Page 226 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 197

4.1 Sieving in the Middle Ages (from Agricola, 1556).

lognormal characteristics; if plotted on lognormal paper, the graphs will assume
normal proportions.
The need to cope with very small measurements is only partly satisfied by
micrometre measurements, which are unwieldy in the finer sizings (e.g. 1 m ˆ
0.001 mm). Krumbein (1941) found it more convenient to express each division
of the Wentworth scale in terms of the negative logarithm to the base 2 of the
diameter d of the particle in millimetres.
2
' ˆ ÿlog d 4.1
Krumbein introduced the phi (') unit of measurement, expressing each division
of the Wentworth scale as one ' unit, while retaining its essential features in a
simplified form. Whole ' numbers are substituted for the small Wentworth
fractions and because the negative logarithm is used, the ' sizings increase