284 
VADOSE ORE DEPOSITION 
the hypothesis that in arid regions the permanent water-level 
changes from time to time. In central Nevada, for instance, 
A. C. Lawson ascribes such occurrences in the Robinson Mining 
District, near Ely, to a higher groundwater-level when the climate 
was much moister than at present. Others, notably J. E. Spurr,^^ 
lay particular stress upon the wet climatic conditions during 
former geologic periods, as the Glacial Epoch, for example. 
Plausible as is this explanation, it does not appear to be satis¬ 
factorily supported by unquestionable data. Changes controlled 
by climate are apt to be excessively slow. Until it is shown that 
these various occurrences of sulphidic ores within the oxidized 
zone are not the direct result of local impounding of waters due to 
favorable geologic structures, it may be seriously questioned 
whether the changes in groundwater-level have been so rapid or so 
marked as has been sometimes inferred. In a number of cases 
in which it was at first thought that decided oscillations of the 
groundwater-level had occurred it was found upon further inves¬ 
tigation that the sulphide-bodies were disposed in basins in which, 
owing to peculiarities of geologic structure, impounding conditions 
of the groundwaters prevailed, producing, as it were, all the neces¬ 
sary factors characterizing the permanent groundwater zone. 
That it is possible for groundwater-level slowly to change can¬ 
not be disputed. That the rate of such change is sufficiently rapid 
to be recorded in the oxidation of ore-bodies may be seriously 
questioned. That its oscillations are directly referable to great 
climatic swings between excessive precipitation and excessive 
drought is an assumption which may be safely denied. The 
seasonal variations in the volumes of water encountered in mines 
— as is notably the case of the Joplin zinc district, for example — 
have little critical bearing upon this point, since the extensive 
pumping of water from the mines produces artificial conditions. 
In moist climates the rainfall is so copious that the zone of per¬ 
manent water saturation is always quite near the surface of the 
ground, and any oscillation of the groundwater-level would be 
hardly noticeable even under the most favorable circumstances. 
In arid lands where the vadose zone presents great extent the 
opportunity for recording oscillations of the permanent water- 
table is unexcelled; yet direct and unquestionable evidence in sup- 
20 Bull. Dept. Geol., Univ. California, Vol. IV, p. 331, 1906. 
21 Bull. Geol. Soc. America, Vol. XII, p. 266, 1901. 
