1074 A TREATISE ON METAMORPHISM. 



Under normal conditions of increase of temperature of one degree for 

 30 meters, the temperature would be 100° C. at a depth of 3,000 meters; 

 therefore, where the circulation is deep, increase in temperature with depth, 

 even under normal conditions, is of great consequence. But in many 

 regions, during the segregation of ore deposits, igneous rocks have been 

 intruded or extruded, or important orogenic movements have taken place, 

 or both. Wherever igneous rocks have been intruded in the upper part of 

 the lithosphere, or poured out upon the lithosphere during the time when 

 ore deposits are forming, the temperature of the underground water is 

 higher, and may be much higher than normal, so that even where ore 

 deposits have been produced by a relatively shallow circulation the waters 

 may have had the advantage of a high temperature. The temperature may 

 increase with more than normal rapidity in consequence of mechanical and 

 chemical action. Where there is no evidence of igneous intrusion during 

 the time of ore deposition, but mechanical or chemical action has taken 

 place upon a great scale, the temperature of the underground water may 

 be raised considerably, and thus the deposition of the ores even in such 

 cases be accomplished by solutions at higher temperatures than normal. 

 For the underground solutions which occur in nature, pressure usually 

 promotes solution. (See Chapter III.) 



The particular metals and the amounts of them which are taken into 

 solution also depend greatly upon the nature of solutions. To illustrate, 

 where the solutions contain strong acids they are likely to dissolve the 

 metals; where oxygen is abundant the sulphides are likely to be oxydized 

 into sulphates and taken into solution; abundant carbonic acid forms car- 

 bonates; where alkaline sulphides and carbonates are present and the 

 solutions come into contact with sulphides, these are somewhat readily 

 dissolved as such. 



While all of these compounds favor solutions, in the early part of the 

 journey the most important single factor in the process of solution of the 

 valuable metals is the presence of oxygen in the water. This is of great 

 consequence, because many of the valuable metals, both as original com- 

 pounds in the igneous rocks and as secondary products, are as sulphides. 

 Where oxidizing waters come into contact with sulphides they are trans- 

 formed to sulphates, and thus sulphates of most of the valuable metals may 

 be formed, as, for instance, those of silver, copper, mercury, lead, zinc, iron, 



