It must be remarked, that what Woodward says in this place with respect to specific gravity, must not be looked upon as a general rule, for we find lighter and heavier shells in the same matters; for example, shells of cockles, of oysters, of echini, &c. are found in the same stones and earth; and even in the royal cabinet may be seen a petrified cockle in a cornelian, and echini petrified in an agate, &c. therefore the specific weight of the shells has not influenced so much as Woodward supposes their position in the earth. The reason why such light shells are found more abundantly in chalk is, that chalk is only the ruinated part of shells, and that those of the echini being lighter and thinner than others, would have been most easily reduced into powder or chalk, so that the strata of chalk are only met with in the places where formerly a great abundance of these light shells were collected, the destruction of which formed that chalk, in which we find those shells, which having resisted the frictions, are preserved entire, or at least in parts large enough to discover their species. But this subject is treated more fully in our discourse on minerals; we shall here content ourselves with saying, that a modification must be given to Woodward's expressions: he seems to say, that shells are found in flints, cornelians, in ores, and sulphur, as often, and in as great a number as in other matters; whereas the truth is, that they are very rare in all vitrifiable or purely inflammable substances; and, on the contrary, are in prodigious abundance in chalk, marl, and marbles, insomuch that we cannot absolutely pretend to say, that the lightest and heaviest shells are found in corresponding strata, but only that in general they are oftener found so than otherwise. They are all filled with the substance which surrounds them, whether found in horizontal strata or in perpendicular fissures, because both have been formed by the waters, although at different times and in different manners. Those found in horizontal strata of stone, marble, &c. have been deposited by the motion of the waves of the sea, and those in flints, cornelians, and all matters which are in the perpendicular fissures, have been produced by the particular motion of a small quantity of water, loaded with lapidific or metallic substances. In both cases these matters were reduced into a fine and impalpable powder, which has filled the shells so fully and absolutely, as not to have left the least vacuum. There is therefore in stone, marble, &c. a great multitude of shells which are whole, beautiful, and so little changed, that they may be easily compared with the shells preserved in cabinets, or found on the sea shores.