Theory of the Earth Volume Ii Part 10

The pumice islands form part of a chain of land that may be traced forming a circular line from the cape Missene to the mount Circello at the other side of the Gulf of Gaeta. The islands of Ischia and Procida, which form part of this chain of land, might, from the inspection of the map, be allowed as having once formed a continuation of the land from the continent of Italy, even without the testimony of natural history, that traces this connection from the materials of those ma.s.ses which now are separated.

The pumice islands form the middle part of that chain, and are the farthest removed from that continent of which it is probable they once formed a part. They are connected with the promontory of Missene on the one hand, as being of the same or similar volcanic origin, and on the other with mount Circello, by a curious circ.u.mstance in the island Zanone, which, but a little more of the devouring operation of the sea, would have concealed from our observation.

The island of Ventotiene, which is the nearest of them to Ischia, would appear to be the ancient island of Pendataria, in which Julia was confined. The marks of degradation in this island, I would wish to give in the Chevalier's own words, (p. 52.)

Cette isle continue a etre devoree par la mer, elle l'attaque dans toutes les parties de son contour, ou elle trouve peu de resistance, et elle ne cesse de creuser, princ.i.p.alement, tous les escarpemens du nord.

Il paroit, par les vestiges des antiquites qui sont sur la pointe dite _di Nevola_, que sous l'Empire de Cesar cette isle avoit encore une etendue plus considerable. Il s'y fait journellement des eboulemens; on peut prevoir qu'elle diminuera progressivement, qu'elle se divisera, et que dans les temps a venir elle sera reduite aux rochers de laves qui la supportent, et qui seuls peuvent resister, pendant une longue suite de siecles, a tous les efforts des flots; ce ne sera surement pas la seule terre que le temps et la mer auront devoree, et que les vicissitudes de la nature ont fait disparoitre avant que l'histoire en ait pu constater l'existence.

As the island of Ventotiene connects this group of the pumice islands with the continent of Missene, that of Zanone, on the other side, connects them with the continent at mount Circello. Here is a fact of which our author now gives proper evidence.

It would appear that Mount Circello is composed of an alpine limestone.

But in the north end of the island of Zanone, the Chevalier de Dolomieu finds a small part of a similar limestone in vertical strata, closely united with the volcanic materials of the islands now under consideration. It is impossible that this portion of calcareous rock could be formed in its present situation, and we have but to examine nature in order to be convinced that this limestone part had been once continued from Mount Circello. Here again I beg leave to give this author's own words, (page 141.)

Cette reunion de deux matieres aussi differentes par leur origine que le font celles qui forment l'Isle Zanone, est une circonstance des plus singuliers. La pierre calcaire ne contient point de coquillages; sa densite sa durete; son odeur fetide annonce une origine ancienne; elle n'est point formee par un depot de nouvelle date; elle differe des pierres calcaires-coquilliere qui recouvrent les volcans du Padouan et du Vicentin, et de celles qui se sont meles avec les produits du feu dans les volcans eteintes de la Sicile: les laves ici reposent sur elle: elle paroit donc anterieure a l'epoque des irruptions qui ont eleve les isles ponces. Par sa nature elle est semblable aux pierres du Mont Circe, et a celles de l'interieure de l'Apennin; il semble que cette portion de montagne calcaire, abstraction faite des matieres volcaniques qui lui sont reunies, a appartenu a quelqu'unes des montagnes qui dependent de la chaine qui traverse l'Italie; car il n'est pas possible que ni elle ni le Mont Circe ayent ete formes seules et isoles ainsi que nous les voyons. Mais quand ont-ils ete detachees? etoient-ils deja isoles lorsque les feux ont commence la formation des isles ponces? ou seroit-ce la meme revolution qui les auroit separes du continent, et qui a opere le desordre que nous voyons dans ces isles volcanique? On ne peut former sur toutes ces questions que des conjectures bien vagues.

Our present inquiry is only with regard to the operation of those causes which we now perceive to be acting upon the coasts of the land; which must be considered as having been operating for a long time back, and which must be considered as continuing to operate. One example more I wish to give, not only as it is much to the purpose, and properly described, but because it contains the natural history of a coast well known from the circ.u.mstance of the Giant's Causeway which it contains; a coast composed of stratified chalk indurated and consolidated to a species of marble or lime-stone, and of great ma.s.ses of basaltes or columnar whin-stone. Now, though our present object is not the formation of land, yet, knowing the mineral const.i.tution of this land, the coast of which we are considering as having been worn by the action of the sea, the view here to be given, of the white marble and basaltic cliffs, is satisfactory in the highest degree. It is from Letters concerning the Northern Coast of the County of Antrim, by the Reverend William Hamilton, A. M.

