almost the whole of thesesuperimposed folds now removed from the central regions of theAlps by denudation,
[1] Lugeon, _loc. cit._
[2] Schmidt, _Ec. Geol. Helvetiae_, vol. ix., No. 4.
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and leaving the underlying gneisses rising through the remains ofPermian, Triassic, and Jurassic sediments; while to the north thegreat limestone mountains and further north still, the Préalpes,carved from the remains of the recumbent folds, now stand withalmost as little resemblance to the vanished mountains as thememories of the past have to its former intense reality.
These views as to the origin of the Alps, which are shared at thepresent day by so many distinguished geologists, had their originin the labours of many now gone; dating back to Studer; findingtheir inspiration in the work of Heim, Suess, and MarcelBertrand; and their consummation in that of Lugeon, Schardt,Rothpletz, Schmidt, and many others. Nor must it be forgottenthat nearer home, somewhat similar phenomena, necessarily on asmaller 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 thephenomena we have just been reviewing. The uppermost of thesuperimposed recumbent folds is more extended in its developmentthan those which lie beneath. Passing downwards from the highestof the folds, they are found to be less and less extended both inthe northerly and in the southerly direction, speaking of thespecial case—the Alps—now before us. This feature might bedescribed somewhat differently. We might say that those foldswhich had their roots farther
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to the south were the most drawn-out towards the north: or againwe might say that the synclinal or deep-seated part of the foldhas lagged behind the anticlinal or what was originally thehighest part of the fold, in the advance of the latter to thenorth. The anticline has advanced relatively to the syncline. Tothis law one exception only is observed in the Swiss Alps; thesheet of the Brèche (_Byecciendecke_) falls short, in its northerlyextension, of the underlying fold, which extends to form thePréalpes.
Contemplating such a generalised section as Professor Schmidt's,or, indeed, more particular sections, such as those in the MontBlanc Massif by Marcel Bertrand,[1] of the Dent de Morcles,Diablerets, Wildhorn, and Massif de la Brèche by Lugeon,[2] orfinally Termier's section of the Pelvoux Massif,[3] one isreminded 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 thereremain, the succeeding wave must necessarily find itselfsuperimposed upon the first. But no effects of inertia, nokinetic effects, may be called to our aid in explaining theformation of mountains. Some geologists have accordingly supposedthat 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.
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the recumbent folds and the peculiar phenomena of increasingoverlap, or _déferlement_, an obstacle, fixed and deep-seated, musthave arrested the roots or synclines of the folds, and held themagainst translational motion, while a movement of the upper crustdrew out and carried forward the anticlines. Others havecontented themselves by recording the facts without advancing anyexplanatory hypothesis beyond that embodied in the incontestablestatement that such phenomena must be referred to the effects oftangential forces acting in the Earth's crust.
It would appear that the explanation of the phenomena ofrecumbent folds and their _déferlement_ is to be obtained directlyfrom the temperature conditions prevailing throughout thestressed pile of rocks; and here the subject of mountaintectonics touches that with which we were elsewhere speciallyconcerned—the geological influence of accumulated radioactiveenergy.
As already shown[1], a rise of temperature due to this source ofseveral hundred degrees might be added to such temperatures aswould arise from the mere blanketing of the Earth, and theconsequent upward movement of the geotherms. The time element ishere the most important consideration. The whole sequence ofevents from the first orogenic movements to the final upheaval inPliocene times must probably have occupied not less than tenmillion years.
[1] _Mountain Genesis_, p. 129, et seq.
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Unfortunately the full investigation of the distribution oftemperature after any given time is beset with difficulties; theconditions being extremely complex. If the radioactive heatingwas strictly adiabatic—that is, if all the heat was conserved andnone entered from without—the time required for the attainment ofthe equilibrium radioactive temperature would be just about sixmillion years. The conditions are not, indeed, adiabatic; but, onthe other hand, the rocks upraised by lateral pressure were by nomeans at 0° C. to start with. They must be assumed to havepossessed such temperatures as the prior radiothermal effects,and the conducted heat from the Earth's interior, may haveestablished.
It would from this appear probable that if a duration of tenmillion years was involved, the equilibrium radioactivetemperatures must nearly have been attained. The effects of heatconducted from the underlying earthcrust have to be added,leading to a further rise in temperature of not less than 500° or600° . In such considerations the observed indications of hightemperatures in materials now laid bare by denudation, probablyfind their explanation (P1. XIX).
