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the strata which are lowest in the plains being highest in the mountains. The strata of these mountains are often intermixed with veins of metal, running in all possible directions, and occupying vacuities which appear to be of somewhat later date than the original formation of the mountains. The volcanic mountains intenupt those of every other description, without any regularity, as if their origin were totally independent of all the rest.
The internal constitution of the earth is little known from actual observation, for the depths to which we have penetrated are comparatively very inconsiderable, the deepest mine scarcely descending half a mile perpendicularly. It appears that the strata are more commonly in a direction nearly horizontal than in any other; and their thickness is usually almost equable tor some little distance ; but they are not disposed in the order of their specific gravity, and the opinion of their following each other in a similar series, throughout the greater part of the globe, appears to rest on very slight foundations.
From observations on the attraction cf the mountain Schehallien, Dr. Maskelyne interred the actual mean density of the earth to be to that of water as four and a half to one, judging from the probable density of the internal substance of the 'mountain, which he supposed to be a solid rock. Mr. Cavendish has concluded more directly, from experiments on a mass of lead, that the mean density of the earth is to that of water as five and a half to one. Mr. Cavendish's experiments, which were performed with the apparatus invented and procured by the late Mr. Michell, appear to have been conducted with all possible accuracy, and must undoubtedly be preferred to conclusions drawn from the attraction of a mountain, of which the internal parts are perfectly unknown to us, except by conjectures founded on its external appearance. Supposing both series of experiments and calculations free from error, it will only follow that the internal parts of Schehallien are denser, and perhaps more metallic, than was before imagined. The den«ity assigned by Mr. Cavendish is not at all greater than might be conjectured from observations on the vibration of pendulums; Newton had long ago advanced it as a probable supposition, that the mean density of the earth might be about five or six times as great as that of water, and the perfect agreement of the
result of many modern experiments with this conjecture affords us a new proof, in addition to many others, of the accuracy and penetration of that illustrious philosopher. See Globes.
GEOLOGY has for its object the structure and formation of this globe: it, of course, embraces the consideration of the materials of which it is composed, and the circumstances peculiar to its original formation, as well as the different states under which it has existed, and the various changes which it has undergone.
It necessarily follows, from the very limited depth within which our actual examinations have been made, that our facts and real observations are confined to what may be considered, comparatively, as merely the crust of the globe. With respect to its more internal part, we have hitherto only been aided by conjecture, which, it must be admitted, has too frequently led to theories the most extravagant and absurd. From the experiments of several learned men, it, however, appears, that the density of the globe is greatest towards its centre. Boscovich is of opinion, from his very ingenious calculations, that the centre is a spherical nucleus, possessing an equal degree of density to within some leagues of the earth's surface; but although it is thus concluded, that the interior of the earth is solid, contrary to the conjectures of several ancient philosophers, yet it is by no means pretended, that even in this its more solid parts, there may not exist cavities of a greater or less size, connected, perhaps, with each other, and extending considerably, in all probability, towards the surface.
The solid masses of the globe, which have come within our examination, have been distinguished into primitive and secondary; among the former, were placed the rocks of granite, gneiss, porphyry, serpentine, and limestone, of a peculiar character; and, among the latter, were considered the rocks of secondary lime stone, of phosphate of lime, of gypAum, and of some of the sandstones , of chalk, and of silex. This division is not, however, at present universally adopted; other divisions having been assumed, which have appeared to agree better with the different systems which have been proposed: these divisions we shall therefore more fully notice, after pointing out the peculiarities of these several systems. The water is supposed, at present, to cover about three-fifths of the whole earth; but undoubted evidence exists, of Its having extended over a much wider surface; and it is the opinion of many of the most eminent geologists, of its having covered the whole of the earth. As the necessity of ascertaining this latter circumstance is much urged, by those who have endeavoured to form correct opinions respecting the mode in which this globe was originally formed, it will be proper litre to notice some of the evidence .which baa been adduced respecting this circumstance.
Herodotus relates, that, according to the priests of Vulcan, the whole of Egypt, except in the neighbourhood of Thebes, had been covered with water. Herodotus himself, also, noticed the existence, even in his time, of lakes of salt water in different parts of Egypt, as well as of the saline matter, mingled with the vast tracts of sand with which that country is covered; which observations are confirmed by the accounts that have been given, by those who have examined these parts in modern times. The diminution of the ocean is also rendered iu Uie highest degree probable, from various facts related also by Strabo, Pliny, Diodorus the Sicilian, and several other early writers; and in the present day, the observations of Pallas, Celsius, Liumriis, and others, seem to establish the t'.: -t, of the diminution and sinking both of the Baltic and of the Caspian Seas.
