756 C. R. VAN HISE 



ore deposits have been exhausted or have become so lean as 

 not to warrant working before the 300 meter level is reached ; 

 a large proportion before the 600 meter level is reached ; while 

 comparatively few ore deposits have been found to be so rich 

 as to warrant working at depths greater than 1000 meters. 



There are however some ore deposits which are not known 

 to decrease in richness with depth so far as yet exploited. 

 There are a considerable number of deposits in which after a 

 first rapid decrease in richness maintain their tenor pretty well 

 to the depth of 300, 500 or even 1000 meters, and some few 

 deposits maintain their richness at even greater depths. But we 

 cannot reasonably hope that a deposit will get richer with 

 depth provided we use a 300-meter unit for measurement. The 

 most sanguine view which is ever justified for any deposit is 

 that, using a 300-meter unit, that the second shall be as good 

 as the first, and the third as good as the second. While the 

 above is true there are very great irregularities in the richness of 

 ore deposits, both favorable and unfavorable, due to multifarious 

 causes, which I cannot possibly discuss tonight, but which I con- 

 sidered somewhat fully in the Institute paper. 1 These irregu- 

 larities are especially marked in the upper 300 meters of a 

 deposit ; so that in many cases if the unit of measurement were 

 10 meters or 30 meters, or in a few cases 100 meters even, it 

 might be said that deposits are becoming richer with depth ; 

 although the reverse also occurs in many cases. The truth is 

 that in the uppermost part of an ore deposit the variations in 

 richness with depth are extreme, and no definite rules can be laid 

 down in reference to them. 



Now what is the explanation of these irregularities and of 

 the very general diminution of richness with depth? What is 

 the explanation in some cases of the relatively even values at 

 variable depth ? The last question will be first considered. 



In those instances in which the tenor is maintained or 

 practically maintained from the surface to a great depth the ore 



'Some Principles Controlling the Deposition of Ores, by C. R. Van Hise: Trans. 

 Am. Inst. Min. Eng., Vol. XXX, 1900, pp. 102-112. 