The first fact that we infer from the former existence of such atemperature distribution is the improbability, indeed theimpossibility, that anything resembling a rigid obstacle, ordeep-seated "horst," can have existed beneath the presentsurface-level, and opposed the northerly movement of thedeep-lying synclines. For
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such a horst can only have been constituted of some siliceousrock-material such as we find everywhere rising through theworn-down sediments of the Alps; and the idea that this couldretain rigidity under the prevailing temperature conditions, mustbe dismissed. There is no need to labour this question; the horstcannot have existed. To what, then, is the retardation of thelower 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 hightemperature and viscosity, which render untenable the hypothesisof a rigid obstacle, suffice to afford a full explanation of theretardation of the roots of the folds. For directed translatorymovements cannot be transmitted through a fluid, pressure inwhich is necessarily hydrostatic, and must be exerted equally inevery direction. And this applies, not only to a fluid, but to abody which will yield viscously to an impressed force. There willbe a gradation, according as viscosity gives place to rigidity,between the states in which the applied force resolves itselfinto a purely hydrostatic pressure, and in which it istransmitted through the material as a directed thrust. The natureof the force, in the most general case, of course, has to beconsidered; whether it is suddenly applied and of brief duration,or steady and long-continued. The latter conditions alone applyto the present case.
It follows from this that, although a tangential force
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or pressure be engendered by a crustal movement occurring to thesouth, and the resultant effects be transmitted northwards, thesestresses can only mechanically affect the rigid parts of thecrust into which they are carried. That is to say, they mayresult in folding and crushing, or horizontally transporting, theupper layers of the Earth's crust; but in the deeper-lyingviscous materials they must be resolved into hydrostatic pressurewhich may act to upheave the overlying covering, but must refuseto transmit the horizontal translatory movements affecting therigid materials above.
Between the regions in which these two opposing conditionsprevail there will be no hard and fast line; but with thedownward increase of fluidity there will be a gradual failure ofthe mechanical conditions and an increase of the hydrostatic.Thus while the uppermost layers of the crust may be transportedto the full amount of the crustal displacement acting from thesouth (speaking still of the Alps) deeper down there will be alesser horizontal movement, and still deeper there is noinfluence to urge the viscous rock-materials in a northerlydirection. The consequences of these conditions must be therecumbence of the folds formed under the crust-stress, and their_déferlement_ towards the north. To see this, we must follow theseveral stages of development.
The earliest movements, we may suppose, result in flexures of theJura-Mountain type—that is, in a
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succession of undulations more or less symmetrical. As theorogenic force continues and develops, these undulations giveplace to folds, the limbs of which are approximately vertical,and the synclinal parts of which become ever more and moredepressed into the deeper, and necessarily hotter, underlyingmaterials; the anticlines being probably correspondinglyelevated. These events are slowly developed, and the temperaturebeneath is steadily rising in consequence of the conductedinterior heat, and the steady accumulation of radioactive energyin the sedimentary rocks and in the buried radioactive layer ofthe Earth. The work expended on the crushed and sheared rock alsocontributes to the developing temperature. Thus the geothermsmust move upwards, and the viscous conditions extend from below;continually diminishing the downward range of the translatorymovements progressing in the higher parts. While above the foldedsediments are being carried northward, beneath they are becominganchored in the growing viscosity of the medium. The anticlineswill bend over, and the most southerly of the folds willgradually become pushed or bent over those lying to the north.Finally, the whole upper part of the sheaf will becomehorizontally recumbent; and as the uppermost folds will be thoseexperiencing the greatest effects of the continued displacement,the _déferlement_ or overlap must necessarily arise.
We may follow these stages of mountain evolution
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in a diagram (Fig. 9) in which we eliminate intermediateconditions, and regard the early and final stages of developmentonly. In the upper sketch we suppose the lateral compression muchdeveloped and the upward movement of the geotherms in progress.The dotted line may be assumed to be a geotherm having atemperature of viscosity. If the conditions here shown persist
{Fig. 9}
indefinitely, there is no doubt that the only furtherdevelopments possible are the continued crushing of the sedimentsand the bodily displacement of the whole mass to the north. Thesecond figure is intended to show in what manner these resultsare evaded. The geotherm of viscosity has risen. All above it isaffected mechanically by the continuing stress, and bornenorthwards in varying
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degree depending upon the rigidity. The folds have beenoverthrown and drawn out; those which lay originally most to thesouth have become the uppermost; and, experiencing the maximumamount of displacement, overlap those lying beneath. There hasalso been a certain amount of upthrow owing to the hydrostaticpressure. This last-mentioned element of the phenomena is ofhighly indeterminate character, for we know not the limits towhich the hydrostatic pressure may be transmitted, and where itmay most readily find relief. While, according to some of thepublished sections, the uplifting force would seem to haveinfluenced the final results of the orogenic movements, adiscussion of its effects would not be profitable.
