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of a dark clay, probably of the same age, with a greater or 
less per-centage of limestone boulders, may be found in the 
Wharfe valley ; near Ripon ; around Leeds ; filling up abrupt 
hollows between Wakefield and Doncaster ; in a railway 
cutting near Roj^ston, &c. ; but, instead of these patches 
being remnants of a much more extensive deposit nearly 
removed by denudation, their distance from the sources of 
supply, and their mode of occurrence, would, I think, render 
it more probable that they were never more than patches 
(though at one time probably larger), and that their deposi- 
tion in abrupt hollows and protected situations was owing to 
the extreme activity of the currents sweeping over the areas 
where they are to be found ; for we ought to remember that 
a force of current capable of denuding a deposit would be 
capable of preventing its deposition.* 
2. Yellowish-brown and Variegated Clay. — It is not difficult 
to understand that, as the land subsided (either continuously 
or after a long interval) beneath the level which cut ofi* the 
low-lying source of supply of the blue clay, the shales and 
sandstones of the upper Yoredale and millstone- grit forma- 
tions composing the sides of the valleys and table-lands, 
would furnish materials for the yellowish-brown clay, with its 
boulders. This clay extends from the bottoms of the valleys 
* In the area between "Wakefield and Sheffield there is very little drift, if we 
except a very thin surface deposit with many angular, and occasionally a few 
roxinded, stones, which are not the mere disintegration of the rocks underneath, 
and which may be looked upon as the gleanings left by the great scythe of denuda- 
tion to which the ground owes its present configuration. There are, however, 
a few patches of what is usually called drift — one near Eoyston, already men- 
tioned, and not far from it a considerable expanse of extra-rounded gravel, clay 
with rounded pebbles near Masborough station, a block of ShapfeU granite, 
according to Mr. Green, F.G.S., in Royston, &c. These facts prove the sojourn 
of the sea in the area under consideration, and I think that the comparative 
absence of drift may be explained by supposing the area to have been a theatre 
of very active denudation, compared with the districts north and east, where 
deposition may have been in excess. 
