392 
VADOSE ORE DEPOSITION 
frequently the hanging-wall is much more affected by replacement 
agencies than the foot-wall. It is to this fact that Rickard 
calls attention; and to which Moesta refers in his account of 
Chilean silver deposits. Numerous other examples might be 
mentioned. The subject of mural replacements in the vadose 
zone requires special elaboration. 
Recapitulating, the consideration of the vadose zone as dis¬ 
played under climatic conditions of aridity discloses a number 
of novel elements bearing directly upon the genesis of ore de¬ 
posits generally: 
(1) . Absence in the arid regions of appreciable rock-decay 
introduces several hitherto entirely unvaluated factors concern¬ 
ing the formation of the so-called gossans. 
(2) . Within the greatly expanded vadose zone of dry climates 
ore-forming conditions, processes and products constitute a dis- 
distinctive class, comparable on the one hand to those of the 
profound zone and on the other hand to that on the surface of the 
ground. > 
(3) . Establishment of well-defined subzones in the vadose 
region promises to be of the greatest practical value in mine- 
development throughout the arid regions. 
(4) . Close association of the form of ore-bodies with tectonic 
features gives at once a rational foundation for a ready classi¬ 
fication of a large group of ore deposits, and in the field a practical 
clue to their exploration and exploitation. 
(5) . Clear distinction is to be made between ordinary gossan 
phenomena of lands having abundant moisture and the somewhat 
simulating phenomena of the zone above groundwater-level in 
arid, regions. 
(6) . Impondment of groundwaters within the vadose zone 
of desert regions, by locally imposing profound conditions, is a 
determining factor in the localization of many ore-bodies. 
(7) . Formation of sulphidic ores within the oxidized zone 
may take place as readily under local impondment conditions as 
it is accomplished in the profound zone. Only in exceptional 
instances does oscillation of groundwater-level, probably account 
for position of sulphidic ore-bodies high up in the vadose zone. 
43 Trans. American Inst. Mining EJng., Vol. XXVI, p. 193, 1897. 
44 Ueber das Vorkommen der Chlor-, Brom-, und Jod-verbindungen des Silbers in 
der Natur, Marburg, 1870. 
