Page images
PDF
EPUB

GEO

the strata which are lowest in the plains result of many modern experiments with being highest in the mountains. The strata this conjecture affords us a new proof, in of these mountains are often intermixed addition to many others, of the accuracy with veins of metal, running in all possible and penetration of that illustrious philosodirections, and occupying vacuities which pher. See Globes. appear to be of somewhat later date than GEOLOGY has for its object the structhe original forniation of the mountains. ture and forniation of this globe : it, of The volcanic mountains interrupt those course, embraces the consideration of the of every other description, without any materials of which it is composed, and the regularity, as if their origin were totally circumstances peculiar to its original formaindependent of all the rest.

tion, as well as the different states under The internal constitution of the earth is which it has existed, and the various changes little known from actual observation, for which it has undergone. the depths to which we have penetrated It necessarily follows, from the very liare comparatively very inconsiderable, the mited depth within which our actual exadeepest mine scarcely descending half a minations have been made, that our facts mile perpendicularly. It appears that the and real observations are confined to what strata are more commonly in a direction may be considered, comparatively, as nearly horizontal than in any other; and inerely the crust of the globe. With retheir thickness is usually almost equable spect to its more internal part, we have hifor some little distance; but they are not therto only been aided by conjecture, disposed in the order of their specific gra- which, it must be admitted, has too frevity, and the opinion of their following quently led to theories the most extravaeach other in a similar series, throughout gant and absurd. From the experiments the greater part of the globe, appears to of several learned men, it, however, aprest on very slight foundations.

pears, that the density of the globe is greatFrom observations on the attraction of est towards its centre. Boscovich is of the mountain Schehallien, Dr. Maskelyne opinion, from his very ingenious calculainferred the actual mean density of the tions, that the centre is a spherical nucleus, earth to be to that of water as four and a possessing an equal degree of density to balf to one, judging from the probable den- within some leagues of the earth's surface ; sity of the internal substance of the ‘moun- but although it is thus concluded, that the tain, which he supposed to be a solid rock. interior of the earth is solid, contrary to Mr. Cavendish has concluded more direct. the conjectures of several ancient philosoly, from experiments on a mass of lead, phers, yet it is by no means pretended, that the mean density of the earth is to that even in this its more solid parts, there that of water as tive and a half to one. may not exist cavities of a greater or less Mr. Cavendish's experiments, which were size, conuected, perhaps, with each other, performed with the apparatus invented and extending considerably, in all probabiand procured by the late Mr. Michell, ap- lity, towards the surface. pear to have been conducted with all pos- The solid masses of the globe, which have sible accuracy, and must undoubtedly be come within our examination, have been preferred to conclusions drawn from the distinguished into primitive and secondary; attraction of a mountain, of which the in- among the former, were placed the rocks ternal parts are perfectly unknown to us, of granite, gneiss, porphyry, serpentine, except by conjectures founded on its ex- and limestone, of a peculiar character; and, ternal appearance. Supposing both series among the latter, were considered the of experiments and calculations free from rocks of secondary lime-stone, of phosphate error, it will only follow that the internal of lime, of gypsum, and of some of the sandparts of Schehallien are denser, and per- stones ; of chalk, and of silex. This divihaps piore metallic, than was before ima. sion is not, however, at present universally gined. The density assigned by Mr. adopted; other divisions having been asCavendish is not at all greater than might sumed, which have appeared to agree betbe conjectured from observations on the ter with the ditferent systems which have vibration of pendulums; Newton had long been proposed: these divisions we shall ago advanced it as a probable supposition, therefore more fully notice, after pointing that the mean density of the earth might out the peculiarities of these several systems. be about five or six times as great as that The water is supposed, at present, to cover of water, and the perfect agreement of the about three-fifths of the whole earth ; but undoubted evidence exists, of its having ex- may still remain, and appear to manifest an tended over a much wider surtace; and it increase of the dry land: on precipitous is the opinion of many of the most eminent shores, the reverse of this is observable; ungeologists, of its having covered the whole dermined by the continual and powerful acof the earth. As the necessity of ascer- tion of the waves, large masses are perpe-, taining this latter circumstance is much tually falling, and, broken by their fall and urged, by those who have endeavoured to by the action of the water, are so reduced form correct opinions respecting the mode as to easily allow of their removal by the in which this globe was originally formed, waves: thus is occasioned a considerable it will be proper here to notice some of the reduction of the level of the shore; and evidence which has been adduced respect- thus an opportunity is given for the extening this circumstance.

