plumbago and of all the diamonds which have ever been derived from those and other sources, would not equal a hundredth part of the weight of coal which is daily quarried in Great Britain. Suppose now that the case had been reversed; and what would have been the consequence! diamond and plumbago, though really combustible substances, yet from their slow combustibility could never have answered, in the place of coal, as a fuel for general purposes; and, on the other hand, without that large supply of coal which nature has provided, what would have become of the domestic comforts and commercial speculations of the greater part of Europe, during the two last centuries?

The value of the diamond is not derived solely from its transparency and lustre. Its remarkable hardness is another and a most useful property belonging to it: for, in consequence of its great degree of hardness, it is capable of cutting and polishing not only the hardest glass, but even the hardest gems: and if we consider how usuful a substance glass is, how universally employed as a means of at the same time admitting light and excluding the air from the interior of our houses; but that in consequence of its hardness and brittleness it would with great difficulty be divided by any common mechanical instrument, so as accurately to fit the frames in which it is fixed for the above purposes, we at once see the value of a substance which easily and readily accomplishes that end. A small diamond no larger than a mustard seed, brought to a point and fixed in a convenient handle, enables the glazier to cut a plate of glass into pieces of any shape that he pleases: and the same instrument will serve his daily use for many successive years. Nor is it among the least of the glories of this gem, that it gave occasion to that remarkable conjecture of Sir Isaac Newton respecting its chemical nature. That philosopher having observed, that the refractive power of transparent substances is in general proportional to their density; but that, of substances of equal density, those which are combustible possess the refractive power in a higher degree than those which are not, concluded from a comparison of the density and refractive power of the diamond, that it contained an inflammable principle; which opinion was subsequently confirmed by direct experiment. It will be remembered by the chemical reader that on the same ground he made the same conjecture with respect to water, and with the same success. And never, perhaps, did the eye of philosophy penetrate more unexpectedly the thick veil which is so often found to hide the real character of various forms of matter: for, imperishable as from its name the adamant was supposed to be, who would have antecedently expected that it might be dissipated into air by the process of combustion? and, with respect to the other subject of his conjecture, if any principle was opposed to combustibility in the opinion of mankind it was water-" Aquæ contrarius ignis."

SECTION IV.

The Distribution and relative Proportions of Sea and Land; and the geological Arrangement and physical Character of some of the superficial Strata of the Earth.

As it is clearly a just object of the present treatise to select the most familiar and most obvious instances of the principle intended to be illustrated, I shall in entering upon the abstruse department of geology, consider only those phenomena which offer themselves to the eye in every part of the world; and which are either at once intelligible, or easily demonstrable, to the commonest observer.

Of such phenomena the most prominent are the general distribution of the sea and the land; and the relative proportions of their superficial extent. With reference to the sea, although we may never know all the ends which are answered by its saltness, and why its depth should be greater in some parts than others; and although we can perhaps form no more than a conjecture as to the advantages derivable from the tides; (the prevention, for instance, of a stagnant state of the water;) or from the accumulation of ice near the poles; (the cooling, probably, of the general mass of the atmosphere, and the consequent production of currents of air;) yet of its mode of distribution, and of the relative extent of its surface, we readily apprehend the reason; simply in considering that all those forms of water which contribute to the fertilization of the earth, or the support of animal life, are derived from the ocean. Were the superficial extent of this therefore much less than it is, the quantity evaporated would not be sufficient for the intended purposes; or, were the distribution different from what it is, were the sea, for instance, to occupy one hemisphere, and the land the other, the water evaporated would not be so equally diffused through the atmosphere as it is at present.

And, with respect to the land, how beautifully does the particular arrangement and character of its surface conspire with its general distribution, to equalize the diffusion of the water that is discharged upon it from the atmosphere! The truth of the proposition contained in those lines,

"Rusticus expectat dum defluat amnis, at ille
Labitur, et labetur in omne volubilis ævum,"

depends on the nature of the particular arrangement and character, to which allusion has just been made. On the one hand, the general surface of the land ascending from the sea on all sides towards some central ridge or district, called the watershed of the country, all the rain that does not sink beneath the surface is accumulated into rivers;

which naturally descend towards, and ultimately reach, the sea: and, on the other hand, the superficial strata being in general incapable of immediately absorbing the rain which falls upon them, the descent of the water is the necessary result of the inclination of the surface. But if, from partial causes, such an inclination of the land is either wanting, or the course of rivers is impeded by the unrepressed growth of reeds and sedge, the adjoining district is overflowed, and at length converted into a stagnant marsh. It is from such a physical cause, that, at this moment, the ancient site of Babylon attests the truth of prophecy; being still, as it has been for ages, "a possession for the bittern, and pools of water."

But that which is called the watershed of any large tract of land is not simply the most elevated portion of the whole surface: it consists also, in a greater or less degree, of ranges of mountains; down the high inclined sides of which the rain immediately descends in numerous torrents, which by their gradual accumulation produce rivers. And, as best calculated to secure the permanent effect, the substance of these mountains is in general so hard, and impermeable to water, that, with reference to the present system of the earth, they may justly be characterised by the epithet "everlasting." But if, instead of being thus durable, they were of a soft or friable substance, they would soon cease to exist as mountains; and if they were porous, instead of compact, they would absorb much of that rain which now contributes to the formation of rivers.

