1218 A TREATISE ON METAMORPHISM. 



district, where the topography is very rugged. It has been pointed out 

 that oxidizing effects are here produced to a depth of a thousand meters, 

 and it can hardly be doubted that the breadth of the belt of effective 

 ascending circulation is great. In the lead and zinc district of south- 

 western Wisconsin the topography is gentle, and here" the effective 

 circulation, ascending and descending, was confined to a vertical distance 

 of not more than 150 meters; but this is partly due to impervious strata. 



EFFECT OF HORIZONTAL ELEMENT. 



The horizontal position of an ore deposit with reference to topography 

 often has an important influence upon its richness and magnitude. If the 

 correct theory of circulation of underground waters and the deposition of 

 ores has been given, certain corollaries follow with reference to this point- 



(1) Commonly ores deposited by ascending waters should be formed 

 below the valleys, or at least below the lower parts of the slopes; for 

 these are the places where waters are ascending in the trunk channels. 

 (2) Commonly ores deposited by descending waters should be formed 

 below the crests or below the upper slopes of elevations; for these are the 

 places where water would be descending-. Probably the upper slopes are 

 more favorable places than the crests; for at an annular belt on the 

 upper slope of an elevation the quantity of descending waters would be 

 greater than at the crests. (3) Commonly ores which receive a first 

 concentration by ascending waters and a second concentration by 

 descending waters should be on the slopes, probably in many instances 

 nearer the valleys than the crests. At such places the meteoric waters 

 falling at the higher elevations have sufficient head to search deeply the 

 zone of fracture for ores. Therefore, the ascending circulation in trunk 

 channels is strong. Further, at such places the level of ground water is a 

 considerable distance below the surface, and abundant descending- waters 

 are concentrated in the upper parts of the openings. (See fig. 26, p. 1076.) 

 The downward migration of the belt of weathering furnishes the final 

 favorable condition for the accumulation of a large amount of second 

 concentrates by descending waters. 



Admirable illustrations of ore deposits corresponding to the second of 

 the corollaries are furnished by the iron-ore deposits of the Lake Superior 

 region. These are the products of descending waters, and the great major- 

 ity of the ore deposits are found near the tops of hills or upon the slopes. 



