644 RECORD OF MEETINGS OF THE 



tions can not spread laterally, but where it is open the impregna- 

 tion may be extensive. The miner has, therefore, 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 barren 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 indi- 

 cate 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 repeated, but 

 they must be anticipated, and for them allowance must be made 

 in any mining operation. 



Ores, therefore, gather along subterranean water-ways. 

 They may fill clean-cut fissures, wall to wall; they may impreg- 

 nate porous wall-rocks on either side ; they may even entirely re- 

 place 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 magmatic 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 volcanoes in active 

 eruption, and have but disproportionate conceptions of the 

 clouds and clouds of watery vapor which they emit. The 



