INLAin) DUNES. 577 



Tschudi observed, in the same desert, two species of dunes, 

 fixed and movable, and lie ascribes a falciform shape to the mova- 

 ble, a conical to the fixed dmies, or medