Chapter XV: Section III (9)

374. The course of the Gulf stream is so fixed and regular, that nuts and plants from the West Indies are annually thrown ashore on the Western Islands of Scotland. The mast of a man of war, burnt at Jamaica, was driven several months afterwards on the Hebrides,[190] after performing a voyage of more than 4000 miles, under the direction of a current, which, in the midst of the ocean, maintains its course as steadily as a river does upon the land.

[Footnote 190: Pennant's Arctic Zoology, Introd. p. 70.]

The great system of currents thus traced through the Atlantic, has no doubt phenomena corresponding to it in the Indian and Pacific Oceans, which the industry of future navigators may discover. The whole appears to be connected with the trade winds, the figure of our continents, the temperature of the seas themselves, and perhaps with some inequalities in the structure of the globe. The disturbance produced by these causes in the equilibrium of the sea, probably reaches to the very bottom of it, and gives rise to those counter currents, which have sometimes been discovered at great depths under the surface.[191]

[Footnote 191: Histoire Naturelle de Buffon, Supplément, tom. ix. p. 479. 8vo.]

The great transportation of materials that must result from the action of these combined currents is obvious, and serves not a little to diminish our wonder, at finding the productions of one climate so frequently included among the fossils of another. Amid all the revolutions of the globe, the economy of nature has been uniform, in this respect, as well as in so many others, and her laws are the only thing that have resisted the general movement. The rivers and the rocks, the seas and the continents, have been changed in all their parts; but the laws which direct those changes, and the rules to which they are subject, have remained invariably the same.

375. Objections have been made to that translation of materials by the waters of the ocean which is supposed in this theory, particularly by Mr Kirwan, in his Geological Essays; and, though I might perhaps content myself with the remark already made, that the Neptunian system involves suppositions concerning the transportation of solid bodies by the sea, in the early ages of the world, as wonderful as those which, according to our theory, are common to all ages, I am unwilling to remain satisfied with a mere _argumentum ad hominem_, where the fallacy of the reasoning is so easily detected.

376. One of Mr Kirwan's objections to the deposition of materials at the bottom of the sea, is thus stated: "FRISI has remarked, in his mathematical discourses, that if any considerable mass of matter were accumulated in the interior of the ocean, the diurnal motion of the globe would be disturbed, and consequently it would be perceptible; a phenomenon, however, of which no history or tradition gives any account."[192]

[Footnote 192: Geol. Essays, p. 441.]

The appeal made here to Frisi is singularly unfortunate, as that philosopher has demonstrated the very contrary of Mr Kirwan's position, and has proved, that the disturbance given to the diurnal motion by the causes here referred to may be real, but cannot be perceptible. Having investigated a formula expressing the law which all such disturbances must necessarily observe, he concludes, "Hàc autem formulâ manifestum fiet, ex iis omnibus variationibus quæ in terrestri superficie observari solent, montium et collium abrasione, dilapsu corporum ponderosiorum in inferiores telluris sinus, nullam oriri posse variationem _sensibilem_ diurni motûs. Nam si statuamus data aliqua annorum periodo terrestrem superficiem ad duos usque pedes abradi undique, eam vero materiæ quantitatem ad profunditatem pedum 1000 dilabi; erit omne quod inde orietur incrementum velocitatis diurni motûs 30000/(19638051)^2 = 1/12855068184."[193]

[Footnote 193: Frisii Opera, tom. iii. p. 269.]

Here, it is evident, that Frisi admits those very changes on the surface which we are contending for, and shows, that their tendency is to accelerate the earth's diurnal motion, but, by a quantity so small, that, in a space of time amounting at least to 200 years, the increase of the diurnal motion would only be such a part of the whole as the preceding fraction is of unity.[194]

[Footnote 194: The time requisite for taking away by waste and erosion two feet from the surface of all our continents, and depositing it at the bottom of the sea, cannot be reckoned less than 200 years. The fraction 1/12855068184, reduced to parts of a day, is 1/148554 of a second; so that it would require 200 years to shorten the length of the day, by the above fraction of a second; and therefore it would require 148554 times 200 years, or 29710800 years, to diminish it an entire second. The accumulated effect, however, of all the diminutions during that period, would amount to much more: and if we had any perfectly uniform standard to compare the motion of the earth with, its difference from that standard would increase as the squares of the time, and the total acceleration would amount to one second in 77080 years. Whatever relation this bears to the age of the globe itself, it exceeds more than ten times the age of any historical record.

Though Frisius concludes, as is stated here, that the acceleration produced in the diurnal motion of the earth, is far too inconsiderable to become the object of astronomical observation, he makes a supposition difficult to be reconciled with this conclusion, namely, that the acceleration has had a sensible effect on the figure of the earth, or rather of the sea, having increased the centrifugal force, and thereby accumulated the waters under the equator, in the present, more than in former ages. Such an accumulation, he thinks agreeable to certain appearances that have been observed respecting the ancient level of the sea. These appearances will be afterwards considered: it is sufficient to remark here, that though the fraction, expressing the increment of the centrifugal force, must be double that which expresses the acceleration, it must be too small to have any perceptible effect in elevating the sea, except after an immense interval of time; and the compensations which arise from other causes, probably must prevent it from becoming sensible in any length of time whatsoever.]

