54 PRINCIPLES OF STRATIGRAPHY 



tected by piles of rock and supplementary pieces placed on the wind- 

 ward side of the affected poles. The telegraph wires of the Trans- 

 Caspian Railway between Aidin and Bala-ischem had to be renewed 

 after eleven years, because their diameter had been diminished one- 

 half by the sand-blast corrasion. (Walther-104: 52.) Even dust 

 from the city streets blown against the tombstones of cemeteries 

 will in time efface their inscriptions. (Kgg\eston-28: 6 §4-6^8.) 



Facetted Pebbles. A highly significant and interesting result of 

 sand corrasion on loose stones is the facetted pebble or windkanter. 

 This generally has three or more faces ground flat, by the sand 

 blown over it at different times, and these faces meet in sharp 

 angles. These angles do not represent the direction of the wind 

 at the time of cutting, but rather the more or less accidental meeting 

 of faces cut at different times. VValther (106) describes "Ein- 

 kanter," Irregular "Windkanter," "Dreikanter," and 'Tarallel- 

 kanter" from the Libyan desert, all formed by wind blowing in one 

 direction only. The einkanter are arranged with the ridge at right 

 angles to the wind direction, the single wind-cut face being nor- 

 mal to and inclined toward that direction.' This Walther regards 

 as the normal type. The polyfacetted condition is brought about 

 by the fact that the sand-laden wind-currents along the bottom of 

 the atmosphere wind in and out among the pebbles strewn over the 

 surface, cutting one here and another there, and so producing faces 

 and angles which have no definite orientation. In some cases the 

 undermining of a pebble on which one face has been cut and its sub- 

 sequent rolling over, expose a new surface to the wind. (Davis- 

 24.) Of between 300 and 400 specimens observed by Wade (loi) 

 in the eastern desert of Egypt, 78 per cent, were set at right angles 

 to the direction of the wind. He found the ridge to be more gener- 

 ally a curved one, and the face presented to the wind not a plane sur- 

 face, but a gently curved one. Measurements taken on over fifty 

 specimens showed that this surface makes an angle with the vertical, 

 which varies between 40° and 50°, most usually approximating 

 45°. Wade determined to his own satisfaction that the air currents 

 were driven upward, carrying the sand against the pebble face. 

 Undermining and consequent change of position of the pebble were 

 found to be the cause of the cutting of the several facets, as postu- 

 lated by Davis. 



The presence of these pebbles in undisturbed condition in older 

 formations is a reliable indication of the subaerial origin of the 

 deposit. Davis (24) has used them to prove the undisturbed sub- 

 aerial character of the apron plains of Cape Cod. They have also 

 been found in the pre-Cambric Torridon sandstone of Scotland, the 



