AGENTS OF ALTERATION OF ORES. 553 



chlorine. Traces of phosphoric acid were found in both of the limestones 

 analyzed. In the Lingula shales, which directly overlie the Blue Lime- 

 stone, 5.14 per cent, phosphoric acid was found in the portion containing 

 casts of Lingula and 0.35 per cent, in that free from these casts. Chlorine 

 was found in each of the sixteen specimens of dolomitic limestones, from 

 different horizons and localities, that were tested for that substance. Bro- 

 mine and traces of iodine have also been detected, but they exist here, as 

 in sea-water, in far smaller proportions than chlorine Organic matter is 

 found abundantly in most of the rocks of the Carboniferous formations. 

 The specimen of Blue Limestone analyzed gave only 0.03 per cent., but 

 in the overlying Weber Grits not only are there frequent beds of carbona- 

 ceous shales, passing at times into actual coal, but the sandstones sometimes 

 contain as high as 4 per cent, of carbonaceous matter. 



Relative richness of galena and cerussite. The greater 1'ichneSS in silver of 



galena over cerussite in this region is very noteworthy. Mr. L. D. Rick- 

 etts, 1 who made a detailed study of the ores of Carbonate Hill, states that 

 the average tenor of cerussite in that locality is less than 40 ounces of silver 

 to the ton, while galena averages 145 ounces of silver to the ton. He also 

 states that assays of five galena nodules, and of the carbonate crusts on 

 each, showed that in proportion to the amount of lead present there was six 

 times as much silver in the galena as in the cerussite. Table XIV, Appen- 

 dix B, which gives the assays of various specimens of ores, vein materials, 

 and adjoining country rocks collected during the investigation, shows a 

 similar relation in the silver contents of galena (No. 15) and of its cerussite 

 crust (No. 16), 420 ounces and 28. 6 ounces, which are in even greater con- 

 trast than in the cases cited above. 



The fact that silver is found disseminated throughout the vein mate- 

 rials and adjoining country rocks, even where little or no lead is found, 

 shows that during secondary alteration silver has been further removed from 

 its original locus 'and more widely disseminated than lead. In fact, it may 

 be assumed that the outlines of the present bodies of lead ore vary but 

 little from those of the original deposits, but it would hardly be safe to 

 make such an assumption in regard to silver ores. It is apparent that this 



'Ores of Leadvillo, p. 87. Priuceton, 1883. 