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OTHER MINDS THAN OURS?
IN the year 1610 Galileo, looking through his telescope thennewly perfected by his own hands, discovered that the planetJupiter was attended by a train of tiny stars which went roundand round him just as the moon goes round the Earth.
It was a revelation too great to be credited by mankind. It wasopposed to the doctrine of the centrality of the Earth, for itsuggested that other worlds constituted like ours might exist inthe heavens.
Some said it was a mere optic illusion; others that he who lookedthrough such a tube did it at the peril of his soul—it was but adelusion of Satan. Galileo converted a few of the unbelievers whohad the courage to look through his telescope. To the others hesaid, he hoped they would see those moons on their way to heaven.Old as this story is it has never lost its pathos or itsteaching.
The spirit which assailed Galileo's discoveries and which finallywas potent to overshadow his declining years, closed in formerdays the mouths of those who asked the question written at thehead of this lecture: "Are we to believe that there are otherminds than ours?"
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Today we consider the question in a very different spirit. Fewwould regard it as either foolish or improper. Its intenseinterest would be admitted by all, and but for the limitationsclosing our way on every side it would, doubtless, attract themost earnest investigation. Even on the mere balance of judgmentbetween the probable and the improbable, we have little to go on.We know nothing definitely as to the conditions under which lifemay originate: whether these are such as to be rare almost toimpossibility, or common almost to certainty. Only within narrowlimits of temperature and in presence of certain of the elements,can life like ours exist, and outside these conditions life, ifsuch there be, must be different from ours. Once originated it isso constituted as to assail the energies around it and to advancefrom less to greater. Do we know more than these vague facts?Yes, we have in our experience one other fact and one involvingmuch.
We know that our world is very old; that life has been for manymillions of years upon it; and that Man as a thinking being isbut of yesterday. Here is then a condition to be fulfilled. Toevery world is physically assigned a limit to the period duringwhich it is habitable according to our knowledge of life and itsnecessities. This limit passed and rationality missed, the chancefor that world is gone for ever, and other minds than oursassuredly will not from it contemplate the universe. Looking atour own world we see that the tree of life has,
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indeed, branched, leaved and, possibly, budded many times; itnever bloomed but once.
All difficulties dissolve and speculations become needless underone condition only: that in which rationality may be inferreddirectly or indirectly by our observations on some sister worldin space, This is just the evidence which in recent years hasbeen claimed as derived from a study of the surface of Mars. Tothat planet our hope of such evidence is restricted. Our surveyin all other directions is barred by insurmountable difficulties.Unless some meteoric record reached our Earth, revelationary ofintelligence on a perished world, our only hope of obtaining suchevidence rests on the observation of Mars' surface features. Tothis subject we confine our attention in what follows.
The observations made during recent years upon the surfacefeatures of Mars have, excusably enough, given rise tosensational reports. We must consider under what circumstancesthese observations have been made.
Mars comes into particularly favourable conditions forobservation every fifteen years. It is true that every two yearsand two months we overtake him in his orbit and he is then in"opposition." That is, the Earth is between him and the sun: heis therefore in the opposite part of the heavens to the sun. NowMars' orbit is very excentric, sometimes he is 139 million milesfrom the sun, and sometimes he as as much as 154 million milesfrom the sun. The Earth's orbit is, by comparison, almost
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a circle. Evidently if we pass him when he is nearest to the sunwe see him at his best; not only because he is then nearest tous, but because he is then also most brightly lit. In suchfavourable oppositions we are within 35 million miles of him; ifMars was in aphelion we would pass him at a distance of 61million miles. Opposition occurs under the most favourablecircumstances every fifteen years. There was one in 1862, anotherin 1877, one in 1892, and so on.
When Mars is 35 million miles off and we apply a telescopemagnifying 1,000 diameters, we see him as if placed 35,000 milesoff. This would be seven times nearer than we see the moon withthe naked eye. As Mars has a diameter about twice as great asthat of the moon, at such a distance he would look fourteen timesthe diameter of the moon. Granting favourable conditions ofatmosphere much should be seen.