377. The instance just given may serve as one of many, to shew what confidence is to be placed in that indigested mass of facts and quotations which Mr Kirwan, without discrimination, and without discussion, has brought together from all quarters. He has no intention, I believe, to deceive his readers; but we may judge, from this specimen, of the precautions he has taken against being deceived himself.

In some respects, the result of Frisi's investigation must be considered as imperfect. If there were no relative motion in the parts of our globe, but that by which things descend from a higher to a lower level, a continual acceleration of its rotation, though extremely slow, would take place, as above computed. But as, in the interior of the earth, there are undoubtedly motions of a tendency opposite to those on the surface, and directed from the centre towards the circumference, they must produce a retardation in the diurnal revolution; and from this must arise an inequality, not uniformly progressive in the same direction, but periodical, and confined within certain limits, as the causes are by which it is produced.[195]

[Footnote 195: Even in the descent of bodies from a higher to a lower level at the surface of the earth, the whole tendency is not to increase the velocity of the earth's rotation, and many compensations take place, which, when the matter is considered only in general, are necessarily overlooked. This will appear evident, if we reflect, that it is not simply the approach of a body towards the centre of the earth, or its removal from that centre, which tends to disturb the rotation of the earth; but its approach to the axis of the earth, or its removal from that axis. The velocity with which a particle of matter revolves, whether on the surface, or in the interior of the globe, is proportional to its distance from the axis of rotation; and therefore, when a body comes nearer to the axis, it loses a part of the motion which it had before; which part, of consequence, is communicated to the whole mass of the earth, and therefore tends to increase the velocity with which it revolves. The contrary happens when a body recedes from the axis; for it then receives an addition to its velocity, which, of course, is taken away from the rotatory motion of the earth.

Hence, bodies moving in a horizontal plane, may increase or diminish the swiftness of the diurnal motion, according as they move towards the poles or towards the equator; and those which descend from a higher to a lower level, disturb the earth's rotation, much more in consequence of their horizontal, than of their perpendicular motion. The Ganges, for instance, though its source is probably elevated no less than 7000 feet above the level of the sea, tends to retard the earth's rotation, by bringing its waters, and the mud contained in them, from the parallel of 31° to that of 22°, and so increasing their distance from the earth's axis by more than 1/12 th part. Had the Ganges flowed towards the north, as the Nile does, its effect would have been just the contrary.

In the same manner, a stone descending from the top of a mountain, may accelerate or retard the earth's rotation, according to the direction in which it descends. If it descend on the side of the elevated pole, it will then produce acceleration, because its distance from the axis will be diminished; but if it descend on the side of the depressed pole, and if the direction in which it is moved, be over a line less inclined, than a line drawn from the same point to the depressed pole, it will then produce a retardation, because its distance from the axis will be increased.

Let us suppose, for example, that the top of Mount Blanc is in latitude 45° 49′, and that its height is 2450 toises above the level of the sea. The point at which a line drawn from the top of this mountain, parallel to the earth's axis, will meet the superficies of the sea, (supposing that superficies continued inland from the Mediterranean), must be about 2382 toises in horizontal distance, or about 2-1/2 minutes south of the summit, that is, in the parallel of 45° 46-1/2′; and if this parallel be continued all round the globe, the points of the earth's surface between it and the equator, are all more distant from the earth's axis than the top of Mount Blanc is; whereas all the points to the north of it are nearer to that axis. A stone, therefore, from the top of Mount Blanc, if carried any where to the south of the above parallel, will retard the earth's diurnal motion; but if carried any where to the north of the same line, will accelerate that motion.

The same quantity of matter, however, carried an equal distance toward the pole, and toward the equator, from any point, will lose more velocity in the former case than it will gain in the latter, as easily follows from the nature of circle. Therefore, supposing an equal dispersion of the detritus of a mountain in all directions, the parts that go toward the pole will most disturb the diurnal motion; and hence a balance on their side, or in favour of acceleration, as already observed.]

378. Mr Kirwan's second objection is founded on the misapprehension of a well-known fact in the natural history of the earth. "Rivers," says this author, "do not carry into the sea the spoils which they bring from the land, but employ them in the formation of deltas of low alluvial land at their mouths, according to what Major Rennell has proved." The fact of the formation of _deltas_ from the spoils which the rivers carry from the higher grounds, is perfectly ascertained; and the detail into which Major Rennell has entered in the passage referred to by Mr Kirwan, does credit to the acuteness and accuracy of that excellent geographer. But it is not there asserted, that rivers employ _all_ the materials which they carry with them, in the formation of those deltas, and deliver none of them into the sea. On the contrary, they carry from the _delta_ itself mud and earth, which they can deposit no where but in the sea; and it is this circumstance chiefly that limits the increase of those alluvial lands, and makes them either cease to increase, or makes them increase very slowly after a certain period, though the supply of earth from the higher grounds remains nearly the same. To make Mr Kirwan's argument conclusive, it would be necessary to prove, that _all_ the mud carried down by the Nile or the Ganges, was deposited on the low lands before these rivers enter the sea; a thing so obviously absurd, that nothing but his haste to obtain a conclusion unfavourable to the Plutonic system, could have prevented him from perceiving it[196].

