390 
VADOSE ORE DEPOSITION 
latively few instances is the enrichment likely to have been 
derived wholly from the superficial portion of a vein. In the 
majority of cases the surrdunding area doubtless largely con¬ 
tributes the ore-materials. In moist climates vein-stones usually 
decay more rapidly than the country-rock. In desert lands veins 
and fault-lines are about the only places where marked chemical 
change goes on. Very local imponding conditions are imposed 
on the veins in the same way as elsewhere; but in the vadose 
zone they are due to geologic structures. Secondary sulphide 
enrichment of a vein, as generally described, is manifestly due 
to the fact that the metal-bearing waters are imponded rather 
than merely intermingled. Examples are innumerable throughout 
the arid region. The majority of the recent descriptions of 
secondary enrichment are of this class. 
Metasomatic replacement is no doubt a far more important 
factor in rock-change throughout the vadose zone than is generally 
imagined. Several of the controlling conditions of metasomatism 
are usually present in the zone above groundwater-level, although 
it is only in arid regions that they become prominent. The 
essential facts regarding the metasomatic alterations associated 
with fissure-veins have been so admirably and so recently sum¬ 
marized by Lindgren and by Vogt that little need be here 
added. 
In the formation of ore-bodies in the vadose zone, metaso¬ 
matism processes are probably even more active than in the 
profound zone. The field is as yet almost untouched. Rickard 
gives us a glimpse of what some of the changes might be. 
Moesta describes certain of the features as presented in the 
excessively dry region of Chanarcillo, Chile. Moricke alludes 
to other aspects as displayed at Cabeza de Vaca and Caracoles, 
in the Atacama desert. 
In the case of the remarkable zinc-carbonate deposits of the 
Magdalena lead-district (New Mexico)^® where entire beds of 
limestone are replaced by zinc carbonate, every detail of texture 
is perfectly preserved. Calyces and stems of crinoids, brachio- 
34 Trans. American Inst. Mining Eng., Vol. XXX, p. 578, 1901. 
35 Ibid., Vol. XXXI, p. 147, 1902. 
36 Trans., Vol. XXVI, p. 193, 1897. 
37 Ueber das Vorkommen der Chlor-, Brom-, und Jod-verbindungen des Silbers 
in der Natur, p. 26, Marburg, 1870. 
38 Die Gold-, Silber-, und Kupfer-Erzlagerstatten von Chile, p. 27, Freiberg, 1898. 
39 Mining Magazine, Vol. XII, p. 109, 1905. 
