VADOSE ORE DEPOSITION 
279 
ore-bodies mined were distinctly of deep-seated origin. Desert 
mining regions, where the vadose zone is so very much more ex¬ 
tensively developed than in moist countries, introduce to us many 
novel conditions. 
The theory of gossan-formation assumes as its fundamental 
premise that the mineral-vein is of profound or magmatic origin 
and that the weathering of its surficial portion gives rise to the 
rusty oxidized ores. In general this is in fact only partially true, 
since the vadose region with which the majority of us are most 
familiar is the quite limited weathered zone of normally wet or 
moist countries. Under such conditions the subject of ore-deposi¬ 
tion receives most attention; yet it is under these very circum¬ 
stances that the distinctive features of the vadose zone are most 
completely intermingled and most thoroughly obscured. 
In moist lands, as is well known, the chemical decay of rock- 
masses usually goes on at the surface of the ground much faster 
than the removal by erosion of the resultant rock-waste. In 
many mineral regions secular decay seldom extends downward 
more than a score of feet. Gossans formed under such condi¬ 
tions are more or less completely leached of their ore-materials. 
The relative small amount of ore not carried away from the upper 
weathered part of a vein collects at the bottom at groundwater- 
level as an insignificant “enriched” layer. 
So inconsequential commercially is the enriched layer often that 
it is in many mining operations passed through unnoticed. When¬ 
ever important ore-bodies are encountered at the groundwater- 
level, their origin is commonly ascribed to causes other than the 
results of strictly vadose deposition as it is now known to be. 
The contrasts between ore-forming conditions in the vadose zone 
in moist and in dry climates are much stronger than might at first 
glance appear. 
In contradistinction to the gossans of normal moist regions the 
vadose zone under conditions of excessively dry climate presents 
several peculiarities. In arid districts the absence of chemical 
rock-decay is especially noteworthy. Rock-weathering, or general 
rock-disintegration, takes.place mainly through spalling or insola¬ 
tion. Great depth is one of*the striking features of the vadose 
zone. Its great thickness gives exceptional control of ore-localiza¬ 
tion by geologic structure. The marked irregularity in depth of 
