Abstract: A carriage movable back and forth along a tank of water has an agitator which washes floatable material and moves it to a floating material conveyor at one end of the tank. Denser, sinkable material washed from the floatable material drops onto a sunk material conveyor coursing along the bottom of the tank. The drive of the carriage is reversible and, when excess resistance is met when the carriage is traveling toward the floating material conveyor, the resistance actuates a control to reverse the drive of the carriage.