Page 292 - Handbook of Gold Exploration and Evaluation

258 Handbook of gold exploration and evaluation

4.35 Typical changes in development of a Tertiary deep lead (after Macdonald,
1983a).

western USA. Dryness in these regions is due to location, either distance from
the ocean or in a rain shadow on the sheltered sides of mountain ranges. Desert
placers are characteristic of basin and range topography with small watersheds
supplying sediment to alluvial fans. Annual precipitation ranges from 10±50 cm.

Aeolean processes
Wind blowing over the surface of the ground gives rise to forces and stresses at
the ground/air interface that are generally similar in nature to those produced by
a river flowing along its bed, but with differences in scale. Air ( a ˆ 1:25 kgm ÿ3
ÿ3
at sea level) has a much lower mass/unit volume than water ( w ˆ 1:00 g/cc ).
Dynamic viscosities are also much lower for air ( a ˆ 3:62 10 ÿ7 mPas at
40 ëF) than for water ( a ˆ 3:24 10 ÿ4 mPas at 40 ëF). Because of these differ-
ences, impact and viscous forces are correspondingly smaller for air than for
water as also are the respective buoyancy effects of the two fluids. Bagnold