sion of the waters of the ocean on such par-, Herodotus relates, that, according to the ticular spots. The balance, however, of priests of Vulcan, the whole of Egypt, ex- this seemingly contradictory evidence, is cept in the neighibourhood of Thebes, had undoubtedly in favour of the opinion, that been covered with water. Herodotus him- the water has considerably diminished, and self, also, noticed the existence, even in his is, perhaps, lessening at the present petime, of lakes of salt water in different parts riod. of Egypt, as well as of the saline matter, Indubitable evidence of the water leaving mingled with the vast tracts of sand with stood over the tops of mountains, which are which that country is covered; which ob- at present much above the level of the servations are contirmed by the accounts ocean, is yielded, by the circumstance of that liave been given, by those who have various organized beings, former inhabitants examined these parts in modern times. of the water, being imbedded in these mounThe diminution of the ocean is also render- tains, and even in their summits. Those ed in the highest degree probable, from va- who contend that the whole of the earth Jious facts related also by Strabo, Pliny, has been covered with water, have reDiodorus the Sicilian, and several other course to the testimony afforded by the early writers; and in the present day, the several chemical and physical properties, observatious of Pallas, Celsius, Linnæus, discoverable in the component parts of and others, seem to establish the fact, of the loftiest mountains; and which prove, the diminution and sinking both of the Bal- in their opinion, that all these substances tic and of the Caspian Seas.

leave obtained their origin from the waters On the other hand, innumerable facts of the ocean, which they suppose to have may be adduced which seem to prove, that invested the whole earth. This mode of the water has actually increased, in its pro- the formation of rocks will not, however, portion, over the dry land. From the rela- be admitted by every geologist, to be suffitions of Plaucus, Bryden, Barral, Fortis, ciently ascertained, to allow of its being ad. and others, there can no doubt exist of the duced as an evidence on the present occaMediterranean Sea having very much en- sion. That they have been thus produced, croached on its shores ; temples, and other there appears, however, to be the greatest edifices of different descriptions, which are reason for supposing; but as their origin known to have been erected at consider still remains a question with many, the tesable distances from the sea, being now bg- timony, on this occasion, must be proporried beneath its waves. In explanation of tionally weakened. this varying evidence, it is necessary to In the following sketch of some of the state, although it may not affect the general most interesting and important systems of question, that it cannot be doubted, that the formation of the world, several facts whilst the land is gaining on the sea, in will be noticed, from which additional evisome parts, similar encroachments are ob- dence will be adduced, of not only the forservable in others, of the sea on the dry mation of the rocks from the contents of land. Instances of this, on the small scale, the primitive waters, but also of the waters may be observed on almost all flat, and on having totally covered the earth; and since many precipitous shores; on the former, most of the important geological facts will large embankments of sand are sometimes come into consideration, whilst taking a suddenly thrown up by considerable and view of the different systems which have violent inundations, and which, in conse- been offered of the formation of the world, quence of alteration in the shape of the and of the several changes which it has un coasts, and of the direction of currents, dergone, it is proposed to appropriate the

remaining part of this article to that pur- alterations of its surface, from which propose.

ceeded the vast cavities for the reception Omitting to notice any further the scrip- of the ocean, and those irregularities which tural account of the creation of the world, divide its surface into bills and vallies. merely on account of the brevity of the Since several of the hypotheses of the narration preventing the disposal of the formation of the world, and the changes events, there related, in a systematic ar- which have brought it to its present state, rangement; we shall only here generally deserve rather to be regarded as ingeremark, that the occurrence of the most niously devised allegories, than systems reprominent circumstances related in that ac- gularly deduced, it is not intended to do count, has been repeatedly inferred by the much more than specify those, the consimost learned writers, who have endeavour. deration of wbich will yield but little infor. ed, from a view of the present state of the mation. In agreement with this rule, we world, and of the various changes which it shall only state, respecting the hypothesis has undergone, to form some conjectures of Des Cartes, that he conceived, that this with respect to its original formation. globe might originally have been composed,

From the very imperfect accounts which like the sun, of the pure element (fire); but have reached us, of the doctrines of the that, by degrees, its less subtle parts had Egyptian philosophers on this subject, we gradnally collected together, and formed can only learn, that they were of opinion, thick and obscure masses at its surface, sithat at the beginning the water had cover- milar to those accumulations which occaed the whole surface of the world; and that sion the spots which we see on the sun. this was proved by the remains of organ- From the gradual, but, at length, complete · ized beings, which were so frequently seen incrustation thus formed, he supposed, that in the substance of the earth. These wa- the whole planet, at length, became coverters, it was supposed, had retired to the in- ed and obfuscated ; that, in this manner, terior cavities of the globe, remaining in different crusts were formed, and that, from this great abyss, ready to issue out and pro- the falling in of parts of the exterior crust duce the most extensive inundations; to into the cavity beneath, the irregularities one of which it was supposed that some of of the eartli's surface were produced. their records referred. The axis of the To tliis liypothesis of Des Cartes, that of globe, they believed to have been originally Leibnitz very nearly approaches, he supparallel with that of the plane of its orbit; posed the crust, of which we have just and whilst it remained thus, they supposed spoken, to have been of a vitreous nature, that a perpetual spring existed; but that, the minute fragments of which are the sand on its inclining, an alteration of seasons that is every where so abundant. The took place.

