280 
VADOSE ORE DEPOSITION 
the vadose zone finds no parallel in normally moist climates. 
Local impounding conditions of mine-waters give rise to the 
paradox of the occurrence of sulphide ore-bodies far above 
ground-water level in some localities, and of oxidized ores appar¬ 
ently far below it in others. A basal bonanza-band of sulphidic 
ores is developed to an extent unknown elsewhere. Recognition 
of distinct subzones is an unexpected aid to the interpretation of 
vadose phenomena. 
In arid and in cold regions, as I have recently pointed out ® in 
considerable detail, rock-masses scarcely suffer any chemical 
change at the surface of the ground. Disintegration of the rocks 
is almost entirely mechanical in character; in comparison the chem¬ 
ical effects are quite insignificant. The slight amount of chem¬ 
ical decomposition which rock-materials undergo is well shown by 
the great talus-slopes and other accumulations of colluvial deposits 
which form veritable rubble-piles often of huge proportions and 
with materials so fresh to all appearance that they seem to have 
come directly from some gigantic rock-crusher. When examined 
under the microscope even the adobe soils of the arid region attest 
the strictly mechanical origin of the finer debris. 
In the desert areas of western United States, for example, only 
in open mineral veins and in the crushed belts attending fault- 
planes does the scanty moisture accumulate and normal signs of 
chemical rock-decay appear. Not only do the rugged mountain 
ranges of the desert disclose little rock-decay but the substructure 
of the intermont lowlands rarely displays conspicuous evidences 
of chemical decomposition. As a factor in the general reduction 
of the land-surface towards sea-level, the chemical decomposition 
of rock-masses under conditions of arid climate may be entirely 
neglected. 
Many of the broad intermont desert plains are covered to slight 
depths only by soil materials; nearly everywhere the beveled edges 
of the rock-strata are presented with little or no indications of 
chemical decay."^ Similar conditions are reported by W J McGee ® 
on the coastal plains of Sonora in Mexico. Passarge’s wide ob¬ 
servations in the South African deserts are of like import.® Such 
6 Bull. Geol. Soc. America, Vol. XXI, p. 569, 1910. 
7 Bull. Geol. Soc. America, Vol. XIX, p. 63, 1908. 
8 Bull. Geol. Soc. America, Vol. VIII, p. 991, 1897. 
9 Zeitschrift der deutchen geologischen Gesellschaft, L,VI. Band. Protokol, p. 196, 
1904. 