The chalky cliffs of the island of Raghery, crowned by a venerable covering of brown rock, form a very beautiful and picturesque appearance as one sails towards them; and, if the turbulence of the sea does not restrain the eyes and fancy from expatiating around, such a striking similitude appears between this and the opposite coast, as readily suggests an idea that the island might once have formed a part of the adjoining country, from whence it has been disunited by some violent shock of nature.

You, to whom demonstration is familiar, will wonder to see two sh.o.r.es, seven or eight miles asunder, so expeditiously connected by such a slender and fanciful middle term as apparent similitude; and yet the likeness is so strong, and attended with such peculiar circ.u.mstances, that I do not entirely despair of prevailing even on you to acknowledge my opinion as a probable one.

It does not appear unreasonable to conclude, that, if two pieces of land, separated from each other by a chasm, be composed of the same kind of materials, similarly arranged, at equal elevations, these different lands might have been originally connected, and the chasm be only accidental. For, let us conceive the materials to be deposited by any of the elements of fire, air, earth, or water, or by any cause whatever, and it is not likely that this cause (otherwise general) should in all its operations regularly stop short at the chasm.

The materials of which the island of Raghery is composed are accurately the same as those of the opposite sh.o.r.e; and the arrangement answers so closely, as almost to demonstrate, at first view, their former union.

But to explain this more clearly, it will be necessary to give you a general sketch of this whole line of coast.

The northern coast of Antrim seems to have been originally a compact body of lime-stone rock, considerably higher than the present level of the sea; over which, at some later period, extensive bodies of vitrifiable stone have been superinduced in a state of softness. The original calcareous stratum appears to be much deranged and interrupted by those inc.u.mbent ma.s.ses. In some places it is depressed greatly below its ancient level; shortly after it is borne down to the water's edge, and can be traced under its surface. By and by it dips entirely, and seems irretrievably lost under the superior ma.s.s. In a short s.p.a.ce, however, it begins to emerge, and, after a similar variation, recovers its original height.

In this manner, and with such repeated vicissitudes of elevation and depression, it pursues a course of forty miles along the coast from Lough Foyle to Lough Larne.

It naturally becomes an object of curiosity to inquire what the substance is from which the lime-stone seems thus to have shrunk, burying itself (as it were in terror) under the covering of the ocean: And, on examination, it appears to be the columnar basaltes, under which the lime-stone stratum is never found; nor indeed does it ever approach near to it without evident signs of derangement.

Thus, for example, the chalky cliffs may be discovered a little eastward from Portrush; after a short course, they are suddenly depressed to the water's edge, under Dunluce Castle, and, soon after, lost entirely in pa.s.sing near the basalt-hill of Dunluce, whose craigs, near the sea, are all columnar. At the river Bush the lime-stone recovers, and skims a moment above the level of the sea, but immediately vanishes in approaching towards the great basalt promontory of Bengore, under which it is completely lost for the s.p.a.ce of more than three miles.

Eastward from thence, beyond Dunsaverock Castle, it again emerges, and, rising to a considerable height, forms a beautiful barrier to White Park Bay and the Ballintoy sh.o.r.e. After this it suffers a temporary depression near the basalt hill of Knocksoghy, and then ranges along the coast as far as Ballycastle Bay.

Fairhead, standing with magnificence on its ma.s.sy columns of basaltes, again exterminates it; and once again it rises to the eastward, and pursues its devious course, forming, on the Glenarm sh.o.r.es, a line of coast the most fantastically beautiful that can be imagined.

If this, tedious expedition have not entirely worn out your patience, let us now take a view of the coast of Ragery itself, from the lofty summit of Fairhead, which overlook it. Westward we see its white cliff rising abruptly from the ocean, corresponding accurately in materials and elevation with those of the opposite sh.o.r.e, and like them, crowned with a venerable load of the same vitrifiable rock. Eastward, we behold it dip to the level of the sea, and soon give place to many beautiful arrangements of basalt pillars which form the eastern end of the island, and lie opposite to the basaltes of Fairhead, affording in every part a reasonable presumption that the two coasts were formerly connected, and that each was created and deranged by the same causes extensively operating over both.

