196 THE PROBLEM OP THE METALLIFEROUS VEINS. 



and different one, and so on until the crack has become filled. Often 

 cavities are left at the center or sides and are lined with beautiful 

 and shining crystals, which flash and sparkle in the rays of a lamp 

 like so many gems. There are quartz veins in California which are 

 mined for gold and which seem to have filled clean-cut crevices, wall 

 to wall, for several feet across. More often there is evidence of 

 decided chemical action upon the walls, which may be impregnated 

 with the ore and gangue for some distance away from the fissure. 

 As the source of supply is left, however, the impregnation becomes 

 less and less rich and finally fades out into barren wall rock. The 

 enrichment of the walls varies also from point to point, since where 

 the rock is tight the solutions can not spread laterally, but where 

 it is open the impregnation may be extensive. The miner has, there- 

 fore, to allow for swells and pinches in his ore. 



Of even greater significance than the lateral enrichment is the 

 peculiar arrangement of the valuable ore in a vein that may itself 

 be continuous for long distances, although in most places too bar- 

 ren for mining. Cases are, indeed, known in which profitable vein 

 matter has been taken out continuously for perhaps a mile along the 

 strike, but they are relatively rare. The usual experience reveals 

 the ore running diagonally down in the vein filling and, more often 

 than not, following the polished grooves in the walls, which are called 

 " slickensides " and which indicate the direction taken by one wall 

 when it moved on the other during the formation of the fracture. 

 The rich places may terminate in depth as well and again may be re- 

 peated, but they must be anticijoated, and for them allowance must 

 be made in any mining operation. 



Ores therefore gather along subterranean waterways. They may 

 fill clean-cut fissures wall to wall ; they may impregnate porous wall 

 rocks on either side; they may even entirely replace soluble rocks 

 like limestones. 



We may now raise the question as to the source of the water which 

 accomplishes these results, and the further question as to the cause 

 of its circulations. 



The nature of the underground waters, which are instrumental 

 in filling the veins, presents one of the most interesting, if not the 

 most interesting, phase of the problem, and one upon which attention 

 has been especially concentrated in later years. The crucial point 

 of the discussion relates to the relative importance of the two kinds 

 of ground waters — the magmatic, or those from the molten igneous 

 rocks, and the meteoric, or those derived from the rains. The mag- 

 matic waters are not phenomena of the daily life and observation of 

 the great majority of civilized peoples, and for this reason they have 

 not received the attention that otherwise would have fallen to their 

 share. Relatively few geologists have the opportunity to view vol- 



