280 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



from the waters in which it is deposited and that isolated or evaporating 

 basins are indispensable conditions of the formation of dolomite. In this 

 particular region these conditions might have been fulfilled, since the 

 Archean land masses certainly inclosed the sea on two sides. His theory 

 requires, however, that all the lime contained in the sea waters should have 

 first been precipitated by the carbonate of soda, which would then act on 

 the chloride of magnesium and throw it down as carbonate. It would 

 seem, however, from the character of the rocks, which are formed of crys- 

 talline grains of the double carbonate, that the two salts were probably 

 precipitated at the same time and that a certain amount of chloride in solu- 

 tion was inclosed in the grains as they crystallized. 



As regards the question whether carbonate of lime is more -readily dis- 

 solved out of a dolomite than carbonate of magnesia, the evidence goes to 

 show that percolating waters act upon the double salt, and not upon its 

 more soluble member alone, since the veins and cavities, such as are shown 

 in the lower specimen on Plate VI (p. 64), which have been refilled by white 

 crystalline material deposited by these waters, are found to have the same 

 composition as the original dolomite. Moreover, where the entire rock has 

 been apparently changed by the action of waters, as in the so-called "lime 

 sand" found in the mines, which is Blue Limestone from which the cement- 

 ing material of the grains has been removed, or in the case of a given 

 bed in the White Limestone of Dyer Mountain, which in one part has, by 

 this action, from a compact light blue rock, become clayey in structure and 

 pink in color, analysis shows that the proportions of carbonate of lime 

 and magnesia remain essentially unchanged, whatever variation there may 

 have been in the other constituents. 1 



In both the above cases the metamorphism, or change in the character 

 of the limestone, must have taken place about the time of the deposition of 

 the ore bodies in these regions, and would therefore not have been pro- 

 duced by surface waters. The action of surface waters, using this term in 

 its ordinary, restricted sense of waters which come from the surface under 

 essentially the same conditions that exist at the present day, is apparently 

 different from the above, judging from the following observation. 



1 Sec Analyses 5, 6, 9, and 10. Table VI, Appendix B. 