But it is not in these larger features alone that the similitude may be traced; the more minute and accidental circ.u.mstances serve equally well to ascertain it.

Thus, an heterogeneous ma.s.s of freestone, coals, iron-ore, etc. which forms the east side of Ballycastle Bay, and appears quite different from the common fossils of the country, may be traced also directly opposite, running under Rathlin, with circ.u.mstances which almost demonstrably ascertain it to be the same vein.

What I would infer from hence is, that this whole coast has undergone considerable changes; that those abrupt promontories, which now run wildly into the ocean, in proud defiance of its boisterous waves, have been rendered broken and irregular by some violent convulsion of nature; and that the island of Ragery, standing as it were in the midst between this and the Scottish coast, may be the surviving fragment of a large tract of country which, at some period of time, has been buried in the deep.

Besides this argument of the gradation from a continent of land to a bare rock, we have another from the consideration of those rocks themselves, so far as these could not be formed by nature in their present state, but must have been portions of a greater ma.s.s. How, for example, could a perpendicular mountain, such as St. Kilda, have been produced in the ocean? Of whatever materials we shall suppose it formed, we never shall find means for the production of such a ma.s.s in its present insulated state. Let us take examples of this kind near our coast, and of known rocks. Staffa and Ailsa, on the west coast, and the Ba.s.s, upon the east, are mountains of either whin-stone or granite, similar to many such mountains within the land; and they are perpendicular around, except perhaps on one part. It is demonstrable that such basaltic rock as contains zeolite and calcareous spar, as most of our whin-stones do, could not have been the eruption of a volcano, consequently those rocks must have been ma.s.ses protruded in a fluid state, under an immense cover of earth at the time of their production; and they could not have risen immediately out of the sea, with all their various minerals, their veins and cutters, their faces and their angles.

In like manner, the east coast of Caithness is a perpendicular cliff of sand-stone, lying in a horizontal position, and thus forming a flat country above the sh.o.r.e. But along this coast there are small islands, pillars, and peninsulas, of the same strata, corresponding perfectly with that which forms the greater ma.s.s. Now, shall we suppose those strata of sand-stone to have been formed in their place, and to have reached no farther eastward into the sea?--It is unsupposable. Or, shall we conceive that the sea, which has made such depredations in land composed of much more solid materials, had spared this, and had not wasted much more than that now pointed out by the ruins which remain?--Impossible; we must suppose that there had once existed much land where nothing now is found but sea. But, if we are to suppose much to have been wasted, where shall we stop in this process of restoring continents? That is the question now to be discussed.

With this view, let us now turn our attention to the north-west coast of Europe, in consulting the general as well as the most particular maps.

Upon the one extremity of Britain, we find Cornwall separating it as it were from the main land; and, from this promontory, the Scilly Isles pointing out what had been destroyed in that direction, which is here to be considered as the line of greatest resistance. But what a quant.i.ty of the soft materials, or less resisting parts on either side, has been destroyed! Upon the other extremity of Britain, we find the country of Scotland, forming itself into promontories and islands, and those islands and rocks pointing out to us what had been the former extent of our continent and land around. But, in following this connection of things, we cannot refuse to acknowledge that Ireland had formerly been in one ma.s.s of land with Britain, in like manner as the Orkneys had been with Scotland[16].

[Footnote 16: I have the most satisfactory evidence of this fact, in finding the schistus of Galloway and of England in the opposite coast of Ireland, corresponding to its direction in stretching from the coasts of Britain.]

It will be still less possible to refuse the junction of England with the continent of France; the testimony of that peculiar body of chalk and flint, which borders each of those opposite coasts, forms an argument which is irrefragable. Now, in order to complete our continent, we have only to connect the Shetland islands with the coast of Norway.

But this is a notion which, however probable it may appear, is not proposed as a fact immediately supported by natural appearances; it is only to be considered as an enlarged view in which we may contemplate the operations of this earth upon a more extended scale; one which may be conceived as a step in our cosmogeny, and one which, while it ill.u.s.trates the theory of the earth already given, is by no means required in order to confirm a theory founded upon appearances which leave no manner of doubt.

CHAP. IX.