But these are just the conditions of atmosphere of which most ofthe European observatories cannot boast. It is to the honour ofSchiaparelli, of Milan, that under comparatively unfavourableconditions and with a small instrument, he so far outstripped hiscontemporaries in the observation of the features of Mars thatthose contemporaries received much of his early discoveries withscepticism. Light and dark outlines and patches on the planet'ssurface had indeed been mapped by others, and even a couple ofthe canals sighted; but at the opposition of 1877 Schiaparellifirst mapped any considerable
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number of the celebrated "canals" and showed that theseconstituted an extraordinary and characteristic feature of theplanet's geography. He called them "canali," meaning thereby"channels." It is remarkable indeed that a mistranslation appearsreally responsible for the initiation of the idea that thesefeatures are canals.
In 1882 Schiaparelli startled the astronomical world by declaringthat he saw some of the canals double—that is appearing as twoparallel lines. As these lines span the planet's surface fordistances of many thousands of miles the announcement naturallygave rise to much surprise and, as I have said, to muchscepticism. But he resolutely stuck to his statement. Here is hismap 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 strangestfeature about this doubling that at times it is conspicuous, attimes invisible. A line which is distinctly seen as a single lineat one time, a few weeks later will appear distinctly to consistof two parallel lines; like railway tracks, but tracks perhaps200 miles apart and up to 3,000 or even 4,000 miles in length.
Many speculations were, of course, made to account for the originof such features. No known surface peculiarity on the Earth ormoon at all resembles these features. The moon's surface as youknow is cracked and
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streaked. But the cracks are what we generally find cracks tobe—either aimless, wandering lines, or, if radiating from acentre, then lines which contract in width as they leave thepoint of rupture. Where will we find cracks accurately parallelto one another sweeping round a planet's face with steadycurvature for, 4,000 miles, and crossing each other as if quiteunhampered by one another's presence? If the phenomenon on Marsbe due to cracks they imply a uniformity in thickness andstrength of crust, a homogeneity, quite beyond all anticipation.We will afterwards see that the course of the lines is itselffurther opposed to the theory that haphazard cracking of thecrust of the planet is responsible for the lines. It was alsosuggested that the surface of the planet was covered with ice andthat these were cracks in the ice. This theory has even greaterdifficulties than the last to contend with. Rivers have beensuggested. A glance at our own maps at once disposes of thishypothesis. Rivers wander just as cracks do and parallel riverslike parallel cracks are unknown.
In time the many suggestions were put aside. One only remained.That the lines are actually the work of intelligence; actuallyare canals, artificially made, constructed for irrigationpurposes on a scale of which we can hardly form any conceptionbased on our own earthly engineering structures.
During the opposition of 1894, Percival Lowell, along with A. E.Douglass, and W. H. Pickering,
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observed the planet from the summit of a mountain in Arizona,using an 18-inch refracting telescope and every resource ofdelicate measurement and spectroscopy. So superb a climatefavoured them that for ten months the planet was kept undercontinual observation. Over 900 drawings were made and not onlywere Schiaparelli's channels confirmed, but they added 116 to his79, on that portion of the planet visible at that opposition.They made the further important discovery that the lines do notstop short at the dark regions of the planet's surface, ashitherto believed, but go right on in many cases; the curvatureof the lines being unaltered.
Lowell is an uncompromising advocate of the "canal" theory. Ifhis arguments are correct we have at once an answer to ourquestion, "Are there other minds than ours?"
We must consider a moment Lowell's arguments; not that it is myintention to combat them. You must form your own conclusions. Ishall lay before you another and, as I venture to think, moreadequate hypothesis in explanation of the channels ofSchiaparelli. We learn, however, much from Lowell's book—it isfull of interest.[1]
Lowell lays a deep foundation. He begins by showing that Mars hasan atmosphere. This must be granted him till some counterobservations are made.
[1] _Mars_, by Percival Lowell (Longmans, Green & Co.), 1896,
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It is generally accepted. What that atmosphere is, is anothermatter. He certainly has made out a good case for the presence ofwater 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 indeedthey are such—with the advance of the Martian summer, had oftenbeen observed. Lowell plots day by day this waning. It is evidentfrom his observations that the snowfall must be light indeed. Wesee in his map the south pole turned towards us. Mars inperihelion always turns his south pole towards the sun andtherefore towards the Earth. We see that between the dates June3rd to August 3rd—or in two months—the polar snow had almostcompletely vanished. This denotes a very scanty covering. It mustbe remembered that Mars even when nearest to the sun receives buthalf our supply of solar heat and light.
