908 Notices of Memoirs—J. Lomas— 
In our neighbourhood the dunes seldom exceed 50 feet in height, 
but in the desert they have been measured up to 350 feet. A favourite 
simile with travellers is to compare dunes with the waves of the sea, 
with crest and trough following each other parallel in sequence or 
arranged in a network pattern. 
One fundamental difference must be kept in mind; in the sea the 
wave motion progresses while the material is stationary. In the 
desert the sand travels with the wave movement. The Arabs say 
that the dunes ‘walk.’ It follows, then, that if the material goes 
forward the country to windward will be left bare unless fresh 
material is forthcoming from that quarter. Deposits accumulated 
by wind action show very characteristic false bedding. The nature of 
this depends on the steepness of the slope over which the sand tumbles 
and the varying directions of the wind. When the slope to leeward 
is very steep the weight of the sand at the top may cause the sand to 
slide down into the hollow, thus puckering and folding the layers 
beneath. 
The distance to which sand may be carried by wind is very great. 
Dust-storms are sometimes met with in the Mid-Atlantic which can 
only have come from the western Sahara. At Las Palmas a ridge of 
sand dunes exists on the low isthmus connecting the Port with Isleta. 
The rocks of the island could not yield such a deposit, and captains of 
ships attribute the sand to winds blowing from the African continent. 
The finest sand dust may be carried in the air for hundreds of miles, 
and when the winds are constant the result will be the building up of 
thick masses of stratified material. Such is the origin attributed to 
the Loess of China, which attains more than a thousand feet in 
thickness, and Richthofen supposes it to have come from the desert 
areas of Asia. Professor N. 8. Shaler! assigns a similar origin to the 
accumulation of fine-grained detritus in the Western Mississippi Valley, 
and he states that it has been derived from the Cordilleras. 
(2) Sifting.—The sifting action of wind may be observed in a dusty 
road or in our local sand-hills. In deserts, however, where the winds 
blow more consistently from one quarter, the action is more perfect. 
In a series of samples taken in the Sahara from the same locality in 
vertical sequence by the late Dr. Isaac Roberts the grains differ in 
size at each horizon, but for the same depth the sifting is so perfect 
that it looks as if each sample had been put through a sieve. Many 
other observers have commented on the sifting action of wind, and it 
is only reasonable to suppose that the size of a grain carried along will 
vary with the velocity of the current which moves it. Sven Hedin 
graphically describes a sand-storm he encountered in Central Asia. 
Near the ground the wind velocity was 403 miles an hour. Six feet 
from the ground it measured 583 miles an hour. Branches, tufts of 
grass, and grains as big as peas whirled in the air and struck his face 
with stinging force. A strong wind such as that described will move 
large and small particles alike, but as it loses velocity the particles 
will drop to the ground as the carrying power of the wind diminishes. 
Thus the sands raised by one storm will be graded horizontally, 
1 U.S, Geol. Survey, 12th Report. 
