542 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



as the peculiar habitat of lead and silver deposits, from the fact that many 

 silver-bearing limestones have been found to be dolomitie, making thus a 

 generalization out of a coincidence. But the evidence here fails to confirm 

 any such coincidence. Not only are the other limestone beds of the region, 

 which do not carry metals in quantity, equally dolomitie, but in the adjoin- 

 ing Ten-Mile district the ore occurs in non-dolomitic limestones, and in 

 the Robinson mine is confined to the upper part of a limestone which is 

 almost chemically pure carbonate of lime, while it does not extend into the 

 lower part of the horizon, which carries nearly 7 per cent, of carbonate of 

 magnesia. 



A reason which might have been offered for the greater susceptibility 

 of dolomite to the decomposing action of percolating waters, viz, the sup- 

 position that the carbonate of lime in a dolomite is first attacked, and that 

 thus a disintegration of the rock is readily commenced, has been disproved 

 in the case of the rocks of this region by the chemical experiments made, 

 which show that the waters act simultaneously upon both carbonates, or 

 rather upon the double carbonate and not upon either of its component parts. 



As regards the minor differences in composition between the diiferent 

 dolomites of the region, the chemical investigations which it lias been pos- 

 sible to make furnish little more than suggestions. The beds whose phys- 

 ical conditions are most similar to those of the Blue Limestoiae, and which 

 are also most frequently found mineral-bearing after it, are those of the 

 Silurian or White Limestone. 



The first striking difference between the two is the darker color of the 

 former, which is presumably due to organic matter and possibly in part to 

 sulphide of iron, as suggested by Mr. Guyard. Chemical analysis confirms 

 the indications given by outward appearances, showing an appreciable 

 amount of organic matter in the Blue Limestone and none in the White. 

 If the metals were brought in in the state of sulphates, the organic matter 

 would promote their reduction to sulphide. On the other hand, the Robin- 

 son limestone, already cited, affords an instance opposed to this view, for 

 there it is the light-colored limestone which carries tlie ore, while the darker 

 limestone, which it may be assumed has more organic matter, is quite bari'en. 

 A second difference between tlie Blue and the White Limestone is that 

 the former contains no silica and the latter over 10 per cent. It may be 