But other evidence exists to show that Mars probably possessesbut little water upon his surface. The dark places are notwater-covered, although they have been named as if they were,indeed, seas and lakes. Various phenomena show this. The canalsshow it. It would never do to imagine canals crossing the seas.No great rivers are visible. There is a striking absence ofclouds. The atmosphere of Mars seems as serene as that of Venusappears to be cloudy. Mists and clouds, however, sometime appearto veil his face and add to the difficulty of
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making observations near the limb of the planet. Lowell concludesit must be a calm and serene atmosphere; probably onlyone-seventh of our own in density. The normal height of thebarometer in Mars would then be but four and a half inches. Thisis a pressure far less than exists on the top of the highestterrestrial mountain. A mountain here must have an altitude ofabout ten miles to possess so low a pressure on its summit. Dropsof water big enough to form rain can hardly collect in such ararefied atmosphere. Moisture will fall as dew or frost upon theground. The days will be hot owing to the unimpeded solarradiation; the nights bitterly cold owing to the free radiationinto space.
We may add that in such a climate the frost will descendprincipally upon the high ground at night time and in theadvancing day it will melt. The freer radiation brings about thisphenomenon among our own mountains in clear and calm weather.
With the progressive melting of the snow upon the pole Lowellconnected many phenomena upon the planet's surface of muchinterest. The dark spaces appear to grow darker and moregreenish. The canals begin to show themselves and reveal theirdouble nature. All this suggests that the moisture liberated bythe melting of the polar snow with the advancing year, iscarrying vitality and springtime over the surface of the planet.But how is the water conveyed?
Lowell believes principally by the canals. These are
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constructed triangulating the surface of the planet in alldirections. What we see, according to Lowell, is not the canalitself, but the broad band of vegetation which springs up on thearrival of the water. This band is perhaps thirty or forty mileswide, but perhaps much less, for Lowell reports that the betterthe conditions of observation the finer the lines appeared, sothat they may be as narrow, possibly, as fifteen miles. It is tobe remarked that a just visible dot on the surface of Mars mustpossess a diameter of 30 miles. But a chain of much smaller dotswill be visible, just as we can see such fine objects as spiders'webs. The widening of the canals is then accounted for, accordingto Lowell, by the growth of a band of vegetation, similar to thatwhich springs into existence when the floods of the Nile irrigatethe plains of Egypt.
If no other explanation of the lines is forthcoming than thatthey are the work of intelligence, all this must be remembered.If all other theories fail us, much must be granted Lowell. Wemust not reason like fishes—as Lowell puts it—and deny thatintelligent beings can thrive in an atmospheric pressure of fourand half inches of mercury. Zurbriggen has recently got to thetop of Aconcagua, a height of 24,000 feet. On the summit of sucha mountain the barometer must stand at about ten inches. Whyshould not beings be developed by evolution with a lung capacitycapable of living at two and a half times this altitude. Thosesteadily
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curved parallel lines are, indeed, very unlike anything we haveexperience of. It would be rather to be expected that anothercivilisation than our own would present many wide differences inits 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 hissurface. But the inhabitants are making a brave fight for it,They have constructed canals right round their world so that thewater, which otherwise would run to waste over the vast deserts,is led from oasis to oasis. Here the great centres ofcivilisation are placed: their Londons, Viennas, New Yorks. Thesegigantic works are the works of despair. A great and civilisedworld finds death staring it in the face. They have had to tripletheir canals so that when the central canal has done its work thewater is turned into the side canals, in order to utilise it asfar as possible. Through their splendid telescopes they must viewour seas and ample rivers; and must die like travellers in thedesert seeing in a mirage the cool waters of a distant lake.
Perhaps that lonely signal reported to have been seen in thetwilight limb of Mars was the outcome of pride in their splendidand perishing civilisation. They would leave some memory of it:they would have us witness how great was that civilisation beforethey perish!
