VADOSE ORE DEPOSITION 
283 
Although the fact of the great unevenness of the upper surface 
of the sulphidic ores in the dry regions is noted by many writers, 
it does not appear that any adequate explanation has ever been 
advanced. The controlling factor of this remarkable unevenness 
is to be sought in the-peculiarities of arid climate. Under these 
conditions, as already shown, chemical decay of the rocks goes on 
only along fault-lines or joint-cracks; and groundwater-level lies 
deep. General rock-wasting proceeding mainly through mechan¬ 
ical means, the rock at the surface of the ground is altered but 
slightly, or not at all. 
The great variation in depth to which oxidation proceeds is well 
illustrated in many mines throughout the arid districts. Some 
extreme cases may be noted. In the Caballos range, in southern 
New Mexico, glistening lead-sulphide bodies of considerable extent 
outcrop, whereas, only a short distance away, a mineral vein is 
completely oxidized to a depth of over 1000 feet. Similar note¬ 
worthy instances occur in the Spring Mountain mining district, 
west of Las Vegas, Nevada. Such marked irregularities often 
occasion no little uncertainty in mining operations. 
Concerning this great and general irregularity of depth of the 
oxidized ores several important observations may be made. In 
consequence of the absence of general chemical decay of the rocks 
old^ sulphidic ore-bodies may be, through the effects of general 
erosion, or deflation, eventually uncovered and exposed at the 
surface of the ground in a perfectly fresh state, or with only a 
thin film or coating of the carbonates. The ancient mineral-vein, 
or ore-body, wastes away in the same way and at the same rate 
as the rest of the rock-mass. 
On the other hand open fault-planes, joint-cracks or other frac¬ 
ture-lines in the rocks permit the meager supplies of meteoric 
waters to penetrate to great depths before finally reaching per¬ 
manent water-level. Sufficient moisture accumulates along these 
paths to facilitate chemical alteration of both the ores and the 
wall-rocks. In dry regions migration of ore-materials doubtless 
takes place chiefly through means of the soluble chlorides in¬ 
stead of the sulphates, as under normal climatic conditions. 
The occurrence of sulphidic ore-bodies high above groundwater- 
level in the very midst of the oxidized ores is often explained on 
19 Economic Geology, Vol. II, p. 774, 1907. 