[Footnote 196: The instance mentioned in the Geological Essays, from the travels of the Abbé Fortis, concerning urns thrown into the Adriatic, upwards of 1400 years ago, and not yet covered with mud, must be explained from peculiar circumstances, or local causes, with which we are unacquainted, as it makes against the deposition of earth near the shore, and in narrow seas; a general fact which, I think, every body admits.]

379. A remark which Major Rennell has made concerning the mouths of rivers, in his Geography of Herodotus, deserves Mr Kirwan's attention, though perhaps he may not be able to put on it an interpretation quite so favourable to his system. The remark is, that the mouths of great rivers are often formed on principles quite opposite to one another, so that some of them have a real delta or triangle of flat land at their mouths, while others have an estuary, or what may not improperly be called a _negative_ delta. Of the latter kind are some of the greatest rivers in the world, the Plata, the Oroonoko and the Maranon, and by far the greatest number of our European rivers. Nobody can doubt, that the three rivers just named carry with them as much earth as the Nile, or the Euphrates, or any other river in the world. All this they have deposited in the sea, and committed to the currents, which sweep along the shore of the American continent, and by these they have been spread out over the unlimited tracts of the ocean.

Indeed, nothing can be more just than Dr Hutton's observation, that where low land is formed at the mouths of rivers, there the rivers bring down more than the sea is able to carry away; but that where such land is not formed, it is because the sea is able to carry off immediately all the deposit which it receives.

380. Mr Kirwan has denied on another principle the power of the sea to carry to a distance the materials delivered into it: "Notwithstanding," says he, "many particles of earth are by rivers conduced to the sea, yet _none are conveyed in any distance_, but are either deposited at their mouths, or rejected by currents or by tides; and the reason is, because the tide of flood is always more impetuous and forcible than the tide of ebb, the advancing waves being pressed forward by the countless number behind them, whereas the retreating are pressed backward by a far smaller number, as must be evident to an attentive spectator; and hence it is that all floating things cast into the sea, are at last thrown on shore, and not conveyed into the mid regions of the sea, as they should be if the reciprocal undulations of the tides were equally powerful "[197]

[Footnote 197: Kirwan's Geol. Essays, p. 439.]

381. But if the _attentive spectator_, instead of trusting to a vague impression, or listening to some crude theory of undulations, reflects on one of the most simple facts respecting the ebbing and flowing of the tides, he will be very little disposed to acquiesce in the above conclusion. He has only to consider, that the flowing of the tide requires just six hours, and the ebbing of it likewise six hours; so that the same body of water flows in upon the shore, and retreats from it, in the same time. The quantity of matter moved, therefore, and the velocity with which it is moved, are in both cases the same; and it remains for Mr Kirwan to show in what the difference of their force can possibly consist.

The force with which the waves usually break upon our shores, does not arise from the velocity of the tide being greater in one direction than in another. In the main ocean, the waves have no progressive motion, and the columns of water alternately rise and fall, without any other than a reciprocating motion: a kind of equilibrium takes place among the undulations, and each wave being equally acted upon by those on opposite sides, remains fixed in its place. Near the shore this cannot happen; the water on the land side from its shallowness being incapable of rising to the height necessary to balance the great undulations which are without. The water runs, therefore, as it were, from a higher to a lower level, spreading itself towards the land side. This produces the breakers on our shores, and the surf of the tropical seas. A rock or a sandbank coming within a certain distance of the surface, is sufficient, in any part of the ocean, to obstruct the natural succession of undulations; and, by destroying the mutual reaction of the waves, to give them a progressive instead of a reciprocating motion.

382. It is, however, but from a small distance, that the waves are impelled against the shore with a progressive motion. The border of breakers that surrounds any coast is narrow, compared with the distance to which the _detritus_ from the land is confessedly carried; the water, while it advances at the surface, flows back at the bottom; and these contrary motions are so nearly equal, that it is but a very momentary accumulation of the water that is ever produced on any shore.

If it were otherwise, and if it were true that the sea throws out every thing, and carries away nothing, we should have a constant accumulation of earth and sand along all shores whatsoever, at least wherever a stream ran into the sea. This, as is abundantly evident, is quite contrary to the fact.

So, also, the bars formed at the mouths of rivers, after having attained a certain magnitude, increase no farther, not because they cease to receive augmentations from the land, but because their diminution from the sea, increasing with their magnitude, becomes at length so great, as completely to balance those augmentations. When properly examined, therefore, the phenomena, which have been proposed as most inconsistent with the indefinite transportation of stony bodies, afford very satisfactory proofs of that operation.

