in Natural History, the several beds or layers of different matters whereof the earth is composed. See GEOLOGY.

The strata whereof the earth is composed are so very different in different countries, that it is impossible to say any thing concerning them that may be generally applicable: and indeed the depths to which we can penetrate are so small, that only a very few can be known to us at any rate; those that lie near the centre, or even a great way from it, being for ever hid. One reason why we cannot penetrate to any great depth is, that as we go down the air becomes foul, loaded with pernicious vapours, inflammable air, fixed air, &c. which destroy the miners, so that there is no possibility of going on. In many places, however, these vapours become pernicious much sooner than in others, particularly where sulphureous minerals abound, as in mines of metal, coal, &c.

But however great differences there may be among the under strata, the upper one is in some respects the same all over the globe, at least in this respect, that it is fit for the support of vegetables, which the others are not, without long exposure to the air. Properly speaking, indeed, the upper stratum of the earth all round, is composed of the pure vegetable mould, though in many places it is mixed with large quantities of other strata, as clay, sand, gravel, &c.; and hence proceed the differences of soils so well known to those who practise agriculture.

It has been supposed, by some naturalists, that the different strata of which the earth is composed were originally formed at the creation, and have continued in a manner immutable ever since: but this cannot possibly have been the case, since we find that many of the strata are strangely intermixed with each other; the bones of animals both marine and terrestrial are frequently found at great depths in the earth; beds of oyster-shells are found of immense extent in several countries; and concerning these and other shell-fish, it is remarkable, that they are generally found much farther from the surface than the bones or teeth either of marine or terrestrial animals. Neither are the shells or other remains of fish found in those countries adjoining to the seas where they grow naturally, but in the most distant regions. Mr Whitehurst, in his Inquiry into the Original State and Formation of the Earth, has given the following account of many different kinds of animals, whose shells and other remains or exuviae are found in England; though at present the living animals are not to be found except in the East and West Indies.

A CATALOGUE of EXTRANEIOUS FOSSILS, showing where they were dug up; also their native Climates. Mostly selected from the curious Cabinet of Mr NEILSON, in King-street, Red-Lion Square.

<table> <tr> <th>Chambered Nautilus</th> <td>Sheppy island; Richmond in Surrey; Sherborne in Dorsetshire.</td> <td>Chinese Ocean, and other Parts of that great sea.</td> </tr> <tr> <th>Teeth of Sharks</th> <td>Sheppy island, Oxfordshire, Middlesex, Surrey, Northamptonshire.</td> <td>East and West Indies.</td> </tr> <tr> <th>Sea-Tortoise</th> <td>Several kinds; the Hawkbill, Loggerhead, and Green species.</td> <td>West Indies.</td> </tr> <tr> <th>Mangrove Tree Oysters</th> <td>Sheppy island.</td> <td>West Indies.</td> </tr> <tr> <th>Coxcomb Tree Oysters</th> <td>Oxfordshire, Gloucestershire, Dorsetshire, and Hanover.</td> <td>Coast of Guinea.</td> </tr> <tr> <th>Vertebrae and Palates of the Orbes</th> <td>Sheppy island, and many other parts of England.</td> <td>East and West Indies.</td> </tr> <tr> <th>Crocodile</th> <td>Germany, Derbyshire, Nottinghamshire, Oxfordshire, and Yorkshire.</td> <td></td> </tr> <tr> <th>Alligator's Teeth</th> <td>Oxfordshire, Sheppy island.</td> <td>East and West Indies.</td> </tr> <tr> <th>The Banded Buccinum</th> <td>Oxfordshire, and the Alps.</td> <td>West Indies.</td> </tr> <tr> <th>The Dipping-Snail, and Star-Fish</th> <td>Sheppy island.</td> <td>West Indies.</td> </tr> <tr> <th>Tail Buccinum</th> <td>Sheppy island, Hordel Cliff, Hampshire.</td> <td>East Indies.</td> </tr> </table>

Nothing has more perplexed those who undertake to form theories of the earth than these appearances. Some have at once boldly asserted, from these and other phenomena, that the world is eternal. Others have had recourse to the universal deluge. Some, among whom is the Count de Buffon, endeavour to prove that the ocean and dry land are perpetually changing places; that for many ages the highest mountains have been covered with water, in consequence of which the marine animals just mentioned were generated in such vast quantities; that the waters will again cover these mountains, the habitable part of the earth become sea, and the sea become dry land as before, &c. Others have imagined that they might be occasioned by volcanoes, earthquakes, &c. which confound the different strata, and often intermix the productions of the sea with those of the dry land.

But for a view of the different strata so far as they are known, as well as for a view of some of the theories which have been proposed to account for the formation and changes of the earth, see GEOLOGY.

Mr Forster has given an account of some of the strata of the South-sea islands, the substance of which may be seen in the following table.

SOUTH GEORGIA.

1. No soil, except in a few crevices of the rocks. 2. Ponderous slate, with some irony particles, in horizontal strata, perpendicularly intersected with veins of quartz.

Southern Isle of New Zealand. 1. Fine light black mould, in some places nine inches deep, but generally not so much. 2. An argillaceous substance, nearly related to the clays of Talcons, turned into earth by the action of the air. 3. The same substance farther indurated, in oblique strata, generally dipping to the south.