I close this dramatic picture with the poor comfort
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that several philanthropic people have suggested signalling tothem as a mark of sympathy. It is said that a fortune wasbequeathed to the French Academy for the purpose of communicatingwith the Martians. It has been suggested that we could flashsignals to them by means of gigantic mirrors reflecting the lightof our Sun. Or, again, that we might light bonfires on asufficiently large scale. They would have to be about ten milesin diameter! A writer in the Pall Mall Gazette suggested thatthere need really be no difficulty in the matter. With the kindcooperation of the London Gas Companies (this was before the daysof electric lighting) a signal might be sent without anyadditional expense if the gas companies would consent tosimultaneously turn off the gas at intervals of five minutes overthe whole of London, a signal which would be visible to theastronomers in Mars would result. He adds, naively: "If onlytried for an hour each night some results might be obtained."
II
We have reviewed the theory of the artificial construction of theMartian lines. The amount of consideration we are disposed togive to the supposition that there are upon Mars other minds thanours will—as I have stated—necessarily depend upon whether or notwe can assign a probable explanation of the lines upon purelyphysical grounds. If it is apparent that such
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lines would be formed with great probability under certainconditions, which conditions are themselves probable, then theargument by exclusion for the existence of civilisation on Mars,at once breaks down.
{Fig. 10}
As a romance writer is sometimes under the necessity oftransporting his readers to other scenes, so I must now ask youto consent to be transported some millions
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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 quiteout of keeping with the orderly succession of worlds outward fromthe Sun. A society was formed at the close of the last centuryfor the detection of the missing world. On the first day of thelast century, Piazzi—who, by the way, was not a member of thesociety—discovered a tiny world in the vacant gap. Althougheagerly welcomed, as better than nothing, it was a disappointingfind. The new world was a mere rock. A speck of about 160 milesin diameter. It was obviously never intended that such a bodyshould have all this space to itself. And, sure enough, shortlyafter, another small world was discovered. Then another wasfound, and another, and so on; and now more than 400 of thesestrange little worlds are known.
But whence came such bodies? The generally accepted belief isthat these really represent a misbegotten world. When the Sun wasyounger he shed off the several worlds of our system as so manyrings. Each ring then coalesced into a world. Neptune being thefirst born; Mercury the youngest born.
After Jupiter was thrown off, and the Sun had shrunk away inwardssome 20o million miles, he shed off another ring. Meaning thatthis offspring of his should grow up like the rest, develop intoa stable world with the
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potentiality even, it may be, of becoming the abode of rationalbeings. But something went wrong. It broke up into a ring oflittle bodies, circulating around him.
It is probable on this hypothesis that the number we areacquainted with does not nearly represent the actual number ofpast and present asteroids. It would take 125,000 of the biggestof them to make up a globe as big as our world. They, so far asthey are known, vary in size from 10 miles to 160 miles indiameter. It is probable then—on the assumption that this failureof a world was intended to be about the mass of our Earth—thatthey numbered, and possibly number, many hundreds of thousands.
Some of these little bodies are very peculiar in respect to theorbits they move in. This peculiarity is sometimes in theeccentricity of their orbits, sometimes in the manner in whichtheir orbits are tilted to the general plane of the ecliptic, inwhich all the other planets move.
The eccentricity, according to Proctor, in some cases may attainsuch extremes as to bring the little world inside Mars' meandistance from the sun. This, as you will remember, is very muchless than his greatest distance from the sun. The entire belt ofasteroids—as known—lie much nearer to Mars than to Jupiter.
As regards the tilt of their orbits, some are actually as much as34 degrees inclined to the ecliptic, so that in fact they areseen from the Earth among our polar constellations.
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From all this you see that Mars occupies a rather hot comer inthe solar system. Is it not possible that more than once in theremote past Mars may have encountered one of these wanderers? Ifhe came within a certain distance of the small body his greatmass would sway it from its orbit, and under certain conditionshe would pick up a satellite in this manner. That his presentsatellites were actually so acquired is the suggestion of Newton,of Yale College.
Mars' satellites are indeed suspiciously and most abnormallysmall. I have not time to prove this to you by comparison withthe other worlds of the solar system. In fact, they were notdiscovered till 1877—although they were predicted in a mostcurious manner, with the most uncannily accurate details, bySwift.
One of these bodies is about 36 miles in diameter. This isPhobos. Phobos is only 3.700 miles from the surface of Mars. Theother is smaller and further off. He is named Deimos, and hisdiameter is only 10 miles. He is 12,500 miles from Mars' surface.With the exception of Phobos the next smallest satellite known inthe solar system is one of Saturn's—Hyperion; almost 800 miles indiameter. The inner one goes all round Mars in 7½ hours. This isPhobos' 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 thecourse of a day and night; lie would apparently cross the sky
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going against the other satellite; that is, he would moveapparently from west to east.
