1206 A TREATISE ON METAMORPH1SM. 



more soluble than the other rocks." But other factors enter into the 

 matter. It has already been explained that country rock, especially lime- 

 stone, may furnish solutions which react on the mineral-bearing solu- 

 tions and thus cause precipitation or furnish the metallic material. (See 

 pp. 1086-1087.) Furthermore, where limestone and stronger rocks are 

 deformed together, the limestone, having less strength, is more likely to be 

 crushed and broken in a complex manner and thus furnish trunk channels 

 for circulation. 



Change in the level of ground water may be an important factor in 

 opening up rooms in which ores may be deposited later. It is explained 

 (pp. 409-411, 565-566) that for various reasons the level of ground 

 water may vary greatly, and that above it large openings, such as caves, 

 are formed. (See pp. 484-487.) If, later, the level of ground water for 

 any reason rises, these openings pass into the belt of cementation and 

 in them large ore bodies, including sulphides, may be deposited. This 

 may be the history of the great cave-like openings, lined with gigantic 

 geodal crystals, which are found in the lead and zinc districts of Missouri, 

 and which were below the level of ground water before pumping was begun. 

 While the larger caves apparently have not been especially favorable places 

 for ore deposition, in some of the smaller ones ores have been deposited. 

 Probably this history applies to + he Butte district. Emmons 6 says 

 that for this district the level of ground water was once very much lower 

 than now, possibly as much as 300 to 600 meters. Doubtless much of the 

 solution occurred when the water was at a lower level. Later, when the 

 ground-water level was raised, the openings formed by solution were utilized 

 by the ore precipitated from descending waters transporting material from 

 higher levels. This process may have been important in the development 

 of the large sulphide bodies now below the level of ground water. 



In conclusion, I hold that ores deposited from aqueous solutions form 

 where there existed original trunk channels of circulation. Ore deposits 



« Wendt, Arthur F., The copper ores of the Southwest: Trans. Am. Inst. Min. Eng., vol. 15, 1887, 

 pp. 25-77. Curtis, J. S., Silver-lead deposits of Eureka, Nevada: Mon. U. S. Geol. Survey, vol. 7, 

 1884, pp. 64-79. Emmons, S. F., Geology and mining industry of Leadville: Mon. IT. S. Geol. Survey, 

 vol. 12, p. 540-543. 



5 Emmons, S. F., The secondary enrichment of ore deposits: Genesis of ore deposits, Am. Inst. 

 Min. Eng., 2d ed., 1902, pp. 444-445. See also Weed, W. H., The enrichment of gold and silver veins: 

 loc. cit, p. 497. 



