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MR. A. M. EINLAYSON ON ORE-DEPOSITION [May I9IO, 



matter is a less active precipitant of lead and zinc than of the gold 

 and silver, which are extremely sensitive to its influence, as shown 

 by the experiments of Mr. T. A. Eickard, 1 and emphasized in an 

 important paper by Dr. W. P. Jenney. 2 Further, the influence of 

 different materials must vary greatly according to the metals carried 

 in solution. In this connexion mny be mentioned a striking case 

 of selective deposition recorded from Ku Shau Tzu (Mongolia) by 

 J. A. Church. 3 Here the veins carry galena in the limestones, 

 and native silver and tetrahedrite in the overlying shales. In 

 general, the influence of organic matter seems to be most marked 

 in the case of those metals the compounds of which are least stable ; 

 while it is less active in the presence of those basic metals which 

 form more stable compounds. 



The form in which the metals were deposited was not examined 

 in the case of the silicate and the shales. Mr. E. C. Sullivan, in a 

 long series of experiments on the deposition of various mejbals by 

 natural silicates, 4 has shown that the principle of adsorption, 

 advocated by Dr. E. Kohler, 5 does not generally apply, and that, in 

 each instance, an equivalent quantity of base from the silicate 

 passes into solution, in exchange for the metal deposited. Even 

 with the most refractory silicates, this exchange or replacement of 

 bases was found to take place, while it was also observed that the 

 small quantities of metals deposited went into combination as 

 silicates, an exact parallel with the formation of the carbonates in 

 the above- described experiments. Ore-deposition thus resolves 

 itself, in all cases, into a process of osmosis, there being no simple 

 precipitation of metals without an exchange. This has been 

 previously maintained on theoretical grounds by H. P. Gillette. 6 



VII. Secondary Alteration. 



The greatest depths to which mining extends in the British 

 lead and zinc districts are at Eoxdale (2010 feet) and at Laxey 

 (1800 feet), in the Isle of Man. At these depths, the ore is 

 becoming poorer, and it is probable that the limit of ore-deposition 

 does not greatly exceed 2500 feet beneath the surface in any of the 

 districts considered. This range is comparable to that of other 

 well-known lead and zinc fields, such as Freiberg (2134 feet), the 

 Upper Harz (2788 feet), Linares (2000 feet), and Eureka, Nevada 

 (3000 feet). Since the formation of the veins, great denudation, 

 amounting to an average of 2000 feet vertical, has taken place. 

 Thus it appears that nearly one-half of the original height of the 

 veins has been shorn off by denudation. 



Accompanying this denudation there has been much secondary 



1 ' The Enterprise Mine, Rico, Colorado ' Trans. Amer. Inst. Min. Eng. 

 vol. xxvi (1896-97) p. 978. 



2 ' The Chemistry of Ore-Deposition' Ibid. vol. xxxiii (1903) p. 445. 



3 Ibid. vol. xxxiii (1903) p. 1065. 



4 'Interaction between Minerals & Water-Solutions' Bull. U.S. Geol. Surv. 

 No. 312, 1907. 



' Adsorptionsprozesse als Eaktoren der Lagerstattenbildung & Litho- 

 genesis ' Zeitschr. f. prakt. Greol. vol. xi (1903) p. 49. 



6 ' Osmosis as a Factor in Ore-Formation ' Trans. Amer. Inst. Min. Eng. 

 vol. xxxiv (1904) p. 710. 