We may at least assume as probable that other satellites havebeen 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 axialrotation. Others, on the chances, would be retrograde: that is,would move against his axial rotation. They would describe orbitsmaking the same various angles with the ecliptic as do theasteroids; and we may be sure they would be of the same varyingdimensions.
We go on to inquire what would be the consequence to Mars of suchcaptures.
A satellite captured in this manner is very likely to be pulledinto the Planet. This is a probable end of a satellite in anycase. It will probably be the end of our satellite too. Thesatellite Phobos is indeed believed to be about to take this veryplunge into his planet. But in the case when the satellite pickedup happens to be rotating round the planet in the oppositedirection to the axial rotation of the planet, it is prettycertain that its career as a satellite will be a brief one. Thereasons for this I cannot now give. If, then, Mars picked upsatellites he is very sure to have absorbed them sooner or later.Sooner if they happened to be retrograde satellites, later ifdirect satellites. His present satellites are recent additions.They are direct.
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The path of an expiring satellite will be a slow spiral describedround the planet. The spiral will at last, after many years,bring the satellite down upon the surface of the primary. Itsfinal approach will be accelerated if the planet possesses anatmosphere, as Mars probably does. A satellite of the dimensionsof Phobos—that is 36 miles in diameter—would hardly survive morethan 30 to 60 years within seventy miles of Mars' surface. Itwill then be rotating round Mars in an hour and forty minutes,moving, in fact, at the rate of 2.2 miles per second. In thecourse of this 30 or 60 years it will, therefore, get roundperhaps 200,000 times, before it finally crashes down upon theMartians. During this closing history of the satellite there isreason to believe, however, that it would by no means pursuecontinually the same path over the surface of the planet. Thereare many disturbing factors to be considered. Being so small anylarge surface features of Mars would probably act to perturb theorbit of the satellite.
The explanation of Mars' lines which I suggest, is that they wereformed by the approach of such satellites in former times. I donot mean that they are lines cut into his surface by the actualinfall of a satellite. The final end of the satellite would betoo rapid for this, I think. But I hope to be able to show youthat there is reason to believe that the mere passage of thesatellite, say at 70 miles above the surface of the planet, will,in itself, give rise to effects on the crust of the planetcapable
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of accounting for just such single or parallel lines as we see.
In the first place we have to consider the stability of thesatellite. Even in the case of a small satellite we cannotoverlook the fact that the half of the satellite near the planetis pulled towards the planet by a gravitational force greaterthan that attracting the outer half, and that the centrifugalforce is less on the inner than on the outer hemisphere. Hencethere exists a force tending to tear the satellite asunder on theequatorial 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 bycalculation, however, that if Phobos has the strength of basaltor glass there would remain a considerable coefficient of safetyin favour of the satellite's stability; even when the surfaces ofplanet and satellite were separated by only five miles.
We have now to consider some things which we expect will happenbefore the satellite takes its final plunge into the planet.
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This diagram (Fig. 11) shows you the satellite travelling abovethe surface of the planet. The satellite is advancing towards, oraway from, the spectator. The planet is supposed to show itssolid crust in cross section, which may be a few miles inthickness. Below this is such a hot plastic magma as we havereason to believe underlies much of the solid crust of our ownEarth. Now there is an attraction between the satellite and thecrust of the planet; the same gravitational attraction whichexists between every particle of matter in the universe. Let usconsider how this attraction will affect the planet's crust. Ihave drawn little arrows to show how we may consider theattraction of the satellite pulling the crust of the planet notonly upwards, but also pulling it inwards beneath the satellite.I have made these arrows longer where calculation shows thestress is greater. You see that the greatest lifting stress isjust beneath the satellite, whereas the greatest stress pullingthe crust in under the satellite is at a point which lies outfrom under the satellite, at a considerable distance. At eachside of the satellite there is a point where the stress pullingon the crust is the greatest. Of the two stresses the liftingstress will tend to raise the crust a little; the pulling stressmay in certain cases actually tear the crust across; as at A andB.
It is possible to calculate the amount of the stress at the pointat each side of the satellite where the stress is at itsgreatest. We must assume the satellite to be a certain size anddensity; we must also assume the crust of