560 



GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



Kaolin and Chinese talc. These names are given in the mines to certain 

 substances, evidently alteration products of porphyry, occurring with great 

 persistency along the contact of limestone and porphyry, where they some- 

 times form the only vein material, and also within the ore bodies, some- 

 times at quite a distance from the contact. In the latter case they probably 

 result in most cases from small offshoots of the porphyry, such as have been 

 mentioned as occurring in the Little Pittsburgh mine, and, as are shown 

 in Fig. 1, Plate XXII, penetrating the as yet unaltered limestone. The 

 characteristic Chinese talc is compact, with conchoidal fracture, somewhat 

 translucent, with a sort of opalescent luster, and is easily cut by the finger- 

 nail when fresh, but becomes opaque and hardens on exposure to the air. 

 White when pure, it is generally more or less discolored and veined by 

 oxides of iron and manganese. The miners often carve it into pipes and 

 figures. The so-called kaolin is white, opaque, and generally plastic, but 

 also hardens on exposure. No true kaolin was found among the specimens 

 collected. In the following table, I and II would be considered kaolins, 

 and III, IV, and V, Chinese talcs. VI, VII, and VIII are specimens from 

 the Lower Waterloo mine, contributed by Mr. L. D. Ricketts ; in spite of 

 their different composition, they are not to be distinguished in the hand 

 specimen from the Chinese talcs. 



Kanlin and Chinese talc. 



The compositions given above show that these substances are mixtures 

 of hydrated silicates of alumina, with more or less sulphate of alumina, 

 which, in the case o f the last three, is replaced by silicate of zinc. As is 

 generally the case with such alteration products, it is difficult to consider 

 them distinct minerals. The occurrence of zinc in the last three is some- 