_The Theory Ill.u.s.trated, with a View of the Summits of the Alps._

There are two different directions in which we may observe the destruction of our land to proceed; in the one of these, the basis of our continent is diminished by the incroachment of the sea; in the other, again, it is the height of the land above the level of the sea that is lowered. We have been considering the incroachment of the sea upon the continent; let us now examine how far there may also appear sufficient doc.u.ments, by which we may be led to conclude a long progress in time past, for the destruction of the solid ma.s.s of earth above the sea, without diminis.h.i.+ng its basis.

If we shall suppose this earth composed of horizontal strata, and of one level surface, without the least protuberance remaining by which we might be informed of what had been removed by time in the operation of second causes, we should be ignorant of every thing of cosmogeny but this, That the strata of the globe had been originally formed (by the sea) in the same shape as we had found them on the surface of the land.

But this is not the shape of the surface of our continent: We have every where abundance of eminences, sufficient to give us great information with regard to what had pa.s.sed in former periods of time, if the strata of the globe were in that regular shape which they had originally a.s.sumed in being deposited at the bottom the sea.

The strata, however, are not in that regular shape and position from whence we might learn, by examining the remaining portions, what had been carried away from the surface in general; they are found variously inclined to the horizon; and this we find both occasioned from the fracture and flexure of those bodies, thus changed from their natural horizontal state. Thus, though there are in many places immense ma.s.ses of strata cut off abruptly, and exposed to view, without the remainder appearing, we cannot from hence form any estimate of the general quant.i.ty of destruction; at the same time, it must be evident, from a general inspection, that there has been an immense quant.i.ty removed; and that an immense time had been required in bringing about those revolutions of things, which are not done by violent changes, but by slow degrees.

Besides that general conclusion with regard to the destruction of the strata, there is also in many places a demonstration of that fact, from a measured minimum of the quant.i.ty which had been removed. It is to the mining business chiefly that we are indebted for that demonstration of which we now shall give an example.

The coal strata, about Newcastle upon Tyne, dip to the south-east at the rate of one in twelve, or thereabouts. This is but little removed from the horizontal position; at the same time, the strata come all up to the soil or surface in a country which is level, or with little risings. But in those strata there is a slip, or hitch, which runs from north-east to south-west, for 17 or 18 miles in a straight line; the surface on each side of this line is perfectly equal, and nothing distinguishable in the soil above; but, in sinking mines, the same strata are found at the distance of 70 fathoms from each other. Here therefore is a demonstration, that there had been worn away, and removed into the sea, 70 fathoms more from the country on the one side of this line, than from that on the other. It is far from having given us all the height of country which has been washed away, but it gives us a minimum of that quant.i.ty.

The examination of what is commonly called a secondary country is not sufficient to give us an idea of the immense operation of time in wearing the surface of this earth. It is not that those countries of inferior hardness and elevation have been spared in the course of time, but because we have not, in those levelled countries, such great remainders, by which we are to judge the quant.i.ty of what is lost. In the alpine country, again, though it be the same system of things with that which takes place in the lower country, the revolution of things is more marked for our view; and the ravages of time, in destroying the solid parts of the globe, in order to make soil of that which is removed, may be seen in all the steps of that important operation; whereas, in the more level countries, the scale of elevation is imperceptible, and that of time is so slow as renders our examination fruitless. It is the Alps, therefore, chiefly that we are to take for an example, in tracing this operation of nature upon the surface of this earth, and forming some idea of the course of time that must have flowed during that operation in which the height of our land had been diminished.

On whatever side we approach the Alps, we find some great river discharging the waters which had been gathered above, and with that water all the waste of earth and stone which had been made among those lofty ma.s.ses of decaying rock. Now, we find this river running in a valley proportioned, in general, to this vehicle, in which is travelled the wreck of ruinous mountains. s.p.a.cious plains attend those mighty streams; and, tho' sometimes we find the greatest rivers much confined between approaching hills of solid rock, the valley opens again, and, on the whole, is always corresponding to the current of water which has successively run in all the quarters of this plain. Here a question occurs; Has this valley been made by the operation of the river itself, or has it been the effect of other causes? Let us now resolve that question.

If the valley was made for the river by any other natural cause, either we should tell by what means this work had been performed, or all reasoning upon the subject is at an end, and fancy subst.i.tuted in its place. If again the river be considered as the means employed by nature in making this valley, then all the solid parts between the bounding mountains must have been removed, and the fertile plains must have been formed by the water depositing those materials which we find in the soil, and which had come originally from the solid mountains. There is no occasion to enter into any argument to prove this fact; n.o.body that examines the matter will find any reason to doubt; and it would be as unreasonable for those to doubt who have not examined, as for those who find no reasonable subject of doubt to disbelieve.

