This gravel, sand, and earth which the waters from mountains convey into plains form strata, which must not be confounded with the ancient and original strata of the globe. In the former class must be placed those of soft stone, gravel, and sand, the grains of which are washed and rounded; to these may be added, the strata of stones which are formed by a kind of incrustation; neither of which owe their origin to the motion or sediments of the sea. In these strata of soft and imperfect stones are found an infinity of vegetables, leaves, land and river shells, and small bones of terrestrial animals, but never sea shells, or other marine productions; which evidently proves, together with their want of solidity, that these strata are formed on the surface of the dry land; and that they are more modern than those of marble and other stones which contain shells, and were originally formed by the sea. All these modern stones appear to be hard and solid when they are first hewn out, but when, exposed to the weather, the air and rain presently dissolve them; their substance is so different from true stone, that when reduced into minute parts, to make sand of them, they are converted into a kind of earth or clay. Stalactites, and other stony concretions, which Tournefort took for vegetated marble, are not real stones, no more than those formed by the incrustations. We have already shewn that tufa is not of ancient formation, and must not be ranked in the class of stones. Tufa is an imperfect matter, differing from stone or earth, but which derives its origin from both by the means of rain water, as incrustations derive theirs from the deposit of the water of certain springs; therefore the strata of these matters are not ancient, nor been formed like the rest, by the sediments of the sea. The strata of turf are also modern, and have been produced by the successive assemblage of leaves and other perishable vegetables, and which are only preserved by a bitumous earth. Among these modern strata we never meet with any marine production; but, on the contrary, many vegetables, bones of land animals, and land and river shells, as may be seen in the meadows, near Ashby, in the county of Northampton, where a great number of snail shells, plants, herbs, and many river shells are found all in good preservation, some feet deep in the earth, but not a single marine shell among them.[AM] [AM] See Philosophical Trans. Abridg. XI. page 271.