Easter Island. 1. Reddish-brown dusty mould, looking as if it had been burnt. 2. Burnt rocks, resembling flags or dros and other volcanic matters.

Marquesas. 1. Clay mixed with mould. 2. An earthy argillaceous substance mixed with tarras and puzzolana.

Otaheite. The shores are coral rock, extending from the reef encircling these isles to the very high water-mark. There begins the sand, formed in some places from small shells and rubbed pieces of coral; but in others the shores are covered with blackish sand, consisting of the former sort mixed with black, sometimes glittering, particles of mica, and here and there some particles of the refractory iron ores called in England Skim, the ferrum micaceum of Linnaeus, and Kall, the molybdenum/pumna lupi of the same author. The plains from the shores to the foot of the hills are covered with a very fine thick stratum of black mould, mixed with the above-mentioned sand, which the natives manure with shells. The first and lower range of hills are formed of a red ochreous earth, sometimes so intensely red, that the natives use it to paint their canoes and cloth. The higher hills consist of a hard, compact, and stiff clayey substance, hardening into stone when out of the reach of the sun and air. At the top of the valleys, along the banks of the rivers, are large masses of coarse granite stones of various mixtures; in one place are pillars of a gray, solid basaltes; and, in several others, fragments of black basaltes.

Friendly Islands and New Hebrides. The same with the above.

Mallicollo. Yellowish clay mixed with common sand.

Tanna, a Volcanic Island. The chief strata here are clay mixed with aluminous earth, interperforated with lumps of pure chalk. The strata of the clay are about six inches, deviating very little from the horizontal line.

New Caledonia and the adjacent Isles. The shores consist of shell-sand, and particles of quartz; the soil in the plains a black mould mixed with this sand. The sides of the hills composed of a yellow ochreous clay, richly spangled with small particles of cat-filver, or a whitish kind of daze, the mica argentea of Linnaeus. The higher parts of the hills consist of a stone called by the German miners gegetlefein, composed of quartz and great lumps of the above cat-filver. The latter is sometimes of an intensely red or orange colour, by means of an iron ochre.

"From the above account, says Mr Forster, it appears, I think, evidently, that all the high tropical isles of the South sea have been subject to the action of volcanoes. Pyritical and sulphureous substances, together with a few iron-stones, and some vestiges of copper, are no doubt found in several of them: but the mountains of New Caledonia are the most likely to contain the richest metallic veins; and the same opinion, I suspect, may be formed of the mountains in New Zealand."

In the city of Modena in Italy, and for some miles round that place, there is the most singular arrangement of strata perhaps in the whole world. From the surface of the ground to the depth of 14 feet, they meet with nothing but the ruins of an ancient city. Being come to that depth, they find paved streets, artificers shops, floors of houses, and several pieces of inlaid work. After these ruins they find a very solid earth, which one would think had never been removed; but a little lower they find it black and marshy, and full of briars. Signor Ramazzini in one place found a heap of wheat entire at the depth of 24 feet; in another, he found filbert-trees with their nuts. At the depth of about 28 feet, they find a bed of chalk, about 11 feet deep, which cuts very easily; after this a bed of marshy earth of about two feet, mixed with rushes, leaves, and branches. After this bed comes another of chalk, nearly of the same thickness; and which ends at the depth of 42 feet. This is followed by another bed of marshy earth like the former; after which comes a new chalk-bed, but thinner, which also has a marshy bed underneath it. This ends at the depth of 63 feet; after which they find sand mingled with small gravel, and several marine shells. This stratum is usually about five feet deep, and underneath it is a vast reservoir of water. It is on account of this water that the soil is so frequently dug, and the strata so well known in this part of the world. After coming to the sandy bottom above mentioned, the workmen pierce the ground with a torebra or augre, when the water immediately springs up with great force, and fills the well to the brim. The flow is perpetual, and neither increases by rain, nor decreases by drought. Sometimes the augre meets with great trees, which give the workmen much trouble; they also sometimes see at the bottom of these wells great bones, coals, flints, and pieces of iron.

It has been asserted by some, that the specific gravity of the strata constantly increased with the depth from the surface. But Dr Leigh, in his Natural History of Lancashire, speaking of the coal-pits, denies the strata to lie according to the laws of gravitation; observing, that the strata there are first a bed of marle, then free-stone, next iron-stone, then coal, or channel mire, then some other strata, then coal again, &c. This determined Mr Derham to make a nicer inquiry into the matter: accordingly, in 1712, he caused divers places to be bored, laying the several strata by themselves; themselves; and afterwards determined very carefully their specific gravity. The result was, that in his yard the strata were gradually specifically heavier and heavier the lower and lower they went; but in another place in his fields, he could not perceive any difference in the specific gravities.

Acquainting the Royal Society therewith, their operator Mr Haukbece was ordered to try the strata of a coal pit, which he did to the depth of 30 strata: the thickness and specific gravity of each whereof he gives us in a table in the Philosophical Transactions; and from the whole makes this inference, that it evidently appears the gravities of the several strata are in no manner of order, but purely casual, as if mixed by chance.