STRATA, in natural history, the seve ral beds or layers of different matters whereof the earth is composed. The strata, whereof the earth is composed, are so very (Were nt in different countries, that it is impossible to say any thing concern ing them, that may be generally applica ble: 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 firom it, being for ever hid. One rea son why we cannot penetrate to any great depth is, that as we go down, the air be comes foul, loaded with pernicious va pours, inflammable air, fixed air, Scc.w hie..h destroy the miners, and there is no possi bility of going on. In many places, how ever, these vapours become pernicious much sooner than in others, particularly where sulphureous minerals abound, as in mines of metal, coal, &c. But, how ever 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 dif ferent strata of which the earth is compos ed were originally formed at the creation, and have continued in a manner immuta ble ever since : but this cannot possibly have been the case, since we find that many of the strata are strangely intermix ed with each other ; the bones of animals, both marine and terrestrial, are frequent ly found at great depths in the earth ; beds of oyster shells are found of im mense extent in several countries; and concerning these and other shellfish, 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, hut in the most distant regions. Mr. Whitelturut, in his Inquiry into the Ori ginal State and Formation of the Earth, has given an account of many different kinds of animals, whose shells and other remains, or exuvim, are found in England, though at present the living animals are not to be found, except in the East or West Indies. Nothing has more perplex ed 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 re course to the universal deluge. Some, among whom is the Count de Buffon, en deavour to prove that the ocean and dry land are perpetually changing places ; that for many ages the highest moun tains have been covered with water, in consequence of which the marine am mals just mentioned were generated in such vast quantities ; that the waters will again cover these mountains, the habit able part of the earth become sea, and the sea become dry land, as before, 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 hind.