affinity of our earth to the sun, has been The Chaldæans, like the Egyptians, are

more strictly asserted by Buffon, who insupposed, by Diodorus Siculus, to have be- forms us, that the earth was originally sepalieved the earth to be hollow; and that, in rated from the sun, by the stroke which the the early ages of its formation, a perpetual sun received from the falling in of a comet, spring had existed. The Indians also be- that tbis fragment, during its cooling, aclieved in the existence of a vast abyss in quired, from its rotation, a spheroidal form, the centre of the earth, for the reception of cavities being, at the same time, formed in the water, which remained after the conso- its interior part, whilst its vapours conJidation of the crust of the earth : they also densed, and formed the waters of the ocean. believed in a general deluge of the earth, Bicher entertained the opinion, that there and in a subsequent retiring of the waters, existed in the centre of the globe a cavity,

The opinions of the Epicureans, as deli- which contained an accumulation of sulvered to us by Lucretins, appear to have phurous, bituminous, and other mineral been, that by the separatioo and appro. principles, which, raised in the state of va. priate re-union of accordant atoms, the dif- pours, by the internal heat, formed the vaferent elements were formed, which, by rious mineral substances which are containthe regulating influence of gravity, were se- ed in the substance of the earth. This hy. parated from each other, and disposed in pothesis, so little supported by probability, their allotted regions. One of the pro- has been nearly adopted in modern times, cesses which was thus performed, was the by Gensanne, in his “ History of Langueformation of the earth itself'; which, being doc;" who imagines the existence of a cen. then variously acted upon, underweut those tral fire, by the infinence of which numerous mineral principles are raised, in a state be equal in length, and a uniform season to of vapour, through the different clefts of have existed, resembling a perpetual spring; the earth, until they arrive near to its sur- but on the crust of the earth drying, from face, where they enter into various combi, the ardency of the heat, it became violently nations, the result of this is the produc- rent asunder, falling into, and giving opention of the numerous mineral substances ings for the vast abyss of waters beneath: which the earth contains.

hence the axis of the globe became inBesides, these, who consider an inherent clined, occasioning those changes of the sea. or central fire as necessary to the formation sons, and of the length of the days and nights, and continuation of this globe, there are which now exist; and thus also were proothers who refer the particular modifica duced the beds of the ocean, with the vallies tion of the form of its surface to the opera- and the numerous mountainous elevations. tion of subterraneous fires, acting partially Mr. Whiston conjectured, that the earth by the incalescence of pyrites and volcanic was originally a comet, which, at the period eruptions, with accompanying earthquakes; mentioned in the Mosaic account, as that amongst those who have adopted this opi- of the creation of the world, had its orbit nion, may be mentioned Steno, Lazare, rendered nearly circular, and such an ar. Moro, and Ray.

rangement formed of its component parts, To produce the vast effects necessary to as made it fit for the existence of the vegegive form to a planet, or to modify its sur- table and animal creation : having existed face anew, must of course require the most in this state its allotted time, he supposes a powerful physical agents. In the various comet to have passed so near to the earth systems, therefore, which human ingenuity as to have involved it in the vapours formhas devised, with the hope of pointing out ing its tail, and which, being condensed, the natural means which have been employ- fell in torrents, and produced the deluge ed in these prodigions operations, the pow. described by Moses; the action of the coerful agency of fire or of water has been gene. met on the earth itself, having been suffirally referred to ; and hence geologists have cient to produce, at the same time, those been rather whimsically named, according irregularities of its surface, which form to the particular agency which they have chains of mountains and the vast beds of supported in their discussions, Plutonists, the ocean. and Neptunists. The systems already here Mr. Pallas baving assumed the formation noticed, it is obvious, are those in which of the sea and the primitive rocks, supfire has been adopted, as almost the sole posed that, with the sand produced by agent; in those which next will engage our their constant disintegration, the sea must attention, recourse has been had to the have deposited such inflammable and fercombined powers of both agents.

