6i2 THE POPULAR SCIENCE MONTHLY. 



not be different from that of the present deposits of the ocean ; the 

 innumerable remains of fossilized marine animals are a still more elo- 

 quent 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 in- 

 casing parts. Having risen from very deep regions, they are desig- 

 nated 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 prop- 

 erty, 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 re(jinie of subterranean waters is exhibited in simple and clear 

 characters in the deposits known as the ancient alluviumSj^ 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 frac- 

 tion (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 Franco, nappe des puits (well-water) and nappe cC 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^^Z/rg- 

 atic. In a horizontal direction, the phreatic waters may occupy exten- 

 sive 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. Tlie great sheet of the plain of 

 Lombardy thus discharges itself into the beds of the rivers which 

 plow the land in such a Avay that, after the streams have been drained 

 by numerous irrigation-canals, they rise again spontaneously a little 



