This paper presents the results of a series of 10 experiments made in the laboratory with a wooden trough, supplemented by 10 cases taken from actual placer maps. These data give the following concordant results, checking with each other:1. Deposition areas and erosion areas alternate with each other.2. The ore-concentration zones coincide with the erosion areas, whereas the barren zones coincide with the sedimentational areas.3. The ore-concentration zones occur in a logarithmic sequence as established by log-log diagrams.In consequence, a law may be postulated as follows: There is a logarithmic sequence of rich erosion areas alternating with barren sedimentational areas.This will prove useful in the interpretation of placer maps, in the checking of the quality of a prospecting, and in the explanation of irregularities, where interference factors are to be taken into consideration.