383. It is true, that bodies which float in the water, when carried along on the tops of the waves towards a shelving beach, having acquired a certain velocity, are thrown farther in upon the land than the distance they would have floated to, if they had been simply sustained by the water. The depth of water, therefore, at the place where they take the ground, is not likely to be such as to float them again, and to carry them out towards the sea. They are, therefore, left behind; and this produces an appearance of a force impelling floating bodies towards the land, much greater and more general than really takes place.

These observations may serve to show, how unsound the principles are from which Mr Kirwan's conclusions are deduced: they are perhaps more than is necessary for that purpose: it might have been sufficient to observe, that the increase of land on the sea shore is limited, though the augmentation from the land is certainly indefinite, a proof that the diminution from the sea is constant and equal to the increase.

384. "Mariners," says Mr Kirwan, "were accustomed, for some centuries back, to discover their situation, by the kind of earth or sand brought up by their sounding plummets; a method which would prove fallacious, if the surface of the bottom did not continue invariably the same."[198]

[Footnote 198: Geol. Essays, p. 440.]

The fact here stated, that mariners, when navigation was more imperfect than it is now, had very frequent recourse to this method, and that they still use it occasionally, is very true. But from this, the only inference that can be fairly deduced is, that the changes at the bottom of the sea are very slow, and the variation but little; not merely from one year to another, but even from one century to another. The rules by which the mariner judged of his position from the quality of the earth which the lead brought up, and which were deduced no doubt from observations made at no very great distance of time, might be sufficient for his purpose, though a slow change had been all the while going forward. Such observations could at best have little accuracy, and could not be affected by small variations. It is the slowness of the change, that makes the experience of one age applicable, in this, as in innumerable other instances, to the observations of the next. If a long interval is taken, we will look in vain for the same uniformity of results. A pilot, who would at present judge of his position in the German Ocean, by comparing his soundings with those taken by PYTHEAS, (supposing them known) in his navigation of that sea, more than 2000 years ago, could hardly be expected to determine his latitude and longitude with great exactness; and I know not if the most zealous advocate for the immutability of the earth's surface, would be willing to trust his safety in a ship that was guided by such antiquated rules.

NOTE XX. § 118.

_Inequalities in the Planetary Motions._

385. The assertion that, in the planetary motions, we discover no mark, either of the commencement or termination of the present order, refers to the late discoveries of LA GRANGE and LA PLACE, which have contributed so much to the perfection of physical astronomy. From the principle of universal gravitation, these mathematicians have demonstrated, that all the variations in our system are periodical; that they are confined within certain limits; and consist of alternate diminution and increase. The orbits of the planets change not only their position, but even their magnitude and their form: the longer axis of each has a slow angular motion; and, though its length remains fixed, the shorter axis increases and diminishes, so that the form of the orbit approaches to that of a circle, and recedes from it by turns. In the same manner, the obliquity of the ecliptic, and the inclination of the planetary orbits, are subject to change; but the changes are small, and, being first in one direction, and then in the opposite, they can never accumulate so as to produce a permanent or a progressive alteration. Thus, in the celestial motions, no room is left for the introduction of disorder; no irregularity or disturbance, arising from the mutual action of the planets, is permitted to increase beyond certain limits, but each of them, in time, affords a correction for itself. The general order is constant, in the midst of the variation of the parts; and, in the language of La Place, there is a certain mean condition, about which our system perpetually _oscillates_ performing small vibrations on each side of it, and never receding from it far.[199] The system is thus endowed with a stability, which can refill the lapse of unlimited duration; it can only perish by an external cause, and by the introduction of laws, of which at present no vestige is to be traced.

[Footnote 199: Exposition du Systéme du Monde, par La Place, Livre iv. chap. 6. p. 199. 2d edit.]

386. The same _calculus_ to which we are indebted for these sublime conclusions, informs us of two circumstances, which mark the law here treated of as an effect of wise design, to the entire exclusion both of necessity and chance. One of these circumstances consists in the planetary motions being all in the same direction, or all _in consequentia_ as it is called by the astronomers. This is essential to the compensation and stability above mentioned:[200] had one planet circulated round the sun in a direction from east to west, and another in a direction from west to east, the disturbances they would have produced on one another's motion would not necessarily have been periodical; their irregularities might have continually increased, and they might have deviated in the course of ages from their original condition, beyond any limits that can be assigned.

[Footnote 200: La Place, _ibid._]

The other circumstance, on which the stability of our system depends, is the small eccentricity of the planetary orbits, or their near approach to circles. Were their orbits very eccentric, an opening would be given to progressive change, that might so far increase, as to prove the destruction of the whole. But neither the movement of all the planets in the same direction, nor the small eccentricity of their orbits, can be ascribed to accident, since that either of these should happen by chance, in as many instances as there are planets, both primary and secondary, is almost infinitely improbable. Again, that any necessity in the nature of things should have either determined the _direction_ of the planetary motions, or proportioned the _quantity_ of them to the intensity of the central force, cannot be admitted, as these are things unavoidably conceived to be quite independent of one another. It remains, therefore, that we consider the laws, which make the disturbances in our system correct themselves, and by that means give firmness and permanence to it, as a proof of the consummate wisdom with which the whole is constructed.

