Origin of ptygmatic features

A definition is suggested for ptygmatic features. Five explanations, with minor variations, have been offered: 1. (1) Injection along meandering cracks (Read, 1931). 2. (2) Viscous flow (Sederholm, 1926). 3. (3) Injection into an incompetent host and crumpling where the ribbon of magma meets a resistance (Wilson, 1952). 4. (4) Anatectic growth in a passive host (Dietrich, 1959). 5. (5) Crumpling of an originally planar vein by subsequent compression (Milch, 1900). 1. (1) is ruled out among others by mis-fit of the walls, 2. (2) is based on unsound flow mechanics, 3. (3) is excluded by variations in thickness of single veins, by structureless hosts, and by the impossibility of hydrostatic overpressure to force the magma into the more mobile host, 4. (4) by its failure to explain uniform wall distance and because of the necessity to assume resorption of vein material concomitant with deposition exactly on opposite sides. (1) and (4) fail to explain the rough relation between thickness and wave length. Analogous structures produced by deformation of sedimentary beds, sand dikes, or quartz veins, and in experiments support Milch's view 5. (5). It is a matter of definition whether these are included under the term ptygmatic. A search for the correct interpretation is important because ptygmatic features may mean either zero reduction in thickness of the host rock, or, conversely, up to six-fold thinning.