1220 A TREATISE ON METAMORPHISM. 



high peaks, rising to 4,000 and 4,300 meters. Below the mines are the 

 towns at altitudes from 2,300 to 2,700 meters — that is, the outcrops of 

 the veins are 700 to 1,000 meters above the main valleys. 



Another illustration of deposits which are on slopes and are the result 

 of two concentrations is furnished by the lead and zinc district of the upper 

 Mississippi Valley. Chamberlin" notes that in the valleys of the Wisconsin 

 part of the district the waters generally ascend to the surface; therefore, at 

 such places only a first concentration would be expected. It is the general 

 impression among miners in this district that a lode makes better on the 

 slope of a hill "than at the summit or at the foot of a hill." 6 Also, it is 

 held by the miners that the lodes which run parallel to a contour of a hill 

 "like an eave trough" 6 are more likely to be rich than those which run 

 toward the summit of the hill. When considered in connection with the 

 topography both of these practical conclusions of the miners are fully 

 explained by the theory of a first concentration by ascending waters and a 

 second concentration by descending waters. This case of the Wisconsin 

 lead and zinc district must not be too strongly insisted upon, for at many 

 places the streams have cut through the lead-bearing limestone into the 

 St. Peters sandstone below, which is a barren formation. 



In regions in which there have been recent important changes in the 

 positions of the drainage lines and elevations, the generalizations concern- 

 ing the relations of ore deposits and topography are only partly applicable. 

 It is well known that in consequence of the varying hardness of rocks, 

 of structure, of the unequal strength of streams and of unequal declivity, 

 drainage lines are almost constantly shifting*, in many regions somewhat 

 rapidly. Another way in which the position of the drainage lines with 

 reference to ore deposits may be shifted is by base leveling and subsequent 

 uplift. After a region has been cut down to the level of the sea and is 

 again uplifted it is well known that some of the streams are likely to be 

 along old drainage lines, but that others will be in new positions. It fol- 

 lows that when the second cycle of erosion is well inaugurated an ore deposit 

 which during the first cycle of erosion was in a valley may be on a crest, 

 and vice versa. When it is l-emembered that there have been various 



«Chamberlin, T. C, The pre deposits of southwestern Wisconsin: Geology of Wisconsin, vol. 4, 

 1882, p. 565. 



!> Chamberlin, cit. p. 563. '-. . 