On the other hand, innumerable facts may be adduced which seem to prove, that the water has actually increased, in its proportion, over the dry land. From the relations of Plaucus, Bryden, Barral, Fortis, and others, there can no doubt exist of the Mediterranean Sea having very much encroached on its shores; temples, and other edifices of different descriptions, which are known to have been erected at considerable distances from the sea, being now buried beneath its waves. In explanation of this varying evidence, it is necessary to state, although it may not affect the general question, that it cannot be doubted, that whilst the land is gaining on the sea, in some parts, similar encroachments arc observable in others, of the sea on the dry land. Instances of this, on the small scale, may be observed on almost all flat, and on many precipitous shores; on the former, large embankments of sand are sometimes suddenly thrown up by considerable and violent inundations, and which, in consequence of alteration in the shape of the coasts, and of the direction of currents,
may still remain, and appear to manifest an increase of the dry land: on precipitous shores, the reverse of this is observable; undermined by the continual and powerful action of the waves, large masses are perpetually falling, and, broken by their fall and by the action of the water, are so reduced as to easily allow of their removal by the waves: thus is occasioned a considerable reduction of the level of the shore; and thus an opportunity is given for the extension of the waters of the ocean on such particular spots. The balance, however, of this seemingly contradictory evidence, is undoubtedly in favour of the opinion, that the water has considerably diminished, and is, perhaps, lessening at the present period.
Indubitable evidence of the. water having stood over the tops of mountains, which are at present much above the level of the ocean, is yielded, by the circumstance of various organized beings, former inhabitants of the water, being imbedded in these mountains, and even in their summits. Those who contend that the whole of the earth has been covered with water, have recourse to the testimony afforded by the several chemical and physical properties, discoverable in the component parts of the loftiest mountains; and which prove, in their opinion, that all these substances have obtained their origin from the waters of the ocean, which they suppose to have invested the whole earth. This mode of the formation of rocks will not, however, be admitted by every geologist, to be sufficiently ascertained, to allow of its being adduced as an evidence on the present occasion. That they have been thus produced, there appears, however, to be the greatest reason for supposing; but as their origin still remains a question with many, the testimony, on this occasion, must be proportionally weakened.
In the following sketch of some of the most interesting and important systems of the formation of the world, several facts will be noticed, from which additional evidence will be adduced, of not only the formation of the rocks from the contents of the primitive waters, but also of the waters having totally covered the earth ; and since most of the important geological facts will come into consideration, whilst taking a view of the different systems which have been offered of the formation of the world, and of the several changes which it has un dergone, it is proposed to appropriate the remaining part of this article to that purpose.
Omitting to notice any farther the scriptural account of the creation of the world, merely on account of the brevity of the narration preventing the disposal of the events, there related, in a systematic arrangement; we shall only here generally remark, that the occurrence of the most prominent circumstances related in that account, has been repeatedly inferred by the most learned writers, who have endeavoured, from a view of the present state of the world, and of the various changes which it has undergone, to form some conjectures with respect to its original formation.
From the very imperfect accounts which have reached us, of the doctrines of the Egyptian philosophers on this subject, we can only learn, that they were of opinion, that at the beginning the water had covered the whole surface of the world ; and that this, was proved by the remains of organized beings, which were so frequently seen in the substance of the earth. These waters, it was supposed, had retired to the interior cavities of the globe, remaining in this great abyss, ready to issue out and produce the most extensive inundations; to one of which it was supposed that some of their records referred. The axis of the globe, they believed to have been originally parallel with that of the plane of its orbit; and whilst it remained thus, they supposed that it perpetual spring existed; but that, onits inclining, an alteration of seasons took place.
The Chaldxans, like the Egyptians, are supposed, by'Diodorus Siculus, to have believed the earth to be hollow; and that, in the early ages of its formation, a perpetual spring had existed. The Indians also believed in the existence of a vast abyss in the centre of the earth, for the reception of the water, which remained after the consolidation of the crust of the earth: they also believed in a general deluge of the earth, nut} in a subsequent retiring of the waters.
The opinions of the Epicureans, as delivered to us by Lucretius, appear to have been, that by the separation and appropriate re-union of accordant atoms, the different elements were formed, which, by the regulating influence of gravity, were separated from each other, and disposed in their allotted regions. One. of the processes which was thus performed, was the formation of the earth itself; which, being then variously acted upon, underweut those
alterations of its surface, from which proceeded the vast cavities for the reception of the ocean, and those irregularities which divide its surface into hills and vallies.
