3 6 WILLIAM H. EMMONS 



not everywhere present) may project upward far into the zone of 

 rich oxidized ore, or into the zone of leached oxides, or may outcrop 

 at the surface. The zone of sulphide enrichment nearly always 

 contains considerable primary ore, and very often the secondary 

 ore is merely the primary ore containing in its fractures small 

 seams of rich minerals. The zone of enriched oxides is generally 

 found above the water-table when the latter is at the lowest, and 

 often extends to the outcrop. In regions of rapid erosion, and es- 

 pecially of rugged topography, the conditions for the exposure of rich 

 oxides, or even rich sulphides or primary ore, are more favorable. 

 In places along the outcrop of a deposit where erosion is rapid the 

 richer oxidized or sulphide ores may be exposed, whereas in other 

 places, protected from erosion, and therefore exposed longer to so- 

 lution, the same outcrop is frequently leached. It is evident that 

 the amount of metal remaining in the upper part of the oxidized 

 zone and at the outcrop depends upon the ratio between the rate 

 at which the metal is dissolved, and the rate at which the value- 

 less constituents are dissolved and removed. Under certain 

 conditions gold is removed very slowly, and the removal of value- 

 less constituents may effect a concentration at the very apex of 

 the lode; while under other conditions, favorable to the solution 

 of gold, it is removed more rapidly than silica, iron, etc., and the 

 apex and the oxidized zone are leached. In a country not subject 

 to erosion it would be supposed that the outcrops of manganifer- 

 ous lodes would be everywhere leached; but rapid erosion may 

 remove the upper part of the lode before it is completely leached, 

 and, under favorable conditions, placers accumulate from the debris 

 of the apex. 



It thus appears that all of these zones except that of the pri- 

 mary ore are continually descending; so that ore taken from the 

 outcrop may represent what was once primary ore; afterward, 

 enriched sulphide ore; still later, oxidized enriched sulphide ore; 

 later still, leached oxidized enriched sulphide ore; and finally 

 become the surface ore. Through more rapid erosion at some 

 particular part of the lode, any one of these zones may be exposed; 

 and hence an outcrop ore of any character is possible. Conse- 

 quently, longitudinal assay plans, showing the changes of value 



