STRATA, 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, and 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 mold, 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 carcase are found
in England; though at present the living animals are not to be found except in the East and West Indies.