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Nesteroff discovered, among other interesting facts, that many oceanic turbidites are covered by a thin film of pelagic shells, and he attributed this to deposition from the tail of the turbidity current. Any later pelagic deposit is supposed to have been washed away by the following current, and is therefore missing. Nesteroff's finding is important in itself and throws new light on ancient turbidites. However, the present author favours an explanation through winnowing of the shells from a pelagic stratum by the current that deposited the covering turbidite. This would mean that the shell pavement does not mark the end but the beginning of a turbidite, and that the upper part of the pelitic interval with pteropods and planktonic Foraminifera is a pelagic deposit. Several arguments are presented in favour of the latter view. As Nesteroff and Heezen are led by their interpretation to doubt similarly the presence of pelagic strata between flysch turbidites, a brief survey is made of this subject. It shows that most of the ancient turbidite sequences also contain many beds with a pelagic part towards the top of the pelitic interval. Up to the present, however, no shell pavement has been reported, but careful search will probably reveal examples of such fossil pavements.

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.