Page images

tainly admit of a plausible explanation on one or other of these three principles. In conclusion, he thus puts his brother geologists to the question :

Where a rock is stratified, is it necessarily bound by parallel surfaces ? if so, let us hear no more of mantle-shaped, saddle-shaped, shield-shaped, basin-shaped, trough-shaped stratification. — Are its surfaces necessarily parallel to those of the adjoining rock ? If so, let us hear no more of unconformable and overlying stratification. Is it sufficient that parallelism shall be found in a portion of the rock ? Let us never hear of substances being unstratified? Or must it ex. tend through the entire mass ? Let us hear no more of strata. The laminæ of flagstone, the folia of slate, are these strata ? Are masses of four hundred feet thick strata ? Is there any assignable limit to their thickness or tenuity ? - When one set of parallel planes crosses another, are both sets to be called strata, or neither, or only one of them ? And if one only, by what rule are we to be guided in distinguishing the real from the counterfeit ? – Must the beds be so arranged, as to convey to the observer the idea of deposition alternately suspended and renewed ? If this is not necessary, how is the parallelism derived from stratification, to be distinguished from paral. lelism resulting from other causes ? and of what use is it to know whether a substance is stratified or not? If it is necessary, where two observers have imbibed contrary impressions, how shall we de. termine which of the two is right ? — Let him who can answer these questions rest assured that he has a distinct idea of stratification.'

In geology, as in many other sciences, the loose use of words is the great source of perplexity. Until the precise import of the term stratification, for example, be settled and understood, the positive assertion of one observer will be met by the positive contradiction of another: the combatants will continue to waste their strength in air, and the truth will only be made more inextricable by their contention. The more general term disposition, may, perhaps, be sometimes employed with less risk of ambiguity; and the definitions which some of the French writers have given of couche, lit, banc, &c. may, probably, suggest some useful distinctions. While, on the whole, we cordially concur in the general spirit, and in the style of reasoning manifested in this important Essay, we may be permitted to express a desire, that a few of the arguments which are so formal. ly enounced, had been somewhat more fully developed.

Essay II. is on the Figure of the Earth. On the supposition that the earth's surface was originally more or less fluid, the result of rotation on its axis would be such a figure as the observations of philosophers have proved that it actually possesses; namely, a spheroid flattened at the poles. Hence a strong presumptive argument in favour of the original fluidity of its super

ficial materials an argunent which is powerfully confirmed by an examination of those materials, which bear evident marks of having once existed in a soft or fluid state, and most of them in aqueous solution or suspension. The quantity of water requisite for such a condition of things, and its subsequent disappearance, may be points of difficult explanation ; but how few of the phenomena of nature are we capable of explaining in a satisfactory manner? The Huttonians, indeed, profess not to go back to the original state of our planet, and, therefore, dispense with a former prevalence of waters; but then they are more pressed with difficulties than other theorists, when they labour to deduce the present figure of the earth from the constant tear and wear of its surface, and from the production of new lands, elevated, at indefinite periods, from the bottom of the sea-two causes which, it should seem, would balance each other, and, consequently, produce no effect.

In regard to the actual figure of the earth, or the inequalities on its surface, Mr Greenough first endeavours to show its pros ximate, and, afterwards, its more remote causes,—keeping, however, out of view the changes produced by volcanoes, coral reefs, drifting of sands, and calcareous concretions, as he perfectly acquiesces in Cuvier's account of these partial irregularities. From a very copious induction, he arrives at the general conclusion, that the interstices between mountains and hills have beery produced, for the most part, by the removal of matter which previously occupied them. Advancing a step farther, he demonstrates the inadequacy of our present seas and rivers to effect the excavation of extensive valleys, whence he is led to infer the operation of a deluge, or violent rush of waters, which has swept over every part of the globe. The consideration of these positions necessarily involves that of the agreement of strata ánd rocks, on opposite sides of valleys, rivers, and channels of the sea, as well as the transference of masses of granite to the detached and problematical spots on which they are now found : and both these topics are discussed with ability and candour. Another argument is deduced from the nature of bowlderstones, and alluvial deposites, which, every where, indicate the traces of running water, and seem to have proceeded from the breaking up of rocks at a higher level than themselves. It has likewise been observed, that the larger masses of these substances are generally found nearest to the parent rock; and that those blocks, or pebbles, which are more distant from their native place, are composed of the hardest and most indestructible materials. It is added, that • Substances which break into

cubic or hexagonal blocks, are found at a greater distance * from their native place than those which break into blocks, • the angles of which are acute.' The enumeration of granite bowlders in various quarters of the world, evinces the futility of the theory which slides them into the north of Germany on the ice.

One of the most striking of the quotations by which he endeavours to discredit the notion, of Rivers being sufficient to account for the transportation of such bodies, is the following from a late traveller in Spain, who bestowed much attention on this subject, and thinks, that rivers, flowing under ordinary circumstances, are incompetent to transport to any distance, not only colossal blocks, but moderately-sized gravel.

6“ Froin the singularity of their appearance,” he says, “ there are few pebbles which it would be so easy to recognise, as those in the bed of the Henares, near St Fernandez. If they ever moved at all, they ought, in the course of ages, to have found their way into the Tagus a little way off; but there is not one of them in the Tagus.

. At Sacedon, the Tagus is full of limestone pebbles : lower down, at Aranjuez, there are none. Nobody has ever seen granite pebbles, large or small, in the Ebro, nor blue stones veined with white; yet the Cinca, just before it joins the Ebro, abounds in them.

o White and red pebbles of quartz are found in the bed of the Noxera, which likewise falls into the Ebro; but in the Ebro is found nothing of the kind. The Guadiana in different parts of its course flows over pebbles, similar to those found in the strata of the adjacent hills; but those which occur half a league up the stream, never mix with those which occur half a league down; and at Badajos, stones of this kind, being no longer found in the cliffs, are no longer found in the river. - At the source of the Loire are pebbles innumerable ; lower down, at Nevers, only sand. - In the Yonne river, above Sens, are flints in abundance; for they abound in the banks of the Yonne, about Joigny. The Yonne falls into the Seine above Paris ; but who ever saw any of these flints at the Pont-neuf, or any pebble whatever, round or angular ?

