VADOSE ORE DEPOSITION 
281 
conditions could hardly exist did* rock-decay proceed to any 
marked extent. 
Preconceived notions concerning the gossans of moist regions 
accord not at all with the facts observed in arid lands. Ore- 
bodies are formed that manifestly have no association whatever 
with thermal waters from the depths. “Enriched” ore-bodies it 
has been conclusively proven, often gather their metallic materials 
not from the remnants of former veins but from a wide area of 
the country rocks themselves, even when the latter are not or¬ 
dinarily regarded as metalliferous. The ground-waters with their 
metallic salts in solution are directed along paths that are as 
immediately dependent upon definite geologic structures as are 
accumulations of rock-oil.^® This I have shown to be also true 
in moist regions. Were Sandberger’s principles of lateral secre¬ 
tion of ore deposits literally and alone applied to the vadose zone 
of arid regions, they would as truly meet actual conditions as does 
the ascension hypothesis the profound conditions.^^ 
Under normal climatic conditions the continual breaking down 
of rock-masses at the surface of the ground is a process that is, 
as is well known, both chemical and mechanical in character. Al¬ 
though genetically entirely unrelated clear distinction between the 
two is not always made. In moist climates chemical decomposi¬ 
tion of the rocks so greatly predominates over their mechanical 
disintegration that the effects of the latter process are in compari¬ 
son scarcely noticeable. In the arid regions not only is the very 
reverse true but it is so to a preeminent degree. 
The expansion and contraction which exposed rock-masses, 
rock-fragments and the components of the soils of the desert 
‘undergo on account of the great diurnal range of temperatures 
appears to be the principal method of rock-weathering. Insola¬ 
tion it is appropriately termed. The rock-surfaces are constantly 
kept fresh through the continual removal of the finer debris by the 
winds. In its larger phases Streeruwitz graphically describes 
the process as it operates in the hot, cloudless deserts of Trans- 
Pecos Texas. 
Wind-scour, or deflation, is in fact far more potent than water- 
10 Economic Geology, Vol. II, p. 780, 1907. 
11 Eng. and Mining Jour,, Vol. EXXXVII, p. 1048, 1909. 
12 Trans. American Inst. Mining Eng., Vol. XEI, p. 140, 1910. 
13 Texas Geol. Surv., 4th Ann. Kept., p. 144, 1892. 
