UNDERGROUND WATERS AND MINERAL VEINS. 639 



depth, the veins are prolonged indefinitely, and mining excava- 

 tions, however deeply they may be carried, fail to reach their 

 lower limit. 



At first sight, metalliferous veins contrast in their minera,l 

 composition with the incasing rocks, to whatever category they 

 may belong, even when they are welded to them. They are 

 formed of very different minerals ; and it is necessary to distin- 

 guish in them the useful substances or minerals, and the stony 

 matter or gangue. The last very often occur in a decidedly pre- 

 dominating proportion, and much that is unforeseen in the re- 

 turns of a mining operation results from their variations in quan- 

 tity and richness. The various substances constituting veins 

 sometimes assume a symmetrical disposition in respect to the two 

 walls, showing that they result from deposits made successively 

 one upon another, as happens in a crystallizing pan or in fountain- 

 pipes that are incrusted with stony substance. 



Metalliferous veins are rarely isolated, but usually form sys- 

 tems or groups, connected by a bond of parallelism and by simi- 

 larity of composition. They occur exclusively in regions that 

 have suffered dislocations, of which they appear as if they were a 

 consequence. The constitution of the soil of France well brings 

 out this correlation. While veins are wanting in the districts 

 in which the beds have nearly preserved their original horizontal- 

 ity, they are found by thousands, although of inferior richness, in 

 the central plateau, the Vosges, the Pyrenees, and the Breton 

 peninsula. They often border upon the eruptive rocks, with 

 which they are visibly connected as if by a bond of relationship. 

 Many countries famous for their metallic riches, like Cornwall, 

 Hungary, and the State of Nevada, furnish striking examples of 

 this last alliance. 



Metalliferous veins betray their origin by their forms and the 

 independent manner in which they cut rocks of every kind. Their 

 formation is due to large vertical breaks, called faults, which have 

 given an outlet to the substances and have ultimately been filled 

 with them. The concomitance of the veins and of grand disloca- 

 tions sufficiently testifies that metallic matters and their gangues 

 have been brought from down up, that is, from the deeper regions 

 of the globe toward the surface. From this fact, it was at first in- 

 ferred that the ascension of the vein-filling minerals was accom- 

 plished by sublimation or at least by fusion, but this has been 

 shown by many circumstances to be inexact. The specimens of 

 the collections teach of themselves alone that their various min- 

 erals have been precipitated one after another, distributing them- 

 selves in an order quite different from their degree of fusibility 

 and volatility. It should also be observed that the greater part of 

 them are found outside of the veins, and in circumstances in 



