REVIEWS 771 



have a necessary connection with that level, and may under favorable 

 conditions extend below that level for a distance as yet undetermined, 

 the most important favoring conditions appearing to be recent or post- 

 mineral fractures, which have admitted a relatively free and uninter- 

 rupted descent of these waters. 



In conclusion, students are cautioned against making the inference 

 too sweeping. "Until a much larger number of ore deposits have 

 been studied with a definite purpose of determining how far they have 

 been subjected to secondary enrichment, it does not seem safe to draw 

 any far-reaching conclusions from the observations and suggestions 

 noted above. It has long been recognized that the superficial altera- 

 tion of ore deposits has often produced a very considerable modifica- 

 tion of the original constitution of the deposit, and its alteration has 

 so frequently been in the nature of an enrichment in the more valu- 

 able metals relatively to the original tenor of the ores, that it has 

 given rise to the very hasty decision that all ore-deposits necessarily 

 become poorer in depth, which is almost as unjustifiable as the old 

 assumption by the miner, that the nearer he got to the source of his ore 

 in the unknown deposits, the richer it would become. 



"The fact that ores under some conditions may be removed and 

 redeposited as sulphides, even below groundwater-level, opens a wide 

 field of possibility in accounting for the unusually rich bodies of ore 

 that are in some mines found in the middle levels, and have been fruit- 

 lessly sought for at greater depth. In many cases these have undoubt- 

 edly resulted from a concentration of material leached down from the 

 upper portions of the deposit as they have been worn down and car- 

 ried away by denudation. Especially in the case of large bodies of 

 pyritous ore carrying small proportions of more valuable metals, is a 

 concentration of those metals by downward percolating solution to be 

 looked for. It is, however, not yet safe to say that all rich bonanzas in 

 vein deposits have necessarily been formed in this way." 



The paper by Mr. Weed gives a brief statement of the theory enun- 

 ciated in a former contribution. The principles are applied more 

 particularly to the deposits of the precious metals, with special empha- 

 sis laid upon the dependence of such enrichments upon the presence 

 of iron sulphide in the primary ore, and upon the structural features 

 which control the circulation of the enriching solutions below ground- 

 water-level. The discussion is largely of Montana deposits, which the 

 author has been engaged in studying for several years past. Regarding 



