194 THE PROBLEM OF THE METALLIFEROUS VEINS. 



metals, and second, the places where the precipitation of them takes 

 place. We may then inquire more particularly into the source of 

 the agent and the methods of its ojDeration. In order to do this in the 

 time at command I must remorselessly focus attention on the large 

 and essential features, resolutely avoiding every side issue or minor 

 point, however inviting. 



The one solvent which is sufficiently abundant is water, and prac- 

 tically all observers are agreed that for the vast majority of ore 

 deposits it has been the vehicle of concentration. Of course it need 

 not operate alone. On the contrary, easily dissolved and ever-present 

 materials like alkalies may, and undoubtedly do, increase its effi- 

 ciency. It does not operate necessarily as cold water. On the con- 

 trary, we all know that the earth grows hotter as we go down, so 

 that descending waters could not go far without feeling this in- 

 fluence. Volcanoes, too, indicate to us that there are localities where 

 heat is developed in enormous amounts and not far below the surface. 

 There is therefore no lack of heat and we need only be familiar 

 with the western country to know that there is no lack of hot springs 

 when we take a comprehensive view. As solvents, hot waters are so 

 incomparably superior to cold waters that they appeal to us strongly. 

 We may therefore take it as Avell established that water is the vehicle. 

 The chemical compounds which constitute the ores naturally differ 

 widely in solubility and no sweeping statements can be made regard- 

 ing them. Iron, for example, yields very soluble salts and is widely, 

 one might almost say miiversally, 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 combined with sul- 

 phur. Although almost all of them have oxidized compounds, the 

 latter are on the whole very subordinate contributors to our furnaces. 



Iron is everywhere present in the rocks and when exposed to the 

 natural reagents it is one of their most Aulnerable elements. It 

 therefore presents few difficulties in the way of solution and concen- 

 tration 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, quick- 

 silver, antimon3^ 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 important 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 water- 

 way. It may be a small crack, or a large fracture, or a porous 



