636 THE POPULAR SCIENCE MONTHLY. 



feet of a molecular substitution^ gradual and slow, wliicli has pre- 

 served to us the most delicate organs of various plants. A liquid, 

 such as water, has been able of itself to produce these substitu- 

 tions 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, al- 

 though 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 im- 

 bedded 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 par- 

 ticularly 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 bisulphu- 

 ret of iron, and abound in the chalk, the plastic clay, and the car- 

 boniferous 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 aerolithes. 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 tend- 

 ency 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 de- 

 posit is wholly inorganic ; water, branching out by capillary at- 

 traction 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 lit- 

 tle 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 



