548 



GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



The earthy bases, alumina, lime, and magnesia, preserve a certain pro- 

 portion between each other in the three analyses. This accords with the 

 assumption that the deposits are a replacement of limestone, and the fact 

 that the apparently pure cerussite from the porphyry in the Adelaide ground 

 contains as much magnesia and lime as the Little Chief ore is in so far a 

 confirmation of the assumption on structural grounds (see p. 407) that the 

 ore bodies in the former mine are the replacement of fragments of Blue 

 Limestone caught up by the porphyry intrusion. 



Silica is in relatively higher proportion in the mixture (4) as compared 

 with the earthy bases, but preserves a comparatively even relation to iron 

 oxide, which would be expected from their association in the ores in general. 

 It is a question whether the silica is in part in actual combination with iron 

 or whether it all exists as free silica. Mr. Guyard suggests that some of 

 the lead may exist as silicate, but this also is not definitely proved. 



chloride ores. In the following table is given the proportional amounts 

 of chlorine, bromine, and iodine, respectively, in three typical chloride ores 

 relatively free from other minerals, tested for the purpose of determining 

 whether sufficiently definite quantitative relations exist between these sub- 

 stances to justify the recognition of more than one mineral species. I and 

 II were obtained from the pale-green mineral, and III from a colorless 

 specimen. 



Chlorine, bromine, and iodine. 



The results are somewhat negative, and, so far as they go, lead to the 

 conclusion that the chloride ores are merely mixtures, in varying propor- 

 tions, of chloride, bromide, and iodide of silver. The green chlorides, 

 which are of very common occurrence, are generally called embolite, and 

 this application of the term is justifiable if this mineral is considered simply 

 an indefinite mixture. In the analyses of embolite given in Dana's Miner- 