- Near the Perte du Rhone you cross the river of the Valoisine, which is full of pebbles, because the country it flows through is full of them. At one place, this river tumbles into a kind of cả. 'vern: If pebbles were carried down by rivers, the cavern ought to contain them in abundance; it does not contain one. On my way to Geneva, I threw some stones, which I had marked so that I might know them again, into this river, just above its fall; and there I found them on my return. They had not advanced an inch during my absence. - The Rhone; Garonne, and Adour rivers, remarkable for the quantity of pebbles they run over in one part of their course, have only sand at their mouth.'”

On the subject of the larger blocks which have evidently travelled, he afterwards observes,

A late naturalist (M. Deluc), who, dying in the fulness of years, left behind him a name much too respectable to prevent his errors from being contagious, advanced a very extraordinary hypothesis, to explain the blocks so frequent on the Jura, and in Northern Germany; he supposed these blocks to have been thrown up by the expansive power of gas, generated at the time of their formation, and to have fallen where we now find them; that is, resting upon beds of limestone and sandstone, the pedestal on which they rest unshattered. How blocks of such enormous weight and magnitude, could fall upon beds so fragile, without fracturing them, it is not easy to discover ; still less, how such an event could happen before these beds were in existence; for, I suppose, no one will claim for the mountains of Jura so high an antiquity as is conceded to Mont Blanc. - It is some palliation, however, of this hypothesis, that it was constructed at a time when the imaginations of all men were so dazzled by the brilliant discoveries then making, in pneumatic chemistry, that it was almost as difficult to speculate without gas, as to breathe without air. - The circumstance of primitive blocks resting so frequently upon secondary beds, furnishes an argument equally conclusive against the opinion, that these blocks are only the survivors of a catastrophe by which the adjoining parts of the strata to which they belonged were destroyed.'

Mr Greenough meets the material objections to his doctrine with no less vigour than he states the arguments in its fayour; and, although we cannot accompany him through the details, we very earnestly recommend to the perusal of our geological readers, his excellent remarks on fossil, animal, and vegetable remains, and on the hasty and crude conceptions which have been formed of continuous ridges of mountains. The diluvian catastrophe he supposes to have taken place subsequent to the consolidation of the planets of our solar system ; but he admits that we have no positive physical evidence to de termine whether it happened before or after the creation of man. “We have only,” he says, this negative evidence, that

neither any part of a human skeleton, nor any implements of ,art, have been hitherto discovered, either in regular strata, 6 or in diluvian attritus.' Having adverted to the improbability of any adequate cause of such an eventful visitation residing within the limits of our globe, or even of our solar system, he thus concludes

'If, then, we would discover the cause of this catastrophe, we must look for a cause foreign to our globe, foreign to the solar systém, capable of inundating continents, and giving to the waters of the deep unexampled impetuosity, but without altering the interior con

stitution of the earth, or deranging the sister planets : moreover, the cause must be transitory, and one which, having acted its part once, may not have had occasion to repeat it in the long period of five thousand years. Any supposeable cause that would not fulfil these conditions, is insufficient for our purpose.

. Would a comet fulfil them? Much would depend on its bulk and distance. It would not fulfil them if we suppose a comet, large in comparison of the earth, to move in a line joining the centres of the two bodies, so as to produce a direct shock; but, if we suppose one of suitable dimensions to move in such a direction as would allow it only to graze the earth, it is not impossible that the shock of this body, a body, such as we require, out of the solar system, might produce the degree and kind of derangement which we are attempting to account for ; I mean, a great temporary derangement on the surface of the earth, unaccompanied by any material change of its planetary motion. Euler, who, in a treatise entitled De periculo a nimiâ cometæ appropinquatione metuendo,has investigated the changes that would be made in the elements of the earth's orbit by a comet, its equal in bulk, coming almost in contact with it, finds that the attraction of such a comet would indeed alter the length of our year, but only by the addition of seven hours. The maximum effect resulting from the comet's attraction at the time of its passage, would be greater than we should be led to infer from the total result of its at. traction, after its final departure ; for the changes occasioned during its approach, would be in a great measure undone during its retreat : but, even at their maximum, they would not be very great ; because, from the rapidity of the comet's motion, time would be wanting to complete them. A comet grazing the earth would be incompetent, Euler says, to produce even a deluge of our continents, unless the shortness of its stay were compensated by a magnitude of volume, exceeding that upon which he has founded his calculation.

" I shall conclude by remarking, that if the hypothesis of a shock derived from the passage either of a comet or of one of those numerous, important, and long neglected bodies, often of great magnitude and velocity, which occasion meteors, and shower down stones upon the earth, would explain the phenomena of the deluge, (a point upon which I forbear to give any opinion), we need not be deterred from embracing that hypothesis, under an apprehension that there is in it any thing extravagant or absurd. In the limited period of a few centuries, there is little probability of the interference of two bodies so small in comparison with the immensity of space; but the number of these bodies is extremely great ; and it is therefore by no means improbable, says La Place, that such interference should take place in a vast number of years.'

Essay III. On the Inequalities which existed on the Surface o the Earth previously to diluvian action, and on the Causes of these Inequalities. After admitting, that irregular crystallization, par. tial deposition, subsidence, earthquakes and volcanoes, may have

« PreviousContinue »