450 



GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



4. A nearly continuous bed of lime-sand, in which are occasional por- 

 tions of compact Blue Limestone, is found overlying the iron body, extend- 

 ing from near Vulture No. 3 over adjoining portions of the Vulture, Car- 

 boniferous, Chrysolite, New Discovery, and Little Chief claims. 



5. In the Little Chief mine, about the middle of the claim and south 

 of the Gray Porphyry dike, a body of limestone comes in suddenly, occu- 

 pying the greater portion of the ore horizon for a considerable distance, an 

 up-raise having been made through it ; while below it has been proved to 

 extend to the Parting Quartzite by a drift running along the contact of the 

 two in an east-and-west direction. This limestone is partly disintegrated 

 into sand and partly in a compact state. A little farther south the iron body 

 is found resting directly on the Parting Quartzite, affording a direct proof 

 that itis a replacement of the limestone. (See Section J, Atlas Sheet XXXIV.) 



6. The most considerable body found is that cut at the northern end 

 of the third level of the Dunkin mine. The drift runs through this body 

 for a distance of about one hundred feet, the stratification lines showing at 

 first a dip of 45 to the northward ; the angle becomes shallower farther on, 

 which may be due to a change in the strike. This body shows the charac- 

 teristic ribbed structure of the Blue Limestone and contains imperfect casts 

 of fossils. It has, moreover, every external appearance of a solid hard rock, 

 but upon being broken down crumbles at once to fine sand. 



The analyses of these lime-sands show no essential change in compo- 

 sition from the unaltered rock, as regards their contents in carbonate of lime 

 and magnesia. The following are the proportions obtained : 



The disintegration is probably due to the dissolving out of the cement- 

 ing material, which held the grains together, by percolating waters, and from 

 the above analyses it would seem that in these dolomites, as in quartzites, 

 the cementing material was essentially of the same composition as the rock 

 itself. 