ruginous matters, as, being disposed in Dr. Burnet, whose system manifests a beds on the granite, would form the fuel of considerable portion both of ingenuity and volcanoes; these, raising and bursting the judgment, supposes the earth to have ori- solid beds under which they had existed, ginally been a Auid mass, the component and which they must have altered by fusion parts of which became arranged according or calcination, would raise up the mountains to their gravity; hence the heaviest mat. of schist and of lime-stone. The shores of ters were deposited at the centre, and the sea being gradually augmented, the sea above these were disposed, in concentric being diminished and driven back, whilst layers, the substances which were less and its bed was raised in different parts by the less heavy, and on the surface was the power of volcanoes, the formation of the earth, covered all round by the water, mountains containing petrifactions would which was itself invested by an unctuous take place. Lastly, he supposed, after the matter, around which existed the circum- earth had been well stocked with vegetaambient air. By the subsequent intermix- bles and animals, that by some enormous ture of the oily matter and earth, and other eruptions at the bottom of the sea, its waarrangements of its several component ters may have been made to inundate the parts, the crust of earth acquired a smooth whole horizontal surface of the earth, and form, and obtained those qualities which even those mountains which have not exwere necessary for the existence of orga- ceeded one hundred toises in height. nized beings. At this period, the axis of The system of Dr. Hutton resembles, in the globe was supposed to be parallel with many points, that which has been just nothat of its orbit, the days and the nights to ticed; but its several parts are better connected, and it certainly possesses, although After some time, God ordained that the in its tendency it is highly exceptionable, crust should break and fall into the abyss, a more prepossessing appearance, since it and that the water should cover the surascribes the formation of continents, of face. By the great solvent powers of this mountains, vallies, &c. not to accidental water, he supposed that every thing was occurrences, but to the operation of regular again dissolved, and that afterwards they and uniform causes; making the decay of were again precipitated in conceutric layers. one part subservient to the restoration of The surface was then supposed to have another, by successive reproductions. Thus been again broken, by which the waters he supposes this globe to be regulated by again reached the centre, and the broken a system of decay and renovation, and that surface yielded those inequalities which now these are effected by certain processes exist. which bear a uniform relation to each other. De Luc conceived, that in the beginning The solid matter of the earth, especially of the sun did not exist in a luminous state, the rocks and high lands, he supposes to be and that the earth, not feeling its influence, perpetually separating by the reiterated ac- was frozen ; but that, as the sun diffused tion of air and water, and when thus de- its rays, the ice on the earth's surface betached, carried by the streams and rivers, came thawed, and penetrating inwards, disand then deposited in the beds of the solved the earth and other frozen matters ocean. From these deposits, the various to the depth of several leagues below the strata of our earth are supposed to be surface. But the thaw having reached this formed, obtaining their consolidation from point, he supposes that the dissolved subthe action of sub-marine fires ; which being stances became either crystallized or preciplaced at immense depths, must operate on pitated, and that as they solidified they these stratified depositions under the cir- formed the primitive crust of the earth. cumstance of vast pressure ; by which vola. After this, organized beings were created, tilization must be prevented, and such many of which became involved in new changes produced as would not otherwise strata, (the secondary) which were now be effected by the power of heat. The ex- formed at the bottom of the ocean; and pansive power of subterraneous fire, is the thawing of the internal parts of the called also in to explain, by the elevation globe continuing, cavities were formed, in of strata, their various positions. Thus, consequence of the thawed substances poswhilst the ocean is in one part removed by sessing less space than they did whilst frothe accumulation, and the elevation of zen. The whole of the crust, thus losing strata, fresh receptacles are forming for it its support, sunk partially, at different peon other spots, where new strata will be riods, and the external water rushed in to deposited, rendered solid, and elevated, fill the cavities wbich existed, and thus

According to this system, therefore, in caused a considerable diminution of the the present world, which is made up of the waters which covered the earth; whilst, fragments of those which preceded it, the from the overturned fragments, arose the materials are arranging for the formation irregularities of the earth's present surface. of its successor; the system manifesting, as Led by the observation that the Alpine its author avowed, neither vestige of a be- Mountains were frequently composed of ginning nor prospect of an end.

strata obliquely disposed, Saussure imaHaving thus sketched the outlines of the gined, that the surface of the globe, formed most interesting of the systems, which sup- by successive depositions and crystallizapose the formation of this globe . to have tions, was originally covered by the ancient chiefly depended on the agency of fire, we ocean ; but that the crust bursting by the shall now proceed to take a view of those in expansive force of heat, or of elastic fluids, which the same effect is described, as having the interior, or primitive parts of the crust been produced by the influence of water. were turned outwards, and supported by

Woodward, with too little attention to those of secondary formation. By the rafacts, well known at the period at which pid retreat of the waters into the cavities he wrote, supposed that the solid parts of thus formed, be accouets for the enormous the earth were arranged in strata, accord- blocks, now lieing in plains far distant ing to their degrees of specific gravity; the from the rocks from which they were sepawater which had held them in solution, hav- mated. After this retreat of the waters, he ing afterwards retreated to the grand abyss supposes that plants and animals were which he supposed to exist in the centre. 'formed; and that since that period, several

« PreviousContinue »