Columns of rock fragments show a wide variety of form and structure and are associated with many different kinds of ore deposits. The fragments range from angular to rounded and from blocks weighing many tons to the particles that constitute a matrix of rock paste. The columns generally develop on planar structures or at structural intersections mostly in competent rocks.Two broad divisions of mineralized rock fragment columns are proposed, namely pre-hydrothermal and co-hydrothermal. In the pre-hydrothermal type the spatial coincidence between the column and the ore shoots in most cases is poor because the column developed prior to and independently of mineralization, and other controls besides the configuration of the column were operative in localizing the ore. In the co-hydrothermal type there is closer coincidence in time and generally in space between ore and the column because the column results from the corrosive action of the hydrothermal fluids.Tectonic breccias, volcanic vent fillings, intrusive igneous breccias, and filled sink and circle deposits formed by ground water leaching of carbonate rocks, generally come under the heading of pre-hydrothermal structures. Other breccia columns, hydrothermal pebble dikes, and probably most porphyry copper deposits come under the heading of co-hydrothermal structures.The incidence of ore most likely is higher in co-hydrothermal than in pre-hydrothermal structures though important ore deposits occur in both types.