772 REVIEWS 



In the exploitation of the ore-bodies, it all goes to show how 

 vitally important is a full consideration of the geological structures 

 presented at the time of the first concentration, and as subsequently- 

 assumed. 



The keynote of Mr. Emmons' paper is given in one of the opening 

 paragraphs, when he says that "admitting fully the general truth of 

 the statement that the descending surface waters exert an oxidizing 

 action, and hence that oxidation products within the reach of surface 

 waters are the result of the alteration by the latter, I have been led to 

 believe, by observations now extending over a considerable number of 

 years, that under favorable conditions the oxidation products may be 

 changed back again into sulphides and redeposited as such, thus pro- 

 ducing what may be called a sulphide enrichment of the original 



deposits Being rather a searcher after facts than a theorist, I 



am not deterred from accepting what may appear to me the correct 

 reading of observed facts, because it seems to contradict generally 

 accepted theories." 



After briefly discussing the circulating waters of surface origin, the 

 groundwater level, the deposition of oxides below water-level, and the 

 deposition of sulphides, the author goes on to give an account of many 

 cases of secondary enrichment which his wide experience has brought 

 to notice. This account occupies the greater part of the paper. The 

 three propositions following are believed to be substantiated by the 

 geological evidence adduced : 



i. That descending waters not only cause migrations, or transfer- 

 ence and reconcentration, of the alteration products of the original 

 vein-materials in oxidized form, producing in one place an enrich- 

 ment, and in another possibly an impoverishment of the original 

 deposit, but that in their further downward course the oxidized forms 

 are frequently reduced and redeposited as sulphides, thereby producing 

 a sulphide enrichment of the original vein-materials. 



2. That this secondary enrichment of sulphides is not necessarily 

 a reduction in the presence of organic matter, but is frequent where no. 

 organic matter can be supposed to be present. It occurs mainly in 

 contact with the original sulphides of the deposits, and is, presumably, 

 a result of chemical reaction between these sulphides and the materials 

 brought down in solution by the descending waters. 



3. That while this redisposition of sulphides in many cases appears 

 to commence at or near the groundwater-level, it does not appear to 



