282 
VADOSE ORE DEPOSITION 
action in accomplishing the separation of the lighter minerals of 
the finer rock-waste from the heavier materials which tend to be 
left behind.^^ I have lately quite fully discussed the various 
aspects of this subject as observed in the desiccated regions of 
southwestern United States and northern Mexico. 
The remarkable distances downward to which typical vadose 
conditions prevail, despite the extreme irregularities of the zone, 
are especially emphasized in their recent consideration in the 
northern part of the Mexican tableland. In many localities of 
this region, as at Santa Eulalia, in Chihuahua, and in the Sierra 
Mojada, in Coahuila, 1000 and 1500 feet are not uncommon 
depths to be attained before groundwater-level is encountered. 
At Tonapah, Nevada, Spurr reports the Desert Queen mine dry 
at a depth of over 1100 feet. Recent writers, in calling attention 
to the great depth which the so-called gossan often attains in cer¬ 
tain places in the dry belts of western United States, do not seem 
to appreciate the significance of the phenomena in its bearing 
upon ore-genesis: yet the facts recorded supply some of the most 
valuable available data. In this connection such descriptions as 
that by S. E. Emmons,^^ of the Horn Silver mine, near Milford 
Junction, Utah, are especially instructive. 
The great depths to which oxidized ores extend give rise to 
some unusual conditions. In some localities where geologic struc¬ 
tures produce local impounding of mine-waters sulphidic ore- 
bodies are formed high above general groundwater level in the 
very midst of the oxidized ores. The great sulphide ore-lenses at 
Magdalena, New Mexico, which I have recently described in 
some detail, may have been produced in this way. The important 
point is that such sulphide bodies are not the remnants of original¬ 
ly deep-seated ores, but that they are formed within the vadose 
zone contemporaneously with the oxidized ores and from the same 
ore-materials, none of which need be immediately connected with 
primary sulphide ore-bodies. 
The subject of depth as related to the vadose zone in desert 
lands requires especial elaboration which cannot well be given in 
the present connection. 
14 Trans. American Inst. Mining Eng., Vol. XLI, p. 153, 1910. 
15 Bull. Geol. Soc. America, Vol. XXI, p. 565, 1910. 
16 Prof. Pap., U. S. Geol. Surv., No. 42, p. 106, 1905. 
17 Trans. American Inst. Mining Eng., Vol. XXXI, p. 675, 1902. 
18 Mining Magazine, Vol. XII, p. 109, 1905. 
