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POPULAR SCIENCE REVIEW. 
The phenomena presented by coast-dunes are repeated by the 
sand-hills of the desert. They are steepest on the side which 
faces the wind, and the sand often blows into curious crater- 
like forms, somewhat resembling volcanic cones, one side of 
which has been blown away. The crater-like shaped hills would 
appear to be more perfectly formed in inland deserts than on 
coast-dunes. 
Although the surface of sand-hills and sand-deserts is so bare 
and arid, water may frequently be found at no great depth. In 
coast-dunes the sand is generally quite moist just below the 
surface. Where the shape of the dunes is constantly changing, 
fresh sand being blown up from the shore, the water is often 
brackish, but in fixed dunes the water is generally quite fresh. 
If the sand cover any wide area, and especially if it be under- 
lain by an impervious bed, such water may often be got in fair 
quantity. All this may be easily explained by the rain falling 
on the sand and being here retained, partly between the grains 
of sand, and partly held up by the underlying clay. 
But it is not so easy to understand how large quantities of 
water are stored beneath the sands of the Sahara, nor the 
source from whence the water is derived. The Moors have for 
long ages been aware of the existence of this store of water, 
and they have dug wells through the sand to reach it. The 
water occurs at various depths, but it generally rises to the 
surface when the water-bearing bed is reached. The French 
engineers have made a great number of bore-holes in various 
parts of the desert. The quantity of water thus obtained is 
often very great. One bore-hole yielded nearly 1,500,000 
gallons per day ; the water of this was employed to turn a 
water-mill. Small fish are sometimes thrown up with the 
water of these wells and bore-holes. The same fish occur in 
surface pools of water ; it seems therefore probable that there 
may be some communication between the pools and the water- 
bearing stratum.* 
Partly from a want of vegetation, and partly from its greater 
dryness, the sand of inland deserts is more easily moved by the 
wind than is the sand of the coast. In the deserts of Peru the 
crescent-shaped hillocks of sand are called Medanos. They are 
u from ten to twenty feet high, and have an acute crest. The 
inner side is perpendicular, and the outer or bow side forms an 
angle with a steep inclination downwards. When driven by 
violent winds the medanos pass rapidly over the plains. The 
* Professor Ramsay suggests this in a note to his translation of Professor 
Desor’s Memoir on the Sahara , “ Geological Magazine,” vol. i. p. 27. For 
further details on this most interesting region, see Rev. H. B. Tristram’s 
“ Great Sahara,” 1860. 
