20 



SCIENCE. 



[N". S. Vol. XXIII. No. 575. 



liensive view. As solvents, hot waters are 

 so incomparably superior to cold waters 

 that they appeal to us strongly. We may, 

 therefore, take it as well established that 

 water is the vehicle. The chemical com- 

 pounds wdiicli constitute the ores naturally 

 differ widely in solubility and no sweeping 

 statements can be made regarding them. 

 Iron, for example, yields very soluble salts 

 and is widely, one might almost say uni- 

 versally distributed in ordinary waters. 

 Its ores are compounds of the metal with 

 oxygen, and in this respect it differs from 

 nearly all others, which are mostly com- 

 bined with sulphur. Although almost all 

 of them have oxidized compounds, the lat- 

 ter are on the whole very subordinate con- 

 tributors to our furnaces. 



Iron is everywhere present in the rocks 

 and when exposed to the natural reagents 

 it is one of their most vulnerable elements. 

 It, therefore, presents few difficulties in the 

 way of solution and concentration by 

 waters which circulate on or near the sur- 

 face and which perform their reactions un- 

 der our eyes. 



The compounds of copper, lead, zinc, 

 silver, nickel, cobalt, quicksilver, anti- 

 mony and arsenic with sulphur present 

 more difficult problems and ones into 

 whose chemistry it is impossible to enter 

 Iiere in any thorough way, but in general 

 it may be said that the solutions were prob- 

 ably hot, that they were in some cases 

 alkaline, in others acid, and that the pres- 

 sure under which they took up the metals 

 in the depths has been an important fac- 

 tor in the process. The loss of heat and 

 pressure as they rose toward the surface no 

 doubt aided in an important way in the 

 result. 



The first condition for the production of 

 an ore-deposit is a waterway. It may be 

 a small crack, or a large fracture, or a 

 porous stratum, but in some such form it 

 must exist. Naturally porous rock affords 



the simple.st case, and provides an easily 

 understood place of precipitation. For ex- 

 ample, in the decade of the seventies rather 

 large mines at Silver Reef, in southern 

 Utah, were based upon an open-textured 

 sandstone into which and. along certain 

 lines silver-bearing solutions had entered. 

 Wherever they met a fossil leaf or an old 

 stick of wood which had been buried in the 

 rock the dissolved silver was precipitated 

 as sulphide or chloride. Sometimes for 

 no apparent reason the solutions impreg- 

 nated the rock with ore, but the ore seems 

 to follow along certain lines of fracturing. 

 Again at Silver Cliff near Rosita in cen- 

 tral Colorado, the silver solutions had evi- 

 dently at one time soaked throiigh a bed 

 of porous volcanic ash, and had impreg- 

 nated it with ore, which, while it lasted, was 

 quarried out like so much rock. In the 

 copper district of Keweenaw Point on Lake 

 Superior, the copper bearing solutions have 

 penetrated in some places an old gravel 

 bed and impregnated it with copper; in 

 other places they have passed along cer- 

 tain courses in vesicular lava flows, and 

 have yielded up to the cavities, scales and 

 shots of native copper. 



It has happened at times that the ore- 

 bearing solutions, rising through some 

 crevice, have met a stratum charged with 

 lime, and having spread sideways have ap- 

 parently been robbed of their metals be- 

 cause . the lime precipitated the valuable 

 minerals. In the Black Hills of South 

 Dakota, there are sandstones with beds of 

 calcareous mud rocl?s in them. Sohitions 

 bringing gold have come up through in- 

 significant-looking crevices called 'verticals' 

 and have impregnated these mud-rocks 

 with long shoots of valuable gold ores. In 

 prospecting in a promising locality the 

 miner, knowing the systematic arrange- 

 ment of the verticals, and having found 

 the lime shales, drifts along in them, fol- 

 lowing a crevice in the hope of breaking 



