758 C. R. VAN HISE 



material has been removed. However, in this district, a first 

 concentration by ascending waters was adequate, but it is not 

 often that a first concentration produces deposits of such rich- 

 ness as those adjacent to Calumet and Houghton On Keweenaw 

 Point ; and, indeed, this is exceptional even in the Keewe- 

 nawan of the Lake Superior region ; for while concentrations of 

 copper have occurred at many points in the - rocks of this period, 

 as yet at no other locality have those concentrations been found 

 to be so abundant and rich as to warrant exploitation on a large 

 scale. 



I now turn to the question as to the cause of frequent diminu- 

 tion of richness of ore deposits with depth. Many or most of such 

 ore deposits are believed to be the products of two concentrations, 

 the first by ascending, the second by descending waters. In this 

 connection it is necessary to call attention to the fact that a large 

 proportion of the ore deposits which are being exploited are 

 below some part of a slope. It may be said that the reason for 

 this is that the low grounds are more difficult to explore and 

 work ; but giving due allowance for this, it still seems to me 

 that the majority, perhaps the great majority, of very rich 

 deposits are below slopes and crests, and not below the valleys. 

 I believe the richer deposits are below the slopes, because at 

 these places a second concentration is possible and probable. 



Returning now to this chart (Fig. 6) we shall direct our atten- 

 tion to the fissure on the slope. This fissure once extended up 

 through the overlying rocks which have been removed by denu- 

 dation. What has become of the ore in the part of the fissure 

 which has been worn away ? If, for instance, it carried 5 per 

 cent, of copper, what has become of it ? A part of it would 

 have been scattered far and wide through erosive action ; but a 

 part of it would have been taken into solution and redeposited 

 in the same vein deeper down. In the belt of weathering oxi- 

 dized salts, such as sulphates, would form ; the descending 

 waters would carry these products downward ; and it is my belief 

 that they would react upon the solid, lean sulphides below with 

 the result of precipitatingthe metals from the descending solutions. 