387. The geological system of Dr Hutton, resembles, in many respects, that which appears to preside over the heavenly motions. In both, we perceive continual vicissitude and change, but confined within certain limits, and never from a certain, mean condition, which is such, that, in the lapse of time, the deviations from it on the one side, must become just equal to the deviations from it on the other. In both, a provision is made for duration of unlimited extent, and the lapse of time has no effect to wear out or destroy a machine, constructed with so much wisdom. Where the movements are all so perfect, their beginning and end must be alike invisible.

NOTE XXI. § 122.

_Changes in the apparent
Level of the Sea._

388. In speaking of the natural epochs marked out by the phenomena of the mineral kingdom, we have supposed a greater simplicity, and separation of effects from one another, than probably takes place in nature. We have, for instance, abstracted, in speaking of the waste and degradation of the land, from that elevation which may have been carried on at the same time. This appeared necessary to be done, in order to simplify as much as possible the view that was to be given of the whole; but there can be no doubt, that, while the land has been gradually worn down by the operations on its surface, it has been raised up by the expansive forces acting from below. There is even reason to think, that the elevation has not been uniform, but has been subject to a kind of oscillation, insomuch, that the continents have both ascended and descended, or have had their level alternately raised and depressed, independently of all action at the surface, and this within a period comparatively of no great extent.

It will be easily understood, that the facts we are going to state, each taken singly, prove nothing more than a change of the line in which the surface of the sea intersects the surface of the land, leaving it uncertain to which of the two the change ought really to be ascribed. Taken in combination, however, these facts may determine what each of them separately cannot ascertain. I shall first, therefore, mention some of the principal observations relative to the change above mentioned, and shall then compare them, in order to discover whether it is most probable that this change has been produced by the motion of the land or of the sea.

389. If we begin with examining the coasts of our own island, we shall find clear evidence every where, that the sea once reached higher up upon the land than it does at present. The marks of an ancient sea beach are to be seen beyond the present limits of the tide, and beds of sea shells, not mineralized, are found in the loose earth or soil, sometimes as high as thirty feet above the present level of the sea. Some of these on the shores of the Frith of Forth are very well known, and have been often mentioned. Indeed, on the shores of that frith, many monuments appear, which would seem to carry the difference between the present and the ancient level of the sea, to more than forty feet. The ground on which the Botanic Garden of Edinburgh is situated, after a thin covering of soil is removed, consists entirely of sea sand, very regularly stratified, with layers of a black carbonaceous matter, in thin lamellæ, interposed between them. Shells I believe are but rarely found in it, but it has every other appearance of a sea beach. The height of this ground above the present level of the sea is certainly not less than forty feet.

390. On almost every part of the coast where the rocks do not rise quite abrupt and precipitous from the sea, similar marks of the lowering of the sea, or the rising of the land, may be observed. On the shores opposite to ours, the same appearances are remarked. The author of the Lettre Critique to M. de Buffon, tells us, that he had found the bottom of a bason at Dunkirk, which he had reason to think was dug about 950 years ago, ten feet and a half above the present low water mark, though it must have been originally under it. The bottom of this bason is in the native chalk. From this, the same author concludes, that the sea at Dunkirk lowers its level at the rate of an inch nearly in seven years. The observation was made in 1762, (Lettre à M. le Comte de Buffon, &c. p. 55.)[201]

[Footnote 201: In the county of Suffolk, near Wood Bridge, at the distance of seven or eight miles from the sea, are the Crag-pits, in which prodigious quantities of sea shells are discovered, many of them perfect and quite solid, (Pennant's Arctic Zoology, Introd. p. 6.) Lincolnshire affords various proofs of the same kind; but some other circumstances in the appearance of that coast, just about to be taken notice of, indicate changes of a more complicated nature.]

391. The shores of the Low Countries, and of Holland, have been often instanced in proof of the same kind of changes, and it has been supposed, that, independently of those artificial barriers which at present exclude the waters of the ocean from overflowing a great part of this tract, nature herself has brought it nearer to the surface than it had formerly been. It is indeed certain, that those countries, to a very great extent inland, have either been under the sea at some period, by no means remote if compared with the great revolutions of the globe, or that they are entirely alluvial, and of the same sort with the Deltas formed at the mouths of rivers. The relative changes, however, of the sea and land on this tract, have been differently represented, and I am unwilling, on that account, to found any argument on them.

392. If we proceed farther to the north, to the shores of the Baltic for instance, we have undoubted evidence of a change of level in the same direction as on our own shores. The level of this sea has been represented as lowering at so great a rate as 40 inches in a century. Celsius observed, that several rocks which are now above water, were not long ago sunken rocks, and dangerous to navigators; and he particularly took notice of one, which, in the year 1680, was on the surface of the water, and in the year 1791 was 20-1/2 Swedish inches above it. From an inscription near Aspô, in the lake Melar, which communicates with the Baltic, engraved, as is supposed, about five centuries ago, the level of the sea appears to have sunk in that time no less than 13 Swedish feet.[202] All these facts, with many more which it is unnecessary to enumerate, make the gradual depression, not only of the Baltic, but of the whole northern ocean, a matter of certainty.

