34 KEMP'S ORE DEPOSITS. 



bonate, pari passu, just before the metallic sulphide is precipitated. 

 It must be confessed that for enormous bodies of ore, like those of 

 Leadville, the small amount of organic matter present seems hard- 

 ly equal to the task assigned it, and the delicate balance of the lat- 

 ter case causing deposition to tread so closely on the heels of rock 

 removal, in order to avoid assuming an extended cavity makes it 

 appear that the entire chemistry of the process is perhaps hardly 

 understood. 



1.04.12. When silicate rocks are replaced, leaving a siliceous 

 gangue, the process may have been somewhat as suggested by R. 

 C. Hills for the mines of the Summit district, Rio Grande County, 

 Colorado. (See Proc. Colo. Sci. Soc., Vol. I., p. 20.) Alkaline solu- 

 tions remove silica and have slight action on silicates, but solutions 

 acid with sulphuric acid attack silicates, such as feldspar and bio- 

 tite, remove the alumina or change it to kaolin, and cause the sep- 

 aration of free silica. In the alteration products abundant opportu- 

 nity would be afforded for the precipitation of sulphides, which 

 would in part at least replace the rock. Along a crack or line of 

 drainage definite walls would thus easily fade out. Such phenome- 

 na are afforded by innumerable ore deposits (see R. W. Raymond, 

 discussion of S. F. Emmons' " Notes on the Geology of Butte, 

 Mont.," M. J, July, 1877), and often come under the notice of 

 every one familiar with mining. Yet we cannot but hope that 

 in the future our knowledge of the chemical reactions involved 

 will be increased. 



It may again be stated that the formation of ore deposits has 

 proceeded with great slowness, and the solutions bringing the met- 

 als have been, beyond question, very dilute. The extremely small 

 amounts of the metals that have been detected in relatively large 

 amounts of igneous rocks, even by the most refined analytical meth- 

 ods, have necessarily made the progress of solution a protracted 

 one. Curtis records some careful observations on the growth of 

 aragonite at Eureka, Nev., where he found that in three weeks, 

 so long as wet by a drop of water, the crystals increased in one case 

 as a maximum, five eighths of an inch, and in another three eighths. 

 But this was where the whole inclosing mass of rock consisted of the 

 compound deposited. 