Since several of the hypotheses of the formation of the world, and the changes which have brought it to its present state, deserve rather to be regarded as ingeniously devised allegories, than systems regularly deduced, it is not intended to do much more than specify those, the consideration of which will yield but little information. In agreement with this rule, we shall only state, respecting the hypothesis of Des Cartes, that he conceived, that this globe might originally have been composed, like the sun, of the pure element (fire); but that, by degrees, its less subtle parts had gradually collected together, and formed thick and obscure masses at its surface, similar to those accumulations which occasion the spots which we see on the sun. From the gradual, but, at length, complete ■ incrustation thus formed, he supposed, that the whole planet, at length, became covered and obfuscated; that, in this manner, different crusts were formed, and that, from the falling in of parts of the exterior crust into the cavity beneath, the irregularities of the earth's surface were produced.
To this hypothesis of Des Cartes, that of Leibnitz very nearly approaches, he supposed the crust, of which we have just spoken, to have been of a vitreous nature, the minute fragments of which are the sand that is every where so abundant The affinity of nor earth to the sun, has been more strictly asserted by Button, who informs us, that the earth was originally separated from the sun, by the stroke which the sun received from the falling in of a comet, that this fragment, during its cooling, acquired, from its rotation, a spheroidal form, cavities being, at the same time, formed in its interior part, whilst its vapours condensed, and formed the waters of the ocean. Bicher entertained the opinion, that there existed in the centre of the globe a cavity, which contained an accumulation of sulphurous, bituminous, and other mineral principles, which, raised in the state of vapours, by the internal heat, formed the various mineral substances .which are contained in the substance of the earth. This hypothesis, so little supported by probability, has been nearly adopted in modem times, by Gensanne, in his "History of Languedoc ;" who imagines the existence of a central tire, by the influence of which numt rous mineral principles are raised, in a state of vapour, through the different clefts of the earth, until they arrive near to its surface, where they enter into various combinations, the result of this is the production of the numerous mineral substances which the earth contains.
Besides these, who consider an inherent or central fire as necessary to the formation and continuation of this globe, there are others who refer the particular modification of the form of its surface to the operation of subterraneous fires, acting partially by the incalesccnce of pyrites and volcanic eruptions, with accompanying earthquakes; amongst those who have adopted this opinion, may be mentioned Steno, Lazare, Moro, and Ray.
To produce the vast effects necessary to give form to a planet, or to modify its surface anew, must of course require the most powerful physical agents. In the various systems, therefore, which human ingenuity has devised, with the hope of pointing out the natural means which have been employed in these prodigious operations, the powerful agency of fire or of water has been generally referred to •, and hence geologists have been rather whimsically named, according to the particular agency which they have supported in their discussions, Plutonists, and Neptunists. The systems already here noticed, it is obvious, arc those in which fire has been adopted, as almost the sole agent; in those which next will engage our attention, recourse has been had to the combined powers of both agents.
Dr. Burnet, whose system manifests a considerable portion both of ingenuity and judgment, supposes the earth to have originally been a fluid mass, the component parts of which became arranged according to their gravity; hence the heaviest matters were deposited at the centre, and above these were disposed, in concentric layers, the substances which were less and less heavy, and on the surface was the earth,. covered all round by the water, which was itself invested by an unctuous matter, around which existed the circumambient air. By the subsequent intermixture of the oily matter and earth, and other arrangements of its several component parts, the crust of earth acquired a smooth form, and obtained those qualities which were necessary for the existence of organized beings. At this period, the axis of the globe was supposed to be parallel with that of its orbit, the days and the nights to
be equal in length, and it uniform season to have existed, resembling a perpetual spring; but on the crust of the earth drying, from the ardency of the heat, it became violently rent asunder, falling into, and giving openings for the vast abyss of waters beneath: hence the axis of the globe became inclined, occasioning those changes of the seasons, and of the length of the days and nights, which now exist; and thus also were produced the beds of the ocean, with the vallies and the numerous mountainous elevations.
Mr. Wbiston conjectured, that the earth was originally a comet, which, at the period mentioned in the Mosaic account, as that of the creation of the world, had its orbit rendered nearly circular, and such an arrangement formed of its component parts, as made it fit for the existence of the vegetable and animal creation: having existed in this state its allotted time, he supposes the comet to have passed so near to the earth as to have involved it in the vapours funning its tail, and which, being condensed, fell in torrents, and produced the deluge described by Moses; the action of the comet on the earth itself, having been sufficient to produce, at the same time, those irregularities of its surface, which form chains of mountains and the vast beds of the ocean.
