XII] ORIGIN OF METALLIFEROUS DEPOSITS. 233 



ore-deposits, which are naturally divided into two classes ; the 

 first including those which occur in beds, and have been 

 formed contemporaneously with the enclosing earthy sediments. 

 Such are the beds of iron-ores, which often hold embedded 

 shells and other organic remains, and the copper-bearing strata 

 already mentioned, in which the metal must have been de- 

 posited during the decay of the animal or. plant which it 

 incrusts or replaces. But there are other ore-deposits' evidently 

 of more recent formation than the rocky strata which enclose 

 them, which have resulted from a process of infiltration, filling 

 up fissures with the ore, or diffusing it irregularly through the 

 rock. It is not always easy to distinguish between the two 

 classes of deposits. Thus a fissure may in some cases be formed 

 and filled between two sundered beds, from, which may result a 

 vein that may be mistaken for an interposed stratum. Again, a 

 bed may be so porous that infiltrating waters may diffuse through 

 it a metallic ore, or a metal, in such a manner as to leave it 

 doubtful whether the process was contemporaneous with the de- 

 position of the bed, or posterior to it. But I wish to speak of 

 deposits which are evidently posterior, and occupy fissures in 

 previously formed strata, constituting true veins. Whether 

 produced by the great movements of the earth's crust, or by the 

 local contraction of the rocks (and both of these causes have in 

 different cases been in operation), such fissures sometimes extend 

 to great lengths and depths; their arrangement and dimen- 

 sions depending very much on the texture of the rocks which 

 have been subjected to fracture. When a bone in our bodies 

 is broken, nature goes to work to repair the fractured part, and 

 gradually brings to it bony matter, which fills up the little 

 interval, and at length makes the severed parts one again. 

 So when there are fractures in the earth's crust, the circulating 

 waters deposit in the openings mineral matters, which unite 

 the broken portions, and thus make whole again the shattered 

 rocks. Vein-stones are thus formed, and are the work of 

 nature's conservative surgery. 



Water, as we have seen, is a universal solvent, and the 

 matters which it may bring and deposit in the fissures of the 



