” q - 
Parr IL. Sucr i. §1.] SAND DUNES. DUST SHOWERS. 325 
spreads over the desert region between that sea and the Sea of 
Aral, into which latter sheet of water the spread of the sand has 
driven the course of the Oxus, once a tributary of the Caspian. 
In the interior of continents the existence of vast arid wastes of 
loose sand, situated far inland and remote from any sheet of fresh 
water, suggests curious problems in physical geography. In some 
instances these tracts have been at a comparatively recent geological 
period covered by the sea. The desert of the Sahara is no doubt 
in great part a modern sea bottom which has been upraised and 
dried, for shells of the common cockle (Cardiwm edule) are found 
lying on the surface up to heights of 900 -feet. above the level 
of the Mediterranean. Yet the disintegration -of rock in these 
torrid and rainless regions must be great (ante, p. 319), so that 
the existing sand may be partly of subaerial origin. In other 
dry climates it is quite certain that the sand-wastes are entirely 
of this latter character. The sandy deserts of the high plateaux 
of western North America, which have never been under the 
sea for a long series of geological ages, show, as we have already 
found (p. 320), the mode and progress of their formation from 
atmospheric disintegration alone. In Asia'lie the vast deserts of 
Gobi; to the east of the Red Sea stretch the great sand-wastes of 
Arabia; and to the west those of Libya. In the south-east of 
Europe, over the steppes of Southern Russia and the adjacent 
territories, wide areas of sandy desert occur. Captain Sturt found 
vast deserts of sand in the interior of Australia, with long bands of 
dunes 200 feet high, united at the base and stretching in straight 
lines as far as the eye could reach.’ 
Dust-showers, Blood-rain.—bBesides the universal trans- 
port and deposit of dust and sand already described, a phenomenon 
of a more aggravated nature is observed in tropical countries, where 
great droughts are succeeded by violent hurricanes. The dust or 
sand of deserts and of dried lakes or river-beds is then sometimes 
_ borne away into the upper regions of the atmosphere, where, meeting 
with strong aerial currents which transport it for hundreds and even 
thousands of miles, it descends again to the surface, in the form of 
“red-fog,” “sea-dust,” or “sirocco-dust.” ‘This transported material, 
usually of a brick-dust or cinnamon colour, is occasionally so abundant 
as to darken the air and obscure the sun, and to cover the decks, sails, 
and rigging of vessels which may even be hundreds of miles from 
1 For important information regarding the Central Asiatic wastes, see Richthofen’s 
conn, 1. 
2 For accounts of sand-dunes, their extent, progress, structure, and the means 
employed to arrest their progress, the student may consult Andersen’s ‘ Klitformationen,”’ 
1 vol. 8vo. Copenhagens1861; Laval in Annales des Ponts et Chaussées, 1847, 2me sem. ; 
Marsh’s “Man and Nature,” 1864, and the works cited by him. Forchhammer, Edin. 
“New Phil. Journ. xxxi. (1841), p.61. Eliede Beaumont, “ Lecons de Géologie pratique,” 
yol. i. p. 183. Information regarding the sands of the interior of continents will be 
found in Palgrave’s “* Travels in Arabia.” Blake in Union Pacific Railroad Report, v. 
Tristram, “The Great Sahara,” 1860. Desor, “Le Sahara, ses différents types de 
déserts.” Bull. Soc. Sct. Nat. Neufchatel, 1864. Richthofen’s “ China,” i. 
