L. J. Wills— Wind-worn Pebbles in Graveils. 301 
polished. ‘These represent the hollow casts of crinoid stems. It may 
be noted that these silicified limestones are always more or less angular, 
but this does not preclude the possibility that they have been derived 
from the Bunter. 
(4) Gannister (Pl. XXV, Fig. 5).—A very fine specimen of a 
gannister pebble is here figured. It was found by Mr. Willcox, who 
sent it to me. On the reverse side it is smooth but not intensely 
polished, whereas on the side figured a very high glaze is seen. More 
remarkable still is the sharp angular ridge which surrounds the top 
of the stone. The upper surface has an appearance as if the rock 
possessed a slight conchoidal fracture along which pieces had flaked 
off under the influence of frost. ‘The hollows left have been highly 
polished, and the ridges between are quite sharply defined and 
angular. At (a) we can see how the erosive agent has picked out 
a natural joint in the rock and enlarged it. 
This fine specimen gives strong evidence in favour of the conclusion 
that these cases of polishing are due to the action of wind-erosion, 
for it is evident that the wind is the only agent capable of polishing 
concave surfaces and, at the same time, of leaving the ridges between 
the depressions sharp. 
In most of the cases considered above where a dreikanter form has 
been observed, it is due to accidental fracture. But there are instances 
where one is tempted to say that the general shape is the direct result 
of cutting by a sand-blast, the pebble having a roughly tabular form 
with a flattish upper surface surrounded by a sharp ridge. Such 
shapes are common among desert pebbles from Wadi-Halfa in Egypt, 
and may there represent the first stage in the production of a 
dreikanter. A specimen of this type preserved in the Sedgwick 
Museum, Cambridge, is here illustrated on Pl. XXV, Fig. 4, and may 
be compared with Pl. XXV, Fig. 5. 
I¢ is striking that the sand in which the stones lie is composed of 
angular material. But itis at the same time to be remembered that 
wind-rounded sand-grains are almost always of larger size than those 
found here. 
The presence of Welsh erratics among these polished pebbles fixes 
the age of the deposit as either Glacial or post-Glacial. This is of 
interest since, throughout North Germany, wind-cut pebbles are found 
lying on the Boulder-clay at the base of the Loess. In England too 
one wind-cut pebble has already been recorded which was probably 
derived from glacial deposits.! 
The remaining records of wind-polished stones in England (other 
than recent) show that probably their production was a rather local 
phenomenon and not likely to be very widely observed. Further, they 
are not confined to beds of one age. Mr. Clement Reid, to whom my 
thanks are due for help in dealing with this question, mentions some 
from Dewlish, where they appear to be late Pliocene or of the age of 
the Cromer Forest Series.” 
Prestwich? found similar pebbles at Portland. They were there- 
1 F. A. Bather, Proc. Geol. Assoc., 1900, vol. xvi, p. 396. 
2 C. Reid, ‘‘ Pliocene Deposits of Britain”: Mem. Geol. Surv., 1890, p. 206. 
3 J. Prestwich, Quart. Journ. Geol. Soc., 1875, vol. xxxi, p. 29. 
