642 RECORD OF MEETINGS OF THE 



and concentration by waters which circulate on or near the 

 surface and which perform their reactions under our eyes. 



The compounds of copper, lead, zinc, silver, nickel, cobalt, 

 quicksilver, antimony, and arsenic with sulphur present more 

 difficult problems and ones into whose chemistry it is impossible 

 to enter here in any thorough way; but in general it may be 

 said that the solutions were probably hot, that they were in 

 some cases alkaline, in others acid, and that the pressure under 

 which they took up the metals in the depths has been an impor- 

 tant factor 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. Natur- 

 ally porous rock affords the simplest case, and provides an easily 

 understood place of precipitation. For example, 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 impregnated the rock with ore, but the ore seems 

 to follow along certain lines of fracturing. Again at Silver 

 Cliff, near Rosita in central Colorado, the silver solutions had 

 evidently at one time soaked through a bed of porous volcanic 

 ash, and had impregnated 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 solu- 

 tions have penetrated in some places an old gravel bed and 

 impregnated it with copper; in other places they have passed 

 along certain courses in vesicular lava flows, and have yielded 

 up to the cavities scales and shoots 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 apparently been robbed 

 of their£ metals^because the lime precipitated the valuable 



