38 BULLETIN 42, UNITED STATES NATIONAL MUSEUM. 



massive. It varies iu hardness from 3 to 3.5, and in specific gravity 

 from 6.46 to 6.48. It has an adamantine luster and is colgrless or tinged, 

 generally b^^ salts of copper, and is transparent. 



Cerussite is generally associated with, and is the result of the decom- 

 position of galena, its formation generally being subsequent to that of 

 anglesite. It is much more common than the sulphate. It is always 

 more abundant iu the upper levels of a mine. To illustrate the occur- 

 rence of cerussite six specimens are shown. 



(1) Crystallized, separate transparent tabular crystals on galena. Potosi, Wash- 

 ington County, Missouri. (4182.) 



(2) Crystallized, aggregates of small crystals encircling galena. Miller Mine, 

 American Fork district, Salt Lake County, Utah. (180.51.) 



(3) Crystallized, aggregates of slightly coherent crystals. Cave Mine, Milford. 

 Utah. (56034.) 



(4) Crystallized, coarse sand of crystals. Utah. (11502.) 



(5) Crystallized on massive, colored a little by iron. Yosemite Mine, Bingham 

 Canyon, Utah. (55929.) 



(0) Massive. Florence Mine, Leadville, Lake County, Colorado. (56432.) 



Many more specimens, both crystallized and massive, will be found 

 in the silver-lead ore series. 



Silver-Lead Ores. 



The silver-lead ores consist of the various lead minerals carrying suf- 

 ficient silver to render its extraction profitable, as in i he case of all other 

 ores. The amount of silver in a lead ore that will render its extraction 

 profitable varies greatly with circumstances. As low as 10 ounces of 

 silver in an ore containing a high percentage of lead may, under favor- 

 able circumstances, be profitably smelted. In a large majority of cases 

 there are no distinct silver minerals to be observed in these ores, the 

 silver simply replacing the lead in the compound. In a few cases, how- 

 ever, distinct silver minerals have been found, especially where the origi- 

 nal galena has undergone complete decomposition. In some cases large 

 masses of chloride of silver have thus been segregated, as in the case of 

 the Kobert E. Lee Mine, of Leadville, Colorado ; while in others the silver 

 exists in such a condition that a portion of it, at least, will amalgamate. 

 Some of the surface ores at Leadville were treated by amalgamation. 



At the present time large amounts of both lead and silver are pro- 

 duced from ores of this character, and it is only under the most favor- 

 able circumstances that n on argentiferous lead ores can be smelted. 



Galena is frequently associated with other sulphides that are more 

 or less argentiferous, and occasionally auriferous ; the most common of 

 these is the sulphide of zinc (blende). When the lead is undergoing 

 decomposition the other sulphides decompose also; and whether the 

 products of decomposition remain with the lead or are carried away 

 depends upon whether the decomposition product formed is soluble or 

 not. In the case of blende, the decomposition products generally are 

 soluble; and where mixtures of galena and blende have undergone de- 



