Page:Popular Science Monthly Volume 33.djvu/653

Rh mineral is subject to a special influence from its particular planet." Thus, in a little work of only a few pages, and which was simply intended to give the most necessary knowledge to the practical miner, was placed for the same consideration of utility as determines the employment of the compass, the notion of this pretended affinity between the metals and the planets.

The mode of formation, or, as Buff on said, the genesis of minerals is one of the most interesting questions of their history. But the problem could not be approached until geologists had furnished precise data on the conditions of their bearing. Satisfactory solutions have recently been obtained in the case of a certain number of mineral species. Synthetic experiment, placing itself in the circumstances that seem to have presided at their formation, has succeeded in reproducing them, with their crystalline forms, and all their essential characteristics, and has thus completed the demonstration of their origin. By means of this method of demonstration, we have been able to ascertain that many minerals are due to the action of subterranean waters. From the most ancient epochs, these waters have circulated through the crust of the earth, where they have left, at a multitude of points, signs revealing the part they have played, and the course they have taken, even more clearly than contemporary phenomena •have done.

The sedimentary beds, formed like the deposits which the sea spreads every day in the bottom of its basin, are often distinguishable from one another, even at first sight, by certain exterior characters. The differences are, for the most part, produced by the action of subterranean waters, as is demonstrated by the animal and vegetable fossils, which were for a long time designated as petrifactions, or, rather, by the chemical changes which these fossilized bodies have evidently undergone.

Here, shells and corals, showing forms perfectly preserved down to their slightest details, are no longer composed of carbonate of lime, as they certainly were during the life of the animal to which they belonged, but are essentially different substances, quartz having entirely taken the place of the calcium carbonate. There are also other minerals, such as pyrites and sulphate of baryta, which have penetrated and crystallized within the cavities which the bodies of these invertebrates occupied.

The silicified woods, which are very frequently met, assert still more clearly the intervention of a liquid. Not only can the least trained eye recognize their external shape, but the ligneous texture also is still maintained, even to the cells and other inmost parts, as distinctly as in the living wood. It is not, then, a simple molding of silica, performed in the vacant spots that have been left by the disappearance of the vegetable substance, but the