EFFECTS OF DESCENDING WATERS. 1177 



The metals precipitated by the mingling of waters may be contributed 

 by the descending waters, by the ascending waters, or partly by each. In 

 so far as more than an average amount of metallic material is precipitated 

 from the ascending waters, this results in the relatively greater richness 

 of the upper part of veins independently of the material carried down from 

 above. 



The aoove methods of precipitation and enrichment of the upper parts 

 of deposits follow from the reactions of downward-moving waters. Their 

 effect is to precipitate the metals of the ascending water to some extent, and 

 thus assist in the first concentration. But the results of these pi*ocesses can 

 not be discriminated from the second concentration, which is caused by an 

 actual downward transportation of the material of the first concentration. 

 It is believed that the peculiar character of the upper portions of lodes is 

 mainly owing to downward transportation of metals previously precipitated 

 (see pp. 1182-1189); but whether this be so or not, it is certainly due to 

 descending waters. 



SECOND CONCENTRATION FAVORED BY LARGE OPENINGS NEAR THE SURFACE. 



The concentration of large ore bodies in the belt of weathering and in 

 the upper part of the belt of cementation is greatly favored by the abun- 

 dance and size of the openings as compared with the openings at greater 

 depths. 



The openness of the rocks above the level of ground water and the 

 rapid lessening of the volume of the openings below it have already been 

 alluded to as general phenomena, and an explanation has been offered that 

 in the belt of weathering solution is the law, and in the belt of cementation 

 cementation is the law. (See pp. 484-487, 562-565, 612-617.) Of course, 

 it is understood that there is usually not a sudden change in the amount 

 of pore space at the level of ground water, but at and below it the 

 extremely open upper ground grades into the much less open lower ground. 

 In some instances the gradation requires some distance. Thus, so far as 

 the openings are concerned, the conditions for the formation of large ore 

 deposits are more favorable above the level of ground water and as far 

 below it as openings are numerous than at deeper levels. 



This openness of the belt of weathering and the comparative closeness 

 of the belt of cementation are well illustrated by many limestone regions; 



