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pillars, in backs and cutters, in the laminæ of crystals, &c. Besides, the greater or less frequency of the recurrence of
parallel planes depends on the nature of the substances deposited,-granite, porphyry, serpentine trap, salt and chalk, presenting themselves in thick masses, argillite in flakes, and sandstone ánd oolite iu beds of moderate thickness. The larger divisious of rocks, too, are often not parallel to the laminæ of which they are composed : way-boards, or partings, seem to depend no less on the nature of the adjoining rocks, than on the circumstances which may be supposed to have attended their formation. At the junction of two kinds of rock, we often find a mutual impregnation of their respective substances; the ccntemporaneous veins of one stratum sometimes penetrate into that which is contiguous to it; and decomposition or torrefaction will frequently reveal stratification which was formerly latent. From all these circumstances we are warranted to infer, that adjoining strata may, in some instances, be contemporaneous, and that, at all events, stratification is not uniformly the effect of alternate cessations and repetitions of deposition.
Mr G. shows, in like manner, that a great diversity of opinion obtains relative to the position of rocks, and thats though vertical planes occur more frequently among those of primitive than among those of secondary character, yet every rock, in different parts of its course, exhibits both the vertical and horizontal position, as is copiously illustrated by examples.
He then confronts the arguments which have been alleged in favour of the original horizontality of strata, with those which have been urged in support of their original verticality, or, at least, of their high inclination to the horizon; stating, at the same time, with his usual candour, the difficulties which on the different hypotheses which have been advanced with a view to account for such an inclination. This abstract or summary of the conflicting arguments is drawn up with great talent and admirable brevity. The curvatures and angularities of mineral masses and strata, with the consideration of their probable causes, likewise pass under his review.
• It is supposed by Mr Playfair,' he observes, “ that the curvature is generally, if not universally, simple, like the superficies of a cylinder, not double like that of a sphere ;—this is a mistake. - As an instance of curvature extending in both directions, we may mention mantle-shaped strata. This appearance, though it has been most observed in primitive rocks, is by no means peculiar to these : in the north of England the limestone mantles round the slate ; the coal-measures of Derbyshire mantle round the limestone. - When masses or strata decline upon every side towards a certain point, they are said to be basinshaped. Such is the disposition of the mountain limestone at OrmesVOL. XXXIII. NO. 65.
head, of the coal in South Wales, of the chalk in the north of Ireland.. - The clam-shell cave at Staffa was probably so named, from the conchoidal form which it derives from curvature in the strata. It is suposed that on the great Clee hill in Shropshire, there are no less than seven distinct coal-fields ; the principal of them is covered by basalt, which varies in thickness from 60 yards to 0, though this coal field is only two miles in length, and one and a half in breadth: the strata dipping to a common centre, the thickness diminishes towards the circumerence. Another coal-field, a quarter of a mile in diameter, situate on the same hill, crops out in both directions. In all these cases, the curvature is plainly not cylindrical, but spherical.'
In the same spirit, this intrepid reasoner attacks the Huttonian notion of a horizontal elevation of the strata while in a flexible and ductile state; and observes, 1. that such a state could have no existence, there being in these substances no intermediate stage between fluidity and consolidation : 2. that the operation of the alleged cause would have given rise to other indications of disturbance, which do not actually appear : 3. that in many cases, no such cause can have operated, as the curved strata rest on horizontal ones, which betray no symptoms of curvature : 4. that even supposing its operation to have taken place, the effects ought to have been very different from actual appearances : 5. that the hypothesis does not account for curvature in horizontal strata : and, lastly, that the conformity of different strata is another circumstance fatal to this hypothesis. • There is no species of rock in which the
curves are more frequent, or more fantastical, than greywacké • slate : this rock, we know, in many instances, alternates with
conglomerate, the pebbles of which are disposed in such a manner, that it would be impossible for them to remain an instant in the place which they occupy, if the cement which connects them together were to become soft. The conglomerate, therefore, and consequently the slate which alternates 6 with it, could not have been elevated till after its consolida«
tion. If, then, as the Huttonians say, it was not consolidated 6 till after it was curved; neither was it elevated till after it was ' curved: in other words, the effect preceded the cause.
Regarding the principle of crystallization as alike inadequate to explain the phenomenon of curvature, Mr Greenough conjectures, that it may depend sometimes on the inequal effect produced by temperature on the materials of which the masses are composed, sometimes on the motions of the fluid from which they were deposited, and sometimes on the form of the bottom on which they rest; and the cases to which he alludes, cer
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 parallelism 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 determine 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 formally 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 argument 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 pro: 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 been 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 and rocks, on opposite sides of valleys, rivers, and channels of the sea, as well as the transference of masses of granite to the detached an:l 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
" there are
* 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. ““ Froin the singularity of their appearance, he
says, 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
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.
• 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 cavern: If pebbles were carried down by rivers, the cavern ought to contain them in abundance; it does not contain one. 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 my way