786 



SCIENCE. 



[N. S. Vol. XIV. No. 360. 



iron has been taken out above the 1,000- 

 foot level than below it. If time sufi&ced, 

 similar instances from all the old mining re- 

 gions of the world could be cited. 



I by no means assert that the above illus- 

 trations represent an invariable rule. In- 

 deed, this is not the case. But the illustra- 

 tions undoubtedly do represent the average 

 relation of richness and depth. 



Now this story is an entirely different one 

 from the illusion of increased richness with 

 depth based upon the supposition that the 

 metals for the ores are derived from the 

 unknown depths of the earth. If that 

 theory be true, it is natural to believe that 

 ore deposits upon the average will become 

 richer v^ith depth. But if it be a pure 

 unverified hypothesis, not supported by any 

 facts, it is of vital importance for practical 

 mining men to know this, since the knowl- 

 edge will save them vast sums of money 

 expended in exploration under a false 

 theory. So far as I am aware of the facts, 

 I do not know of a mining region in the 

 world which supports the theory of in- 

 creased richness with depth, if the unit of 

 measure be taken as one thousand feet, if 

 the first thousand feet be compared with 

 the second thousand, and the second thou- 

 sand with the third, and so on. In fact, 

 nine mines out of ten, taking the world as 

 a whole, are poorer the second thousand 

 feet than they are the first thousand feet, 

 and are poorer the third thousand feet 

 than they are the second thousand feet. 

 Many ore deposits have been exhausted 

 or have become so lean as not to warrant 

 working before thel,000-footlevel is reached; 

 a large proportion before the 2, 000- foot 

 level is reached ; while comparatively few 

 ore deposits have been found to be so rich 

 as to warrant working at depths greater 

 than 3,000 feet. 



There are, however, some ore deposits 

 which are not known to gradually decrease 

 in richness with depth so far as yet ex- 



ploited. There are a considerable number 

 of deposits which perhaps after a first rapid 

 decrease in richness maintain their tenor 

 pretty well to the depth of 1,000, 2,000 or 

 even 3,000 feet, and some few deposits 

 maintain their richness at even greater 

 depths. Bat we cannot reasonably hope 

 that a deposit will get richer with depth, pro- 

 vided we use a 1,000-foot unit for measure- 

 ment. The most sanguine view which is 

 ever justified for any deposit is that, using 

 a 1,000-foot unit, 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 unfavor- 

 able, due to multifarious causes, which I 

 cannot possibly discuss to-night, but which 

 I considered somewhat fully in my Institute 

 paper.* These irregularities are especially 

 marked in the upper 1,000 feet of a de- 

 posit ; so that in many cases, if the unit of 

 measurement were 25 feet or 100 feet, or in 

 a few cases 500 feet 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 up- 

 per parts of ore deposits the variations in 

 richness with depth are extreme, and no 

 definite rules can be laid down in reference 

 to them. 



This rule of the diminution of richness 

 with depth is one of averages only when 

 considerable depths are taken into account. 

 The factors entering into the production of 

 an individual ore deposit are so numerous, 

 and the irregularities are so great, that the 

 rule cannot be asserted in advance of de- 

 velopment of an individual mine without a 

 study of the conditions there obtaining. 



Now what is the explanation of these ir- 

 regularities and of the very general diminu- 

 tion of richness with depth ? What is the 



* ' Some Principles controlling the Deposition of 

 Ores,' by C. E. Van Hise : Trans. Am. Inst. Min. 

 Eng., Vol. XXX., 1900, pp. 102-112. 



