Gravity flow mixing apparatus for the packed bed flow mixing of particulate matter, such as hot, retorted oil shale, comprises a mixing tower having therein a plurality of parallel mixing tubes. A plurality of three dimensional mixing members are axially spaced apart at about 60.degree. to about 120.degree. rotational separation so that, with packed bed flow through the tubes, each member is below the particulate matter repose surface at the above-adjacent member. Means are included for introducing a cooling fluid, preferably, water, into the tubes just above one or more of such repose surfaces, apertures being provided in the tubes to enable disengagement of gases, for example, steam, from the particulate matter. Comprising each mixing member are primary, secondary and tertiary mixer elements. The primary element has a "W" shaped free edge and the secondary and tertiary elements each have first and second triangular sides connected together to form a linear peak and define a tetrahedral shape, the tertiary element being substantially smaller than the secondary element. The secondary element is joined to the primary element so that the secondary element projects upwardly and outwardly from the primary element. In turn, the tertiary element is joined to the secondary element so that the linear peaks of each meet at an obtuse angle, the tertiary element thereby projecting forwardly and outwardly from the secondary element at upper regions thereof. Corresponding mixing methods are provided.