Abstract: In jigging for purposes of gravity separation of material to be concentrated the material is sorted into a heavy fraction which is withdrawn through a screen (2) and a light fraction withdrawn above the screen (2), by periodically passing flows from the bottom to the top through the feed material. If the material is predominantly of finer character, there will be a ragging (3) on the screen (2) composed of particles which are larger and usually also specifically heavier than those of the feed material. In this manner, however, the separation of feed material will be unsatisfactory in the fine particle size range having upper particle size limits between 300 .mu.m and 1 mm, particularly so if the particle size ratio of the material is rather broad.