762 C. R. VAN HISE 



belt may be partly caused (1) by reactions between the down- 

 ward moving waters carrying 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 part of the 

 veins, carried by descending waters, may be precipitated by mate- 

 rial contained in the rocks below. This material may be organic 

 matter, ferrous substances, etc. So far as precipitating materials 

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

 sulphides, and precipitate the metals in that form. While sul- 

 phides may thus be precipitated either above or below the level of 

 groundwater, they are more likely to be thrown down below the 

 level of groundwater. 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 descending from above, there will probably be a con- 

 siderable belt in which the circulation is slow and irregular, the 

 main current now moving slowly upward and now moving slowly 

 downward, and at all times being disturbed by conventional move- 

 ments. Doubtless this belt of slow general movement and con- 

 ventional 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 

 descending 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 variations in ascending cur- 

 rents result from irregularities of rainfall. 



In the belt of meeting ascending and descending waters (see 

 Fig. 6) conventional mixing of the solutions due to difference in 

 temperature would be an important phenomenon. The waters 



