300 L. J. Wills—Wind-worn Pebbles in Gravels. 
them a more or less facetted appearance. I have not been able to 
convince myself that any large face has actually been produced by 
the wind, but natural fractures and pre-existing faces have been 
ground down, and the sharp intervening ridges have been preserved 
and accentuated. This could not possibly have come about under 
water. 
It may be of interest to examine the manner in which the same 
erosive action has affected different kinds of rocks. The pebbles 
found are capable of derivation either from the Bunter Pebble Beds 
or from glacial deposits. 
1. PEBBLES PROBABLY DERIVED FROM THE Bunter. 
(1) A mottled green and purple grit (Pl. XXV, Fig. 1).—On the side 
shown in the figure a typical dimpled surface with a greasy lustre is 
seen, while the other side preserves the water-worn contours that it 
possessed in Bunter times. The sharp edge seen round the outside 
is to be noticed, since it gives evidence of actual cutting power in the 
sand-blast. 
(2) A small pebble of black felstone with veins of quarts (Pl. XXV, 
Fig. 2).—This is polished all over. ‘The sharp line up the centre, 
shown in the photograph, possibly indicates an edge modified by 
wind-action, as a result of the grinding down and remodelling of 
a natural fracture. In this way a rude dreikanter form is obtained. 
It is possible, though I do not think it probable, that the shaping is 
entirely due to wind-action. The selective action of the sand-blast 
is seen in the way the quartz veins stand out. 
(3) Coarser grits —YThese are chiefly noticeable in regard to the 
projection of the quartz-grains and to polishing of the depressions 
between. 
2. PEBBLES PROBABLY DERIVED FROM GuLAcrIAL Deposits. 
(1) North Welsh Andesitic Ashes.—Boulders of this kind are 
common and may attain a large size (up to 13 or 14 inches through). 
They vary slightly in composition, but they nevertheless show much 
the same external phenomena. They are not so highly polished as 
are the quartzite and quartz pebbles derived from the Bunter, but are 
characterized by an irregular pitting and grooving. The latter is 
really more comparable te the elongation of the pits in a common 
direction than to actual grooving. In a very coarse ash with large 
quartz-grains these stand out and are well polished. 
(2) FKelspathic Grits (probably Carboniferous).—Two specimens of 
very similar grits have been found. One is about 9 inches long and 
shows the effect of the wind-erosion on the rock as a whole, whereas 
the other is quite small and, though equally angular, is instructive as 
illustrating the selective action of the wind on heterogeneous material. 
The sides of the larger stone have the softer parts worn away in more 
or less parallel strips. Its lower surface is irregularly flaked and 
barely polished at all. 
(3) Sclvecfied Crinoidal Limestone (P1. XXV, Fig. 3).—These pebbles 
take a very high polish and look as if they had been rubbed with oil. 
There are occasionally deep round pits in them which are not so well 
a 
