REVIEWS 257 



field relations; for facts about these, therefore, reliance was placed 

 largely on the observations of earlier workers in the area. 



Typical ore specimens from ten mines of the region near Ouray and 

 Telluride are first described. With the co-operation of Mr. Chase 

 Palmer, a chemical study of the hot springs near Ouray was made, but 

 the neutral waters are high in calcium sulphate and yielded no final 

 conclusions as to their origin. 



At several mines it is found that pearceite is a late primary mineral, 

 suggesting that in this form the metal has greater solubility than other 

 silver salts found in these ores. A new silver mineral, apparently inter- 

 mediate between argentite and galena, is noted in several specimens, 

 separating primary galena from secondary argentite. In some of the 

 mines in the Red Mountain district between Ouray and Telluride, replace- 

 ment results in a gradual substitution of silver and copper for lead, 

 zinc, and iron — a change roughly comparable to the position in the 

 electromotive series of the metals concerned. 



In general, these studies seem to show that silver sulpho-salts are 

 important in both primary (hypogene) and secondary (supergene) 

 mineralization. Apparently the sulpharsenides and sulphantimonides 

 of silver tend to be among the later generations of primary minerals. 

 Both downward silver enrichment and the upward transportation of 

 silver minerals take place more readily in neutral or but faintly acid 

 waters. The facts given also illustrate beautifully the importance of 

 relief and erosion in stripping off the enriched zone, as well as the effect 

 of rising solutions, especially those rich in hydrogen sulphide, in retard- 

 ing the downward progress of enrichment. 



More than anything else, however, the facts so well presented illus- 

 trate the growing importance of micropetrography of the ores in solving 

 economic problems. To quote the writer: 



The judgment as to the probable success of deep mining in veins rich in 

 silver near the surface should not be prejudiced by preconceived ideas of the 

 importance of enrichment. Each district and in some districts each mine 

 presents a special problem. The method of microscopic study of the ores 

 worked out in recent years offers a method of determining, roughly at least, the 

 relative importance of primary and secondary processes in the deposition of 

 sUver ores far in advance of the ultimate test by actual development. The 

 practical value of information of this sort is out of aU proportion to the moder- 

 ate cost at which it may be procured. 



Occasionally the reviewer is led to doubt the evidence adduced to 

 prove the secondary character of certain minerals. Thus: 



