_Chemical Work._ The results of the chemical and chemico-physical action of water may be grouped in several more or less distinct categories. 1. The simplest effect is the _subtraction_ of soluble mineral matter. Pure water is in itself a solvent of certain minerals; but the carbonic-acid gas extracted from the atmosphere, and the products of organic decay extracted from the soil make ground-water a much more efficient solvent. Something of the results which it achieves is shown by its composition. All ground-water, whether issuing as springs or drawn out through wells, contains much more mineral matter than the water which falls as rain, and the excess is acquired in its underground course. The subtraction of soluble matter from rock renders it porous. The amount of material dissolved from a given place may be trivial or considerable, according to the character of the rock, the readiness with which water has access to it, and the character of the water. Locally, the subtraction of mineral matter may be the chief, or even the only appreciable, effect of the ground-water. 2. It sometimes happens that ground-water with certain mineral substances in solution exchanges them for other substances extracted from the rock. Thus the process of _substitution_ is effected. By this process the lime carbonate of a shell imbedded in rock may be removed, molecule by molecule, and some other substance, such as silica, left in its place. When the process is complete, the substance of the shell has been completely removed, though its form and structure are still preserved in the new material which has taken the place of the old. Buried logs are sometimes converted into stone by the substitution of mineral matter for the vegetable tissue. This is _petrification_. Petrification is altogether distinct from _incrustation_, which simply means the coating of an object with mineral matter. A bird’s nest may be incrusted with lime carbonate, but it is not thereby petrified. Solution is a necessary antecedent of substitution. 3. The materials which are subtracted from the rock at one point may be added to other rock elsewhere. Thus a third type of change, _addition_, is effected. Rock may at one time and place be rendered porous by the subtraction of some of its substance, and the openings thus formed may subsequently become the receptacles of deposits from solution. This is exemplified in the stalactitic deposits of many caves. Not uncommonly cracks and fissures are filled with mineral matter deposited by the waters which pass through them. Thus arise _veins_ which, for the most part, are nothing more than cracks and crevices filled by mineral matter brought to them in solution, and precipitated on their walls. Most veins of metallic ores have originated in this way.