Mr. Pallas having assumed the formation of the sea and the primitive rocks, supposed that, with the sand produced by their constant disintegration, the sea must have deposited such inflammable and ferruginous matters, as, being disposed in beds on the granite, would form the fuel of volcanoes; these, raising and bursting the solid beds under which they had existed, and which they must have altered by fusion or calcination, would raise up the mountains of schist and of lime-stone. The shores of the sea being gradually augmented, the sea being diminished and driven back, whilst its bed was raised in different parts by the power of volcanoes, the formation of the mountains containing petrifactions would take place. Lastly, he supposed, after the earth had been well stocked with vegetables and animals, that by some enormous eruptions at the bottom of the sea, its waters may have been made to inundate the whole horizontal surface of the earth, and even those mountains which have not exceeded one hundred toises in height.
The system of Dr. Htitton resembles, ia many points, that which has been just noticed; but its several parts are better connected, and it certainly possesses, although in its tendency it is highly exceptionable, a more prepossessing appearance, since it ascribes the formation of continents, of mountains, vallies, ixc. not to accidental occurrences, but to the operation of regular and uniform causes; making the decay of one part subservient to the restoration of another, by successive reproductions. Thus he supposes this globe to be regulated by a system of decay and renovation, and that these are effected by certain processes which bear a uniform relation to each other. The solid matter of the earth, especially of the rocks and high lands, he supposes to be perpetually separating by the reiterated action of air and water, and when thus detached, carried by the streams and rivers, and then deposited in the beds of the ocean. From these deposits, the various strata of our earth are supposed to be formed, obtaining their consolidation from the action of sub-marine tires; which being placed at immense depths, must operate on these stratified depositions under the circumstance of vast pressure; by which Volatilization must be prevented, and such changes produced as would not otherwise be effected by the power of heat. The expansive power of subterraneous fire, is called also in to explain, by the elevation of strata, their various positions. Thus, whilst the ocean is in one part removed by the accumulation, and the elevation of strata, fresh receptacles are forming for it on other spots, where new strata will be deposited, rendered solid, and elevated.
According to this system, therefore, in the present world, which is made up of the fragments of those which preceded it, the materials are arranging for the formation of its successor; the system manifesting, as its author avowed, neither vestige of a beginning nor prospect of an end.
Having thus sketched the outlines of the most interesting of the systems, which suppose the formation of this globe to have chiefly depended on the agency of fire, we shall now proceed to take a view of those in which the same effect is described, as having been produced by the influence of water.
Woodward, with too little attention to facts, well known at the period at which he wrote, supposed that the solid parts of the earth were arranged in strata, according to their degrees of specific gravity; the water which had held them in solution, having afterwards retreated to the grand abyss which he supposed to exist in the centre.
After some time, God ordained that the crust should break and fall into the abyss, and that the water should cover the surface. By the great solvent powers of this water, he supposed that every thing was again dissolved, and that afterwards they were again precipitated in concentric layers. The surface was then supposed to have been again broken, by which the waters again reached the centre, and the broken surface yielded those inequalities which now exist.
De Luc conceived, that in the beginning the sun did not exist in a luminous state, and that the earth, not feeling its influence, was frozen. but that, as the sun diffused its rays, the ice on the earth's surface became thawed, and penetrating inwards, dissolved the earth and other frozen matters to the depth of several leagues below the surface. But the thaw having reached this point, he supposes that the dissolved substances became either crystallized or precipitated, and that as they solidified they formed the primitive crust of the earth. After this, organized beings were created, many of which became involved in new strata, (the secondary) which were now formed at the bottom of the ocean; and the thawing of the internal parts of the globe continuing, cavities were formed, in consequence of the thawed substances possessing less space than they did whilst frozen. The whole of the crust, thus losing its support, sunk partially, at different periods, and the external water rushed in to fill the cavities which existed, and thus caused a considerable diminution of the waters which covered the earth; whilst, from the overturned fragments, arose the irregularities of the earth's present surface. Led by the observation that the Alpine Mountains were frequently composed of strata obliquely disposed, Saussure imagined, that the surface of the globe, formed by successive depositions and crystallizations, was originally covered by the ancient ocean; but that the crust bursting by the expansive force of heat, or of elastic fluids, the interior, or primitive parts of the crust were turned outwards, and supported by those of secondary formation. By the rapid retreat of the waters into the cavities thus formed, he accounts for the enormous' blocks, now lieing in plains far distant from the rocks from which they were separated. After this retreat of the waters, he supposes that plants and animals were 'formed; and that since that period, several