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 favour; 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 determine 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, 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 system, 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 nimia cometa 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 attraction, 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, partial deposition, subsidence, earthquakes and volcanoes, may have

had considerable influence in producing some of the inequalities on the earth's surface, the author is still disposed to attribute by far the greater number of them to the action of running water. The general occurrence of conglomerate and greywacké on the confines of primitive rocks, seems to indicate a deluge similar in kind, though, perhaps, not equal in extent, to that which determined the present outline of the earth. These considerations, which are despatched with much brevity, might, perhaps, with more propriety, have been included in the preceding Essay.

ESSAYS IV. & V. On Formations.-On the Order of Succession in Rocks. In opposition to the popular Wernerian notions of formations, or series of rocks of alleged contemporaneous origin, the author contends, that neither the intermixture of their ingredients, nor their alternations of occurrence, sufficiently justifies the inference of the simultaneous production of mineral substances; for, rocks generally held to be of very different ages, often present intermixtures of their component parts, or pass into one another, while such a mutual blending is frequently not discoverable in others that are reputed to be of the same age. Yet, when two substances are distinctly incorporated in the same mass, it is difficult to conceive of them as generated at different epochs. Examples are also cited of alternating substances which are not regarded as coeval, while those which are deemed coeval, do not always alternate. So many exceptions to the principle of universal and partial formations, are, moreover, adduced, and so many formidable difficulties stated against its probability, that it ought, in fairness, to be abandoned.

Unable to connect similar rocks of distant countries, obliged to connect dissimilar ones in the same neighbourhood, can any one uphold the doctrine of Universal Formations? Let him, who answers in the affirmative, reflect on the consequences which that doctrine involves. He must admit, that, when the particles of quartz, feldspar, and mica, which had heretofore arranged themselves so as to form granite, changed their mode of arrangement so as to form gneiss, that change was conveyed with the rapidity of an electric shock from one end of the world to the other; that the currents of different hemispheres had so equable a motion; that the particles borne along by these currents were so equally assorted; that, within the tropics, and without, the same depositions began and ceased at the same moment; that similar pebbles were detached from their native rocks, at the poles and at the equator, by equal forces acting under the same circumstances; and were deposited and cemented by the same means, and at the same time. All this he must admit, or reject in toto the doctrine of Universal Formations.'

With regard to the Order of Succession in rocks, too, the facts which the author brings forth from his ample stores, are

calculated to shake our faith in the commonly received notions of the Wernerian school. Even the precedence of genealogy assigned to granite has been successfully controverted; for this rock has been found to alternate with gneiss, with mica-slate, and with schistus; nay, killas has been observed passing into it, and dipping beneath it. In some cases, it rests on quartz, on hornstone, on slate; and, in France, not unfrequently, on lime. stone. Again, the term fundamental has, it should seem, been gratuitously predicated of a particular description of granite; for, by the terms of the proposition, the bottom of this formation has never been seen, and consequently we have no means of ascertaining whether it be fundamental or not.' The tables of sections in Ebel's work may suffice to convince us, that equal uncertainty prevails with respect to the relative position of other rocks reputed primitive. Besides, in almost every country, we find what are termed transition rocks in the midst of primitive districts, or vice versa; while the line of demarcation between even the primary and secondary classes, is far less distinct than has been generally supposed.

It is said in the Wernerian theory, that, after the formation of all other strata, an immense deluge suddenly occurred, and as suddenly retired, leaving, behind it, those scattered hummocks of flötztrap, which have, for some years, so greatly engaged the attention of geologists. The proofs of this catastrophe, we are informed, are to be found in the great elevation which these rocks occasionally attain; in their broken stratification; in their unconformable posture; and in the nature of their materials.

But are trap-rocks really more elevated than others? or their stratification more broken? It is time enough to consider inferences when we have established facts. If the posture of trap is often unconformable, so is that of granite, sienite, hornblend rock, porphyry, primitive greenstone, &c.

Every rock without exception lies, sometimes, in a conformable, sometimes in an unconformable posture: and perhaps the different members of the flötz-trap formation, as often exhibit a want of conformity towards each other, as, towards the beds on which they repose. As to the nature of its materials-many of them are precisely the same as those found in other formations. The only rocks which are cited as peculiar to, and characteristic of, the newest flötz-trap, are basalt, wacké, greystone, porphyry-slate, and trap-tuff. I am not sure that I know what greystone is; the only locality given of it by Jameson, is Vesuvius, where it is said to form a portion of the unchanged rocks. The doctrine, that it belongs to the flötz-trap, therefore, is founded on an assumption, that we have the means of distinguishing, in volcanic countries, substances which have been changed by the volcano from those which have not-an assumption somewhat gratuitous. The remaining substances, viz. basalt, wacké,

porphyry-slate, and trap-tuff, are certainly not peculiar to this formation; as in England, Scotland, and Ireland, they are often found interstratified with other formations much older. There is reason to suspect that, in Germany, trap-rocks of very different eras have been referred to the same era, and that much of that which has been supposed the newest flötz-trap in Scotland, and which ought, therefore, to be more modern than the beds of the basin of Paris, is coeval with red sandstone, mountain-limestone, and coal.'

ESSAY VI. & VII. On the Properties of Rocks, as connected with their respective Ages. On the History of Strata, as deduced from their Fossil Contents.-The properties of rocks which are here considered, are their ingredients, structure, specific gravity, consolidation, stratification, posture with regard to the horizon, relative posture to one another, dip and direction, altitude, contained metals, and fossils. On each of these heads the author offers some pertinent remarks; but which our limits will not not permit us to particularize. It is of importance, however, to notice, that the supposed relation between the age of a rock and the fossils which it contains, is often fallacious; and that the various facts which have now been collected concerning the interesting phenomena of organic relics, demonstrate the inaccuracy of some of the opinions which have been adopted by geologists of the first reputation.

ESSAY VIII. On Mineral Veins.-According to our author's views, fissures have been produced principally by shrinkage; but others may have been caused, or enlarged, by the contraction of an adjoining mass, by the shock of an earthquake, or by failure of support, the erosion of subterranean waters occasioning subsidence. These fissures, or chasms, when filled with mineral matter, are called veins. Mr Greenough makes some excellent observations on their varieties, anomalies, and probable indications, which cannot fail to interest both the speculative geologist and the practical miner: but, while he rejects both the Huttonian and Wernerian hypotheses, relative to their formation, he sheds little original light on this obscure subject.

On the whole, however, he possesses the rare merit of stating his facts and opinions in a clear and manly, yet modest and respectful manner, untrammelled by preconceived systems, and unseduced by the fascination of great names. Truth, and truth alone, appears to have been the object of his extensive travels, of years of unwearied study, and of the devotion of an ample fortune to the prosecution of his favourite investigations. Nor will such praiseworthy efforts be without their reward, since they must evidently tend to assuage the angry contentions of conflicting geologists, and to demonstrate the superior va