306 Bernard Smith—Upper Keuper, East Nottinghamshire. 
fragments are very even grained, their different percentages in the 
lamine being doubtless due to oscillations in the amount of supply 
during sedimentation. The smaller size of the dolomite grains, as 
compared with the quartz, may be explained by the higher specific 
gravity of the dolomite. 
The contorted structure shown in Pl. X XVI, Fig. 1 is not so easily 
explained. If the upper part of such aslab is stripped off—as it often is 
by weathering—an irregular lumpy surface, consisting of smooth curves 
and folds, would be exposed. This appearance, taken in conjunction 
with the occurrence of dolomite, is suggestive of concretionary action ; 
I would, however, attribute it mainly to disturbance of the ripples 
while the deposit was still in a soft and pasty condition. In a few 
cases, however, there is a suggestion that in addition to ripple-marks 
the layers of sediment were arranged in rounded curves (somewhat 
like those in some parts of the Cotham Marble), but afterwards disturbed 
by current-action. Freshets of water, producing swirling currents, 
Fic. 1. Stone in wall of Sutton Church, Notts, with contorted lamine. Nat size. 
would sweep along with them a heavy burden of sand, and tear up 
(Fig. 1) or contort underlying ripple-marked layers, embedding them 
in a structureless deposit. When the flood-waters began to come 
to rest a layer of ripple-marked sediment passing upwards into 
drift-bedding (Fig. 2) would be deposited upon the top of the broken ' 
and contorted layer. 
An actual case of the formation of such structures is quoted by 
Mr. A. W. Rogers, of the Cape Geological Survey, with whose per- 
mission the accompanying photograph (Pl. XX VI, Fig. 2) is reproduced. 
“At the mouth of the Prieska ravine a certain bed of alluvium on the 
left bank has its lamine contorted. They often stand vertically, and 
are even curved into the shape of an inverted bulb. Above and below 
this layer the lamine are false-bedded or lie flat.”* He attributes 
! Sorby (Quart. Journ. Geol. Soc., 1908, vol. Ixiv, p. 197, pl. xiv) has shown how 
the sandy ashes of the Langdale slates were deposited sometimes as ripple-drift, 
which became torn up and contorted as the velocity of the sediment-bearig current 
increased. Sorby’s experiments seem to show that a current of about 2 inches per 
second in shallow water would suflice to form ripples in sediment as fine as that of 
these skerries. A velocity of 6 inches per second would wash up and destroy them. 
? Cape of Good Hope, Thirteenth Annual Report of Geol. Commission for 1908, 
p. 107. 
