The following conclusions result from field and laboratory studies of meso- and hypothermal vein matter in many widely separated districts: Quartz can replace country rock, whether directly or through the intermediary of carbonate, but in aluminous rocks such replacement does not normally extend far from the channelways in which solutions flow. Where there are closely spaced surfaces for attack by solutions, as in zones of shearing, sheeting, schistosity or thin bedding, replacement of rock by gangue ranges from partial to thorough. Partial replacement leaves residue of the layers or slices of wall rock interlaminated with the quartz, i.e., book structure. More advanced replacement obliterates original rock texture and leaves only films or septa representing (a) remnants of the book-leaves, (b) difficultly replaceable slips, and (c) material rejected by quartz while crystallizing. The result is ribbon structure. Replacement is comparatively ineffectual against walls of open fissures or large breccia fragments, where ratio of surface to volume is low. Quartz representing open space filling can exist in the same vein or vein system with laminated quartz; the contrasting structures, recording original differences in shape and mechanical origin of channelways, have structural significance.