ABSTRACT

Analyses of grain-size measurements by sieving and settling-tube techniques generally represent the grain size as "sieve diameters" and "sedimentation diameters," artificially replacing the natural, irregular grains with equivalent spheres. Depending on the true grain shape, this, of course. deviates considerably from their actual dimensions. The present study reexamines the technique of sieving, reconfirming that this approach sorts the grains by shape as well as by size. The grain's intermediate axial diameter is the main controlling factor in its passage through sieve screens, and the analysis procedure of this study yields the distribution of grain intermediate diameters from the sieving measurements rather than the "sieve diameter." This permits a direct comparison with size dist ibutions obtained by settling-tube analyses in that an analysis procedure is also presented where measured settling velocities are similarly converted into grain-intermediate diameters rather than to "sedimentation diameters" of spheres. Comparisons of sieving and settling-tube analyses of the same sand sample are found generally to yield nearly the same distributions of intermediate-grain diameters. The remaining differences are due primarily to the heavy mineral contents of the sands, unusual grains such as micas, some uncertainties in the evaluation of settling velocities of fine-grained natural sediments, and, in our particular case, possible errors associated with the sedimentation balance we employed.