[Footnote 202: Frisii Opera, tom. iii. p. 274.]

393. Supposing these changes of level between the sea and land to be sufficiently ascertained, the supposition which at first occurs is, that the motion has been in the sea rather than in the land, and that the former has actually descended to a lower level. The imagination naturally feels less difficulty in conceiving, that an unstable fluid like the sea, which changes its level twice every day, has undergone a permanent depression in its surface, than that the land, the _terra firma_ itself, has admitted of an equal elevation. In all this, however, we are guided much more by fancy than reason; for, in order to depress or elevate the absolute level of the sea, by a given quantity, in any one place, we must depress or elevate it by the same quantity over the whole surface of the earth; whereas no such necessity exists with respect to the elevation or depression of the land. To make the sea subside 30 feet all round the coast of Great Britain, it is necessary to displace a body of water 30 feet deep over the whole surface of the ocean. The quantity of matter to be moved in that way is incomparably greater than if the land itself were to be elevated; for though it is nearly three times less in specific gravity, it is as much greater in bulk, as the surface of the ocean is greater than that of this island.

394. Besides, the sea cannot change its level, without a proportional change in the solid bottom on which it rests. Though there be reason to suppose that such changes in the bottom do actually take place, yet they are probably much slower and more imperceptible than those which we are here considering. It is evident, therefore, that the simplest hypothesis for explaining those changes of level, is, that they proceed from the motion, upwards or downwards, of the land itself, and not from that of the sea. As no elevation or depression of the sea can take place, but over the whole, its level cannot be affected by local causes, and is probably as little subject to variation as any thing to be met with on the surface of the globe.

395. Other observations, however, made on different shores from the preceding, give greater certainty to this conclusion, and make it clear, that the motion or change which we are now treating of is not to be ascribed to the sea itself.

The observations just mentioned prove, that the level of the North Sea is lower now than it was heretofore; but it appears, that in the Mediterranean, the opposite takes place. Very accurate observations made by MANFREDI, render it certain, that the superficies of the Hadriatic was higher about the middle of the last century, than toward the beginning of the Christian era.

Some repairs that were carrying on in the cathedral church of Ravenna, in the year 1731, afforded him an opportunity of observing, that the ancient, and probably original, pavement, was four feet and a half below the present, and nearly a foot under the level of the sea at high water.[203] Now, when the church was built, this cannot have been the position of the pavement, relatively to the level of the sea, for it would have subjected the floor to be under water twice in twenty-four hours, and must have done so the more unavoidably, because at that time (the beginning of the fifth century) the walls of Ravenna were washed by the sea. The fact that this pavement is under the high-water mark, by the quantity just mentioned, was ascertained by actual levelling. This result was confirmed by similar facts, observed by ZENDRINI at Venice.

[Footnote 203: Commentarii Academiæ Bononiensis, tom. ii. pars 1ma, p. 237, &c. and pars 2da, p. 1. &c.]

396. Manfredi himself attributes all this to the elevation of the surface of the sea, and has entered into a long calculation to ascertain at what rate that surface may be supposed to rise, on account of the earth and sand brought down by the rivers, and spread out over the bottom of the sea. But as the fact of the rise of the level of the sea is not general, and as the contrary is observed in the north seas, as already proved, this hypothesis will not explain the apparent rise in the level of the Hadriatic.

397. Though a local subsidence, or settling of the ground, could hardly account for this change, the pavement being perfect in its level, and the walls of the cathedral without any shake, yet a subsidence that has extended to a great tract, as to the whole of Italy, if the mass moved has continued parallel to itself, and changed its place slowly, will agree very well with the appearances. The facts here stated are also the more deserving of attention, that about Ravenna, the land, at the same time that it has sunk in its level, has extended its surface, and has encroached on the sea. Since the time of AUGUSTUS, the line of the coast has been carried farther out by about three miles.[204] This last is the undoubted effect of the degradation of the land by the rivers; and here we have very clear evidence of the forces, both under and above the surface, producing their respective effects at the same time, so that while the surface is raised by earth brought down by the rivers, every given point in the ground is depressed and let down to a lower level.[205]

[Footnote 204: Manfredi, _ibid._]

[Footnote 205: On the coast of Dalmatia also, the rising of the level of the sea has been remarked, particularly at the ruins of Diocletian's palace of Spalatro.]

398. On the southern coast of Italy similar facts have been observed. BREISLAC, in his _Topographia Fisica della Campagnia di Roma_,[206] from certain appearances in the Gulfs of Baja and Naples, concludes, that at the beginning of the Christian era, the level of the sea was lower on that part of the coast than it is now. The facts which he mentions are the following: _1mo_, The remains of an ancient road are now to be seen in the Gulf of Baja at a considerable distance from the land. _2do_, Some ancient buildings belonging to Porto Giulio are at present covered by the sea. _3tio_, Ten columns of granite at the foot of Monte Nuovo, which appear to have belonged to the Temple of the Nymphs, are also nearly covered by the sea. _4to_, The pavement of the Temple of Serapis is now somewhat lower than the high water mark, though it cannot be supposed that this edifice when built was exposed to the inconvenience of having its floor frequently under water. _5to_, The ruins of a palace, built by Tiberius in the island of Caprea, are now entirely covered by the sea.

[Footnote 206: Cap. vi. p. 300.]

Thus, it appears that the level of the sea is sinking in the more northern latitudes, and rising in the Mediterranean, and it is evident that this cannot happen by the motion of the sea itself. The parts of the ocean all communicating with one another, cannot rise in one place and fall in another; but, in order to maintain a level surface, must rise equally or fall equally over the whole of its extent. If, therefore, we place any confidence in the preceding observations, and they are certainly liable to no objection, either from their own nature or the character of the observers, we must consider it as demonstrated, that the relative change of level has proceeded from the elevation or depression of the land itself. This agrees well with the preceding theory, which holds, that our continents are subject to be acted upon by the expansive forces of the mineral regions; that by these forces they have been actually raised up, and are sustained by them in their present situation.

399. According to some other facts stated by the same ingenious author, it appears, that on the coast of Italy the progress of the sea in ascending, or of the land in descending, has not been uniform during the period above mentioned, but that different oscillations have taken place; so that, from about the beginning of the Christian era, till some time in the middle ages, the sea rose to be sixteen feet higher than at present, from which height it has descended till it became lower than it is now, and from that state of depression it is now rising again. Breislac infers this from two facts, which he combines very ingeniously with the preceding, viz. the remains of some ancient buildings, at the foot of Monte Nuovo, five or six feet above the present level of the sea, in which are found the shells of some of those little marine animals that eat into stone: And again, the marble columns of the temple of Serapis, which are also perforated by pholades, to the height of sixteen feet above the ground. All these changes Breislac ascribes to the motion of the sea itself; a supposition which, as we have seen, cannot possibly be admitted, since nothing can permanently affect the level of the sea in one place, which does not affect it in all places whatsoever.

400. Appearances, which indicate such alternations as have just been mentioned in the level of the sea, are to be met with on some other coasts. In England, on the coast of Lincolnshire, the remains of a forest have been observed, which are now entirely covered by the sea.[207] The submarine stratum which contains the remains of this forest, can be traced into the country to a great distance, and is found throughout all the fens of Lincolnshire. The stratum itself is about four feet thick; it is covered in some places by a bed of clay sixteen feet thick, and under it for twenty feet more is a bed of soft mud, like the scourings of a ditch, mixed with shells and silt.

[Footnote 207: Phil. Trans. 1799, p. 145.]

Here then we have a stratum which must have been once uppermost on the surface of the dry land, though one part of it is now immersed under the sea, and another covered with earth, to the depth of sixteen feet. A change of level in the sea itself will not explain these appearances: they can only be explained by supposing the whole tract of land to have subsided, which is the hypothesis adopted by the author of the description in the Transactions, M. CORRIA DE SERRA; the subsidence, however, is not here understood to arise from the mere yielding of some of the strata immediately underneath, but is conceived to be a part of that geological system of alternate depression and elevation of the surface, which probably extends to the whole mineral kingdom. To reconcile all the different facts, I should be tempted to think, that the forest which once covered Lincolnshire, was immersed under the sea by the subsidence of the land to a great depth, and at a period considerably remote; that when so immersed, it was covered over with the bed of clay which now lies on it, by deposition from the sea, and the washing down of earth from the land; that it has emerged from this great depth till a part of it has become dry land; but that it is now sinking again, if the tradition of the country deserves any credit, that the part of it in the sea is deeper under water at present than it was a few years ago. This might also serve to reconcile, in some measure, the phenomena of this submarine forest with the appearances which indicate an extension of the land on the coast of Lincolnshire. Indeed the extension of the land is no direct proof, either of its own elevation, or of the depression of the sea, as we may conclude from the instance of Ravenna already mentioned.

401. We have concluded from the facts stated above, that the level of the sea rises in the Mediterranean, and sinks in the more northern latitudes; and thence some have suspected, that the level of the sea had in general a tendency to rise towards the equator, and to sink towards the poles. This is the notion of Frisi, as has been already remarked, and he suggests, that this rise of the sea may be owing to a slight acceleration in the earth's diurnal motion. But there are facts which show, that between the tropics the relative level of the sea and land has sunk, and is lower at present than it was at some former period, probably not extremely remote. The opinion of Frisi, therefore, is unsupported by observation, and, as has been already shown, cannot be justified from theory.

