NOVEMBEE 22, 1901.] 



SCIENCE. 



789 



occur between the salts of silver and gold 

 and other metals, and the sulphides of lead 

 and zinc. 



Time does not permit the consideration of 

 the various reactions which result in the re- 

 concentration by descending waters. These 

 I shall be obliged to take for granted this 

 evening, but those who care to follow the 

 subject further may find a treatment of this 

 part of it in my full paper upon the depo- 

 sition of ores, published in Volume XXX. 

 of the Transactions of the American Institute 

 of Mining Engineers. 



During the process of reconcentration 

 erosion steadily goes on, perhaps to a depth 

 of 1,000 or 5,000 or even 10,000 feet, or 

 more. As denudation steadily lowers the 

 surface of the country, the material depos- 

 ited by the first concentration is picked up 

 and gradually carried down along the vein 

 by the descendiug waters. This material 

 reacts upon the other materials, and is 

 largely reprecipitated. 



In the foregoing statements the second 

 concentration of metals by solution, down- 

 ward transportation and precipitation by 

 reactions upon the sulphides of an earlier 

 concentration has been emphasized. How- 

 ever, it is not supposed that this is the only 

 process which may result in enrichment of 

 the upper parts of ore deposits by descend- 

 ing waters. The enrichment of this belt 

 may be partly caused (1) by reactions be- 

 tween the downward- moving waters carry- 

 ing metallic compounds and the rocks with 

 which they come in contact, and (2) by 

 reactions due to the meeting and mingling 

 of the waters from above and the waters 

 from below. 



1. The metallic compounds dissolved 

 in the upper parts of the veins, carried by 

 descending water, may be precipitated by 

 material contained in the rocks below. 

 This material may be organic matter, fer- 

 rous substances, etc. So far as precipita- 

 ting materials are reducing agents, they 



are likely to change the sulphates to sul- 

 phides, and precipitate the metals in that 

 form. While sulphides may thus be pre- 

 cipitated either above or below the level 

 of ground- water, they are more likely to be 

 thrown down below the level of ground- 

 water. Other compounds than reducing 

 agents or sulphides may precipitate the 

 downward-moving salts in other forms 

 than sulphides. 



2. In a trunk-channel, where waters 

 ascending from below meet waters descend- 

 ing from above, there will probably be a 

 considerable belt in which the circulation 

 is slow and irregular, the main current 

 now moving slowly upward and now mov- 

 ing slowly downward, and at all times be- 

 ing disturbed by convectional movements. 

 Doubtless this belt of slow general move- 

 ment and convectional circulation would 

 reach a lower level at times and places of 

 abundant rainfall than at other times and 

 places, for under such circumstances the 

 descending currents would be strong. The 

 ascending currents, being controlled by the 

 meteoric waters falling over wider areas, 

 and subject to longer journeys than the de- 

 scending currents, would not so quickly 

 feel the effect of abundant rainfall. 

 Later, the ascending currents might feel 

 the effect of the abundant rainfall and carry 

 the belt of upward movement to a higher 

 level than normal. However, where the 

 circulation is a very deep one, little varia- 

 tion in ascending currents results from irreg- 

 ularities of rainfall. 



In the belt of meeting ascending and 

 descending waters (see Fig. 6) convectional 

 mixing of the solutions due to difierence in 

 temperature would be an important phe- 

 nomenon. The waters from above are cool 

 and dense, while those from below are 

 warm and less dense. In the neutral zone 

 of circulation the waters from above would 

 thus tend to sink downward, while waters 

 from below would tend to rise, and thus 



