OCCURRENCE OF SMALL DESERT TRACTS. 169 
probably better in tract (c) than in either of the other two. 
Faceted stones occasionally occur, and these are of interest in 
the present connection. 
In most deserts, and especially in those whereon the wind- 
direction is constant for months together, the bigger stones 
and pebbles, which successfully resist the pushing power of the 
air current, become faceted by reason of the sand blast action 
of the grit-laden wind. It not uncommonly happens that 
these stones possess three faces separated by three sharp 
edges, for which reason they have been called “‘ Dreikante.” 
The exact mode of formation of Dreikante was an open 
question till five years ago, when Mr. Arthur Wade settled the 
matter in a short Paper to the Geological Magazine. Some 
were of opinion that the keels, or edges, were to be explained 
by the dividing effect which a stone might be supposed. to 
have upon the incident blast, thus causing the simultaneous 
production of two facets, which, in time, would meet along 
an edge in the plane of the wind ; but Wade’s observations, 
made on the seaboard of the Egyptian desert, showed that 
the broad face of Dreikante are opposed to the wind and not 
parallel to its direction, as the above-mentioned theory would 
require. It is not proposed to go deeply into the theory of 
faceted stones in this short Paper, but briefly it may be stated 
thus : The natural sand-blast gradually wears away the stone 
upon which it impinges, producing, in time, a more or less 
smooth face, which exhibits, in profile, a very characteristic 
curve, convex below and concave above (plate V., fig. 1). The 
edge between the facet and the base is also a curve, as shown 
in plate V., fig. 3. Astone loses weight, of course, during the 
process of faceting, and the time comes when the wind is 
able to move it—or perhaps it is moved by some accidental 
circumstance. Sooner or later it takes up a stable position 
with another part of it exposed to the blast, so another face is 
formed, and, in time, they meet each other along a sharp edge. 
In the course of its subsequent movements the stone is almost 
sure to come to rest on one or other of these faces, and then 
maintain itself in stable equilibrium for a long period, wherein, 
unless one face chances to be again opposed to the wind, 
another face is formed. Not all wind-cut stones show three 