Between the tropics, islands are formed from the mere accumulation of coral; and it is the peculiarity of those regions, to produce rocks that have not passed through the usual process of mineral consolidation.[208] The islets, however, which are thus formed, must have their bases laid on a solid rock, though perhaps at a great depth; and it is not probable, that after they are once raised above the surface of the sea, they can still rise farther, except by some elevation of the rock which serves as their foundation.[209] Now, at Palmerston island, which comprehends nine or ten low islets, that may be reckoned the heads of a great reef of coral rock, Captain Cook informs us of his having seen, "far beyond the reach of the sea, even in the most violent storms, elevated coral rocks, which, on examination, appeared to have been perforated in the same manner that the rocks are that now compose the outer edge of the reef. This evidently shows," he adds, "that the sea had formerly reached so far; and some of these perforated rocks were almost in the centre of the island."[210]

[Footnote 208: Dr Foster, in his Voyage round the World, (vol. ii. p. 146,) gives an instance in the South Sea Islands, where the surface of the island, though entirely a coral rock, was raised forty feet above the level of the sea.]

[Footnote 209: A very curious account of the formation of such islands is given by A. Dalrymple, Esq., in the Philosophical Transitions, vol. lvii. p. 394.]

[Footnote 210: Cook's Third Voyage, vol i. p. 221.]

The same excellent navigator, giving an account of the peninsula at Cape Denbigh, remarks: "It appeared to me, that this peninsula must have been an island in remote times; for there were marks of the sea having flowed over the isthmus."

402. We are here touching on one of those subjects, where we feel much the want of accurate and ancient observations, and where it is not from the infancy, but the maturity of science that any thing approaching to certainty can be looked for. The utmost that we can expect at present, is an anticipation, which future ages must certainly modify and correct. The best thing, in the mean time, that can be done for the advancement of this branch of geological knowledge, is to ascertain with exactness the relative level of the sea, and of such points upon the land as can be distinctly marked, and pointed out to succeeding ages. This is not so easy as it may at first appear. Where every object changes, it is difficult to find a measure of change, or a fixed point from which the computation may begin. The astronomers already feel this inconvenience, and when they would refer their observations to an immoveable plane, that shall preserve its position the same in all ages, they meet with difficulties, which cannot be removed but by a profound mathematical investigation.

In geology, we cannot hope to be delivered from this embarrassment in the same manner; and we have no resource but to multiply observations of the difference of level; to make them as exact as possible, and to select points of comparison that have a chance of being long distinguished. The improvements in barometrical measurements, which give such facility to the determination of heights, along with so considerable a degree of accuracy, will furnish an accumulation of facts that must one day be of great value to the geologist.

NOTE XXII. § 123.

_Fossil Bones._

403. The remains of organized bodies, at present included in the solid parts of the globe, may be divided into three classes. The first consists of the shells, corals,-and even bodies of fish, and amphibious animals, which are now converted into stone, and make integrant parts of the solid rock. All these are parts of animals that existed _before the formation of the present land_, or even of the rocks whereof it consists. These remains have been already treated of, and the evidence which they furnish must ever be regarded as of the utmost importance in the theory of the earth. The second class consists of remains, which, by the help of stalactitical concretions, are converted into stone. These are the exuviæ of animals, which existed on the very same continents on which we now dwell, and are no doubt the most ancient among their inhabitants, of which any monument is preserved. In comparison of the first class, they must, nevertheless, be considered as of very modern origin.

404. The third class consists of the bones of animals found in the loose earth or soil; these have not acquired a stony character, and their nature appears to be but little changed, except by the progress of decomposition and of mouldering into earth. No decided line can be drawn between the antiquity of this and the preceding class, as there may be between the preceding and the first. In some instances, the objects of this third class may be coeval with those of the second; in general, they must be accounted of later origin, as they are certainly not preserved in a manner so well fitted for long continuance.

405. The animal remains of the second class, are generally found in the neighbourhood of limestone strata, and are either enveloped or penetrated by calcareous, or sometimes ferruginous matter. Of this sort are the bones found in the rock of Gibraltar, and on the coast of Dalmatia. The latter are peculiarly marked for their number, and the extent of the country over which they are scattered, leaving it doubtful whether they are the work of successive ages, or of some sudden catastrophe that has assembled in one place, and overwhelmed with immediate destruction, a vast multitude of the inhabitants of the globe. These remains are found in greatest abundance in the islands of Cherso and Osero ; end always in what the Abbé FORTIS calls an _ocreo-stalactitic earth_. The bones are often in the state of mere splinters, the broken and confused relics of various animals, concreted with fragments of marble and lime, in clefts and chasms of the strata.[211] Sometimes human bones are said to be found in these confused masses.

[Footnote 211: Travels into Dalmatia, p. 449.]

406. A very remarkable collection of bones in this state is found in the caves of Bayreuth in Franconia. Many of these belong, as is inferred with great certainty from the structure of their teeth, to a carnivorous animal of vast size, and having very little affinity to any of those that are now known. The bones are found in different states, some being without any stalactitical concretion, and having the calcareous earth still united to the phosphoric acid, so that they belong to the third, rather than the second, of the preceding divisions. In others, the phosphoric acid has wholly disappeared, and given place to the carbonic.