396 A. Wade—Formation of ‘Dreikante’. 
something under half a square mile in extent, and noted with regard 
to all the dreikante I could find exactly how they were situated with 
regard to the wind. I noted between 300 and 400 specimens, and 
found that.78 per cent. were set approximately at right angles to the 
direction of the prevailing wind, whilst only 22 per cent. were set 
approximately parallel: to that direction. Evidently the theory 
already stated will not account for these facts. Moreover, not all 
the wind-worn pebbles were three-edged, though there seems. to be 
a tendency towards the Brazil-nut shape in the late stage of erosion 
in all cases. ‘ . jd 
Stones were found in all stages, from the well-rounded, water-worn 
pebbles from the Nubian Sandstone (Pl. XXXI, Fig. 1), or angular 
flints and broken pieces of igneous rock, showing only incipient traces 
of the abrasive action of the wind-blown sand, to dreikante, worn 
down until almost level with the surface of the desert (Pl. XXXI, 
Figs. 7 and 8). The series shown in Pl. XXXI, Figs. 1-8, gives an 
almost complete ‘life’ history of the dreikante from the earlier stages 
to the last. 
The ridge along the crest is rarely a straight line, but is more 
usually a more or less delicate curve. In the Text-figure (Fig. 2) 
Nos. 1 and 2 are the more usual types of curve. The side of the 
stone facing the wind is always beautifully smoothed and polished, 
presenting a clean, fresh surface to the wind. Frequently this face 
is the only one showing signs of cutting by the sand, the remainder of 
the pebble showing the original water-rounding, and is also usually 
stained, probably with iron oxide. This shows that in such cases the 
pebble has not been moved by the wind since it was exposed to the 
action of blowing sand, for if such had been the case other faces would 
have been cut upon it. Such pebbles usually present a ridge-line 
like Text-fig. 2, Nos. 3 or 4. 
The face presented to the wind is not a plane surface, but is also 
gently curved. Measurements taken from over fifty specimens showed 
that this surface makes an angle with the vertical which varies 
between 40° and 50°. It most usually approximates to 45°. The 
curve is shown in Text-fig. 2, No. 4. 
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We are now in a position to consider how the dreikante have been 
formed. Evidently in most cases the travelling sand-grains do not, on 
encountering a pebble, split into two currents which travel along the 
sides of the stone and so form aridge. Instead of this they tend to 
move upwards over the pebble with an eddying motion. The curve 
produced nearest the ground is always convex towards the wind. 