We are now to suppose the great river to have formed the valley and extensive plain in which the water runs,--a valley corresponding to the grandeur of the river by which it has been formed. But, as we ascend this great valley, we find other valleys branching from this main valley; and, in all those subordinate valleys, we find rivers corresponding in like manner with the magnitude of the valley. Here, therefore, is infinitely more than a single river, and a valley corresponding to the river; here is a _system_ of rivers and of valleys, things calculated in perfect wisdom, or properly adapted to each other.

Now it is just as easy, by our theory, to explain this system of rivers and valleys, as it is to understand the single appearance of a river and a valley. But it is only in this manner that such a complicated operation, of a series in rivers and their valleys, is to be explained; and we can neither suppose the land to be formed with this intention by a supernatural cause, nor imagine any other natural cause so arranging things, upon the surface of the earth, as to form this perfect system, which holds of nothing but itself; a system in which is manifested wisdom, so far as all the parts are properly adapted to each other, and thus made to answer that intention which is so visible in the economy of this world.

The direction of the princ.i.p.al valleys of the Alps, or every mountainous region of the globe, may be considered as proceeding from the centre of that region to the plain country in which each river is to terminate; each secondary river with its valley then branches from the primary as from a stem, consequently runs in a direction perpendicular or inclined to the other. But the secondary rivers also have their branches; and subordinate branches still are branched. In thus tracing rivers and their branchings, we come at last to rivulets that only run in times of rain, and at other times are dry. It is here I would wish to carry my reader, in order to be convinced, with his proper observation, of this great fact,--that the rivers, in general, have hollowed out their valleys.

The changes of the valley of the main river are but slow, the plain indeed is wasted in one place, but it is repaired in another, and we do not perceive the place from whence that repairing matter had proceeded.

Therefore, that which here appears does not immediately suggest to the spectator what had been the state of things before the valley had been hollowed out, or before that plain, through which the river runs so naturally as being in the lowest place, was made. But it is otherwise in the valley of the rivulet; no person can examine this subject without seeing that the rivulet carries away matter which cannot be repaired except by wearing away some part of the mountain, or the surface of that place upon which the rain, which forms the stream, is gathered. In those rivulets, or their little plains, we see the detached parts remaining in the soil, and also the place from whence those detached parts were taken. Here we need no long chain of reasoning from effect to cause; the whole operation is in a manner before our eyes. In this case, it requires but little study to replace the removed parts; and thus to see the work of nature, resolving the most hard and solid ma.s.ses by the continued influences of the sun and atmosphere. In this state of things, we are easily made to understand how heavy bodies are travelled along the declivity of the earth, by means of water running from the height.

Such is the system of rivers and their valleys; nor is there upon the continent a spot on which some river has not run. But, in the Alps of Switzerland and Savoy, there is another system of valleys, above that of the rivers, and connected with it. These are valleys of moving ice, instead of water. This icy valley is also found branching from a greater to a lesser, until at last it ends upon the summit of a mountain, covered continually with snow. The motion of things in those icy valleys is commonly exceeding slow, the operation however of protruding bodies, as well as that of fracture and attrition, is extremely powerful.

To ill.u.s.trate those operations of excavating the valleys of rivers and of thus undermining mountains which fall by their proper weight, I shall transcribe some descriptions of what is to be found among the Alps. But first I would wish to carry my reader to the summit of that country, to examine the state of that part which nothing can have affected but the immediate influences of the sun and air. After having thus formed some idea of the summit of this wasting country, we shall next examine the valleys through which the materials of the degraded summit must have travelled.

In order to give a proper idea of this central part of the Alps, which is so interesting a part in the natural history of the earth, M. de Saussure, in the plates of his _Voyages dans les Alpes_, tom. 2. has given us two views, the one in profile, the other in face, of the Mont-Blanc. I have caused copy those plates, which are necessary to be consulted in reading the following description of this centre of the Alps.

This author has taken much pains to form, to himself a proper idea of the object which we have now in view; and he gives a description of the Mont-Blanc as seen from the top of the Cramont. It is that description which I am now to transcribe[17].