fect of a molecular substitution, gradual and slow, which has preserved to us the most delicate organs of various plants. A liquid, such as water, has been able of itself to produce these substitutions of one body for another, by depositing the substances which it held dissolved.
Changes due in like manner to an aqueous influence have induced the formation of the rounded bodies called nodules, which have been sometimes confounded with organic productions, although they are wholly mineral. Flint, which is a variety of quartz, often appears under a tubercular form. Nodules of it are found, in a parallel alignment to the structure of the chalk, in the chalk-beds of England and France. They have been produced subsequently to the deposition of the strata, and have often imbedded fossils upon which they have molded themselves. There are also calcareous nodules that have been produced in a similar manner. The most recent quaternary deposits, like the diluvian clay, or loess, present a large number of them. The same form appears very frequently in the carbonate of iron, which is particularly abundant in the clays of the coal-beds, and is mined in several counties of Great Britain. These balls may be recognized by their metallic luster and brassy color, and their surface spiked with crystalline points. They are formed of pyrites or bisulphuret of iron, and abound in the chalk, the plastic clay, and the carboniferous rocks. When, as the result of denudations, they appear isolated on the surface of rocks of an entirely different nature, people have sometimes been led to suppose them fallen from the sky; and so they have been given, in some parts of France, the vulgar names of thunder-stones or aërolithes. The substance which has been formed into these concretionary forms appears to have been subjected to the influence of a liquid vehicle, like quarry-water, or the water which has been imbibed by rocks. The tendency of dissolved matter to agglomerate, under the influence of attraction, into a spherical shape, has been opposed by the unequal resistance of the mass from which it isolates itself. Hence, the tubercular forms.
In the case of the blackish coatings called dendrites, the forms of which bear a deceptive resemblance to those of mosses, the deposit is wholly inorganic; water, branching out by capillary attraction through extremely minute cracks, has deposited oxide of manganese in them.
The marbles called veined give evidence of another mode of action of subterranean waters. Their varied aspect is due to little veins of white crystalline carbonate of lime winding around in a mass of dark color and amorphous character, but of the same chemical composition. Fissures, intersecting in every direction, are first produced in the rock, under the influence of mechanical
