VADOSE ORE DEPOSITION 
277 
features. Had clear distinction between the two been possible 
I 
early in the serious consideration of the subject of ore-genesis, the 
long controversy over the ascension and lateral-secretion hypothe¬ 
ses might never have taken place, and the true progress of our 
knowledge regarding ore-formation might have been far more 
rapid than it has been. 
Until Franz Posepny ^ so strongly emphasized, a few years 
ago, the fundamental diilerences between ore-forming conditions 
above and those below groundwater-level it was the well-known 
custom to regard the upper ores as merely the weathered parts of 
mineral veins. The distinction thus made was, as S. F. Emmons 
recently observed, one of the most valuable features of Professor 
, Posepny’s now famous memoir. Of late years ores formed in the 
vadose zone have assumed an ever increasing genetic importance. 
No longer are they all regarded as simply the oxidized portions 
of deeper formed ore-bodies. Moreover, they do not necessarily 
have any immediate or direct association with profound veins. 
As an independent genetic group they appear in every way com¬ 
parable to the known ore-bodies formed through deep-seated 
agencies. Commercially it may be questioned whether the strictly 
vadose ores do not constitute the major part of ores mined in the 
world today. 
The genetic association of a universal bonanza zone with vadose 
ore-deposition is distinctly a result of arid-region experience. As 
recently emphasized, under conditions of excessively dry climate 
the vadose zone attains a depth and importance unknown else¬ 
where. From the relatively thin band which it presents in the 
normal moist land it not only expands in the desert to a zone of 
great magnitude but it becomes separable into recognizable sub¬ 
divisions. These certain features which vadose ores present in 
arid regions have special genetic significance. 
At the outset it is with great advantage that certain essential 
differences between the older mining conception of gossan and the 
newer idea of vadose zone are noted. In moist climates, where 
modern mining had its birth and greatest development, and where 
the weathered-rock zone is relatively thin, the rusty surface-ores 
naturally appear to be merely the oxidized portions of sulphidic 
veins and masses that have originated from the depths. As first 
2 Trans. American Inst. Mining Eng., Vol. XXII, p, 213, 1893. 