From that portion of the rain which, in comparatively flat districts, sinks beneath the surface of the earth, reservoirs of water are formed: from which, either spontaneous springs arise, or into which, artificial excavations called wells are sunk and of the utility of such reservoirs, those beds of gravel which occur in every part of the world afford upon the whole the best illustration.

SECTION V.

Beds of Gravel.

Few subjects would at the first view appear more barren of interest than a bed of gravel; consisting, as it usually does, of nothing but fragments of broken pebbles and sand, heaped together in apparently inextricable confusion. Yet such beds, dispersed as they are very generally over the surface of the regular strata, administer materially to the wants of man; in affording him the means of supplying himself readily with that important necessary of life, water.

From the irregularity in the form and size of the component parts of gravel, and from the slight degree of cohesion by which they are united, the whole mass is necessarily porous: and hence, readily transmiting the rain which falls on its surface, becomes charged

with water to an extent proportional to the quantity of rain which has penetrated it; being enabled to retain the water thus accumulated, in consequence of its resting on some substratum, as clay, which is impermeable to water: so that, if an excavation sufficiently deep be made into any part of the gravel, the water immediately drains into this excavation, and rises at length to the level of the general mass of water contained in the whole bed; by which easy process, in such instances at least, those reservoirs, called wells, are formed: and these reservoirs are never exhausted, so long as the whole bed of gravel retains any considerable proportion of water. A very ready illustration of this fact is afforded by the familiar instance of those excavations which children are accustomed to make in the sand of the sea-beach, while yet charged with moisture during the ebbing of the tide.

The inhabitants of a town which, like Oxford, is built partly on a comparatively shallow bed of gravel, and partly on a deep stratum of clay, can well appreciate the value of the former substratum of their habitations, with reference to the facility of procuring water: for while they, whose dwellings are built on the gravel, can readily obtain water by sinking a well immediately on the spot; they whose dwellings are on the clay, .must either procure water from a distance, or incur a very serious, and, finally perhaps, useless expense, in attempting to penetrate the clay. With respect to its general uses, gravel seems only to be employed in the repairing of roads and walks; in the composition of some kinds of mortar; and as a convenient occasional ballast for sailing vessels: so that, if we confine our view to the means afforded by gravel beds of supplying the ordinary wants of man, their history may be comprised in a few words. Not so, if we view them with reference to their origin, and the nature of their occasional contents: and little dreams any one, save the professed geologist, what a mine lies hid, in those confused heaps of ruin, for the exercise of man's intellectual faculties. Few subjects indeed have afforded ampler scope for philosophical reflection. In proof of which I need do no more than refer to the labours and ingenuity of Cuvier on the continent, and of Professor Buckland in our own country of whom the one, by a scientific examination of the organic remains of gravel beds, in addition to those of some of the regular strata, has brought to light not only numerous individual species, but whole families of animals, which have ceased to exist ages and ages since: and the other, with no less labour and ingenuity, has all but exhibited some of these animals to our view in the very act of devouring and digesting their food.

How often, and with what intense interest, has not the scientific

• From the observation of analogous arrangement in the general strata of the earth, namely, that those which are pervious to water alternate with those which are impervious to water, Mr. William Smith, “the father of English geology," became acquainted with the origin of springs, and the true principles of draining.

geologist perused the original essays of Cuvier; in which, setting out from the casual observation of a simple fragment of a fossil bone belonging to some extinct species, he has established not only the class and order, but even the size and proportions of the individual to which it belonged, and the general nature of its food. And how often, in addition to professed geologists, has not an attentive audience of academical students listened with admiration to the clear and vivid eloquence of the other of those philosophers, the Geological Professor of Oxford, while he unfolded that beautiful chain of facts by which he traced his antediluvian animals to their native caves; and exposed to view, to the mental eye at least, and almost to the corporeal, their particular habits, and even the relics of their last meal. And, lest there should be any doubt as to the nature of this meal, he discovered, by a most philosophical, for I will not say fortunate conjecture, unequivocal proofs of the actual remains of it; not only in its original, but also in its digested state. I here allude particularly to his verifications of the masses of digested bone which he has most satisfactorily shown to have passed through the whole tract of the digestive organs of his favourite hyenas; and which are so nearly identical, in every character, with the similar masses that daily traverse the same organs of the living species, as to make it difficult even for an experienced eye to ascertain the difference between them.

It is natural that I should feel a pleasure in recording the wellearned fame of a friend with whom I have lived in habits of intimacy for more than twenty years; and whom, in the commencement of his career, I had the good fortune to lead into that avenue of science, on which he has subsequently thrown more light than perhaps any other English geologist; with the exception indeed of one, the reve rend W. Conybeare, the admiration of whose comprehensive and commanding views, as well in fossil as in general geology, is not confined to his own countrymen; the members of the French Institute having attested their sense of his pre-eminent talents by the high honour of selecting him, a few years since, as one of their foreign associates-an honour particularly distinguished by the uncommon circumstance, that it was not only unsolicited, but unexpected, by himself.

On one point, however, of Professor Buckland's general theory of the organic remains met with in gravel beds, and in certain natural caverns, I not only differ from him, but think it right to express the ground of that difference. Dr. Buckland's arguments in favour of his opinion that the animals of the gravel beds, and the caverns, habitually frequented the spots where these remains are found, are not only ingenious, but are occasionally supported by facts which almost necessarily lead to that conclusion: and it is not intended to attempt to invalidate them. They do not indeed stand in the way of the objection now to be advanced: this objection being applicable to that part of theory only which considers the destruction of