As such, the presence of a strong predictive accuracy for gallium indicates a strong relationship between parent material and soil colour, which is not surprising. An obvious question is there fore why other elements such as vanadium and chromium, the soil contents of which are also related to parent material, are not predicted as accurately. One possible reason for this is that gallium has no biological roles and very few roles in industry (i.e. microelectronics), while elements such as vanadiumand chromium are either more biologically active or have a wider range of uses and therefore potential pollutive sources within industry.