352 
THE CRETACEOUS ROCKS 01' BRITAIN. 
indicated also by the extremely even size of the constituent frag¬ 
ments, and by the layer of nodules which is so generally found at 
its base ; these nodules being indurated and partially phosphatised 
pellets or pebbles of Chalk Marl. 
Now the current which carried away the finer material can 
hardly have been the same current which brought the argillaceous 
material of the marly chalk below. Moreover, the fact that in some 
places, as at Arlesey and at Tring, there are two beds of shelly stone 
divided by a band of marly chalk, is strongly suggestive of two 
currents, the prevalence of the one causing the formation of quite 
a different kind of deposit from that formed under the influence 
of the other. We need not imagine, however, that these currents 
were in actual conflict, because the one may have been a surface 
current, and that which controlled the formation of the Totternhoe 
Stone was certainly a bottom-current. It was probably only a 
temporary cessation or diversion of the ground-current which 
allowed of the deposition of the intercalated bands of chalk marl. 
In the case of the Totternhoe Stone it is noticeable that it attains 
its maximum thickness in the counties of Cambridge, Hertford, and 
Bedford, and that it has not been traced to the south of Chilton, 
in Berkshire, although there are beds which resemble it in North 
Wilts and North Hants. On the other hand, it extends as a thin, 
but continuous bed through Norfolk, Lincolnshire, and Yorkshire, 
where the underlying beds are not marly. These facts seem to 
prove that the current which controlled its formation was quite 
unconnected with the mud-bearing current, and that it did not come 
from the south nor from the north, where the bed is thinner and 
less sandy than in Cambridgeshire. On the whole, it seems most 
probable that the ground current set from some westerly direction, 
either from the south-west or possibly from the inlet opening to 
the north-west; and that in either case it swept eastward and north¬ 
eastward outside the Pennine promontory. 
The layer of pliosphatic nodules which so often occurs at the base 
of the Totternhoe Stone, and the frequency of such nodules and of 
phosphatic fragments in the stone itself, are also evidences of current 
action. In this connection also may be mentioned the occasional 
occurrence of phosphatic nodules about the same horizon elsewhere, 
as at Punfield Cove, on the Dorset coast, and at Compton Bay, in 
the Isle of Wight. 
These phosphatic nodules appear to be similar to those which 
frequently occur in modern sediments of all kinds within a certain 
distance of Continental coast-lines. It has been pointed out by 
Walther* that such phosphatic concretions are most frequent off 
those coasts where, owing to the meeting of warm and cold currents, 
the water is subject to marked and rapid changes of temperature, 
as off the Cape of Good Hope and on the east coast of North America, 
He thinks it probable that large numbers of marine organisms are 
killed by the changes of temperature at such places, and that an 
* Einleitung in die Geologie als historische Wissenschaft, p. 699, Jena, 1894. 
