Page:Popular Science Monthly Volume 32.djvu/632

612 not be different from that of the present deposits of the ocean; the innumerable remains of fossilized marine animals are a still more eloquent testimony to it; and the disposition in beds completes the analogy with contemporaneous sediments. All of these formations may be traversed by mineral masses, disposed in more or less vertical irregular veins, which are usually contrasted in character to the incasing parts. Having risen from very deep regions, they are designated as eruptive rocks.

Some among these various materials are impervious to the passage of water. One of the most so among them is clay, a very abundant hydrated silicate of alumina, which, mixed with carbonate of lime, is also abundant as marl. Granite, and similar rocks, such as the schists, of which slate represents a well-known variety, have the same property, provided the fissures that traverse them are not too open. Thus, although the incessant invasion of water constitutes one of the chief obstacles to the miner's work, there are exploitations that keep quite dry in consequence of the impermeability of the incasing masses.

Other materials are easily permeable by water, as we may observe every day in sand and gravel. The same is the case with rocks which, not being themselves porous, are cut and cross-cut by crevices. Many compact limestones give instantaneous passage to water, which is drained away by their crevices as it would be by artificial conduits.

The regime of subterranean waters is exhibited in simple and clear characters in the deposits known as the ancient alluviums, the drift, and the quaternary deposits, which cover most of the continents as with a carpet. Their gravels and sands, usually associated with clays, greedily absorb water into interstices which represent a notable fraction (perhaps a third) of their total volume. Arrested in its descent by impermeable masses, it accumulates and forms a sheet or shallow body, from which it may be seen to exude through all the openings that may be made into it. This sheet has received several common names: as in France, nappe des puits (well-water) and nappe d'infiltration (infiltration-water); in Germany, Grundwasser; in England, ground-water; and in Italy, acqua di suolo, acqua di livello. A Greek term, which is cosmopolitan, is preferable, and it is found in the word phreatic. In a horizontal direction, the phreatic waters may occupy extensive surfaces, even whole countries, like the arenaceous deposits that serve as their receptacle.

An artificial excavation is not always necessary to make manifest the existence of ground-water. It appears in natural hollows of the soil, takes advantage of ravines of slight depth to issue in springs, which are sometimes impetuous and voluminous enough to constitute considerable streams at their source. The great sheet of the plain of Lombardy thus discharges itself into the beds of the rivers which plow the land in such a way that, after the streams have been drained by numerous irrigation-canals, they rise again spontaneously a little