TRANSPORTATION OF THE METALS. 1081 



It is, of course, understood that the underground circulation in any 

 actual instance is much more complex than that given in the simple ideal 

 case which has been considered. For instance, it is certain that, in the same 

 mineral-bearing area, immediately adjacent trunk channels may have had 

 very different histories. This is especially well shown by the deposits at 

 Butte, Mont., where there are two parallel main zones of mineralization, only 

 a short distance apart, the mineral wealth of one of which is mainly copper, 

 while that of the other is mainly silver." Many of the other special factors 

 which modify the simple general statement above given are discussed on 

 pages 1199-1222. 



PRECIPITATION OF THE METALS. 



The precipitation of metals in the trunk channels by ascending waters 

 is of so much importance in the concentration of ores that this process needs 

 further consideration. 



Precipitation may take place (1) by change in temperature and pressure, 

 (2) by mingling of solutions, and (3) by reactions between solutions and 

 solids. (See pp. 113-123.) 



PRECIPITATIOX BY DECREASE OF TEMPERATURE AXD PRESSURE. 



The general relations of solution and precipitation as a consequence of 

 varying temperature and pressure have been already considered. (See pp. 

 114-116.) Where the increase of temperature with depth is normal, it has 

 been seen that decreasing temperature and pressure due to the ascension 

 of waters from a depth of at least 3,000 meters are favorable to precipitation. 

 Furthermore, the same statement holds even if the increment of tempera- 

 ture be greater than normal, provided the temperature does not greatly exceed 

 100° C. Cases in which water issues at the surface at such temperatures 

 are very rare. The probably decreasing pressure and temperature of solutions 

 rising from depths greater than 3,000 meters are also favorable to precipita- 

 tion. Since it has just been shown that ascending waters are likely to be 

 in trunk channels, lessening temperature and pressure are likely to produce 

 precipitates in the openings of faults, joints, and bedding partings, and the 

 more open places in sandstones, conglomerates, and amygdaloids. 



Since upon the average ascending* solutions are those in which the 

 pressure and temperature are decreasing, precipitation is more likely to 



a Emmons, S. F., Notes on the geology of Butte, Mont: Trans. Am. Inst. Min. Eng., vol. 16, 

 1888, p. 54. 



