• EFFECT OF DESCENDING SOLUTIONS. 1175 



OTHER REACTIONS OF DESCENDING SOLUTIONS. 



In the foregoing pages the second concentration of metals by solution, 

 downward transportation, and precipitation by reactions on the sulphides of 

 the first concentration has been emphasized. However, it is not supposed 

 that this is the only process which may result in enrichment by descending 

 waters of the upper parts of vein deposits. The enrichment of this belt 

 may be caused by reactions between the downward-moving waters carrying 

 metallic compounds and the rocks with which they come in contact, and by 

 reactions due to the meeting and mingling of ascending and descending 

 waters. 



The descending waters carrying metals dissolved in the upper part of 

 the veins may be precipitated by material contained in the rocks below. 

 This material may be organic matter, ferrous salts, etc. So far as precipi- 

 tating materials are reducing agents, they are likely to change the sulphates 

 of most of the metals to sulphides, and precipitate the metals in that form. 

 While sulphides may thus be precipitated to some extent above the level of 

 ground water, because of the deficiency of oxygen they are thrown down 

 much more largely below it. The reducing solutions ordinarily precipitate 

 gold from its solutions in the metallic form. 



In a trunk channel, where ascending and descending waters meet, 

 there is a considerable horizon in which the circulation is slow and 

 irregular, the currents now moving slowly upward and now moving slowlv 

 downward, and at all times being disturbed by convectional movements. 

 Doubtless this belt of slow general movement and convectional circulation 

 reaches lower levels at times and places of abundant rainfall than at other 

 times and places, for under such circumstances the descending currents are 

 strong. The ascending currents, being controlled hj the meteoric waters 

 falling over wider areas, and subject to longer journeys than the descending 

 currents, do not so quickly feel the effect of abundant rainfall. Later, the 

 ascending currents may feel the effect of the increased rainfall and carry 

 the belt of upward movement to a higher level than normal. However, 

 where the circulation is a broad one, little variations in ascending currents 

 result from irregularities of rainfall. 



In the belt where ascending and descending waters meet (see fig. 26), 

 convectional mixing of the solutions due to difference in temperature is an 

 important phenomenon. The waters from above are cool and dense, and 



