Bernard Smith— Upper Keuper, East Nottinghamshire. 309 
remarkably constant (nearly 900 feet), and contrasts strongly with 
the variable thicknesses encountered near the old shore-line near 
Nottingham, a fact which once more emphasizes the uniformity of 
subsidence and isostatic conditions in the centre of the depression. 
The presence of massive gypsum near the top! of the division at 
Newark seems to favour the idea that the water also became 
increasingly saline. The fibrous and platy gypsum in the lower 
skerry-belts was no doubt deposited as the marls dried and got rid of 
their included waters. 
Source of Sediment.—As to the source of the dolomite, its detrital 
nature and manner of deposition show that it was not formed where it 
is now found, but was probably deposited contemporaneously near the 
shore-line and then swept out by current-action. 
Dr. C. G. Cullis recently reported on the occurrence of small isolated 
rhombs of dolomite* in the Keuper Marls of Westbury-on-Severn and 
as far north as Worcester. The crystals are extraordinarily perfect 
and always very minute, and occur in both the red and green marl, 
sometimes in great profusion. A washed residue of the marl contained 
practically nothing but quartz-grains and these dolomite rhombs; 
that is to say, the constituent minerals of the skerries are also 
scattered throughout the shales and marls associated with them. 
Dr. Cullis is inclined to think that the dolomite rhombs were 
precipitated directly from solution. They may have been in the first 
case; in one band at least (from the Waterstones), which was 
examined microscopically, the dolomite seems to have been formed in 
situ. In most cases, however, the rhombs have suffered attrition by 
being drifted some distance together with the quartz and other detrital 
fragments. 
The occurrence of grains of dolomitic sandstone in the higher type 
of skerry is suggestive, as showing that some dolomitic rocks were 
also exposed to denudation at this time. Most of the quartz and 
felspar no doubt was derived from Carboniferous sandstones and grits, 
and to Carboniferous rocks also we must attribute the basis of the 
finer caleareous and dolomitic shales and marls. 
The Finer Deposits.—Just as there are differences in the lithological 
characters of the skerries, so also the marls themselves differ from 
point to point and occur in lithological belts of perhaps as great 
horizontal extent as the skerries. The differences are brought out by 
the character of the soils on the outcrop. It is well known that the 
marls contain a high percentage of silica and are practically fine silts : 
the coarser silts pass into sandy and dolomitic shales or marls, which 
break down into a sandy soil; the finer ones behave as tough homo- 
genecus clays. Above a skerry it is common to find sandy shales pass 
gradually upwards into sandy marl, showing ripple-marks, and then 
into fine homogeneous marl. 
Sorby has shown that such homogeneous clays may be deposited 
either by gentle and uniform currents drifting along fine sediment to 
spots where there is scarcely any current at all, or to quick deposit of 
‘ Massive gypsum is recorded from a lower horizon at Clarborough, north of 
Retford. 
* Rep. Brit. Assoc. for 1907, pp. 506, 507. 
