732 Marvels of the Universe 
out, so that the larger, rocky fragments and particles from which the sand has been derived, have 
been left behind at the edge of the desert. The farther the sand has travelled, the finer the con- 
dition of dust at which it has arrived. The slightest wind-movement raises it again and carries it 
forward, and nowhere is the sand so finely-divided and rounded as in the heart of the desert. The 
wind, where it strikes anything of a rocky, solid nature, uses the sand-dust as an emery-powder 
to wear down all prominences, at the same time adding to itself the material of which it has robbed 
the solid rock. 
Sand-storms are made up of this sand dust. Almost as fine as the impalpable clay-dust which 
blows along our roads, it has the biting, pitting characteristic which sand has, and travellers in such 
a storm can but shield their hands and faces against its onset. 
It is this fine sand-dust which, acting with a proportion of grains of larger size, gives rise to the 
beautiful sand-ripples which characterize the slopes of the larger sand-waves. One is of course 
Photo by) g == = = [H. J. Shepstone. 
SAND-DUNES. 
Heaped up by the prevailing west winds on the fields of the Plain of Sharon. Many a wood or forest has been wrecked, 
and a fertile country made absolutely desolate by these inroads of moving sands. 
accustomed to the rippled sands at the seaside, but those that have been formed directly by the 
wind are even more regular and more beautiful than those formed beneath the rippling waters on 
a sandy shore. It has been found that desert-ripples are formed at any time when there is a direct 
and steady wind, and up to a certain limit they increase in size and regularity so long as the regu- 
larity of the wind is maintained. Given a steady wind of great excavating power and the ripples 
become dunes and sometimes valleys arranged in a similar direction to that of the wind, as in the 
great desert of North-Western India. 
By the continual attrition to which blown sand is subjected, all sharpness in the particles is 
worn away, and under a microscope they are seen to be perfectly rounded. Great as is the wearing- 
action to which sand in a river or on the seashore is subjected, it is but slight as compared with 
that of sand conveyed by the wind. It has been estimated that a hundred years of wind-action 
will produce in sand what would take three thousand years to produce by water action. Conse- 
quently many an ancient rock, when made up of fine, rounded particles, has been judged to have been 
originally laid down as desert-dust, or have been blown out to sea to form sand-shoals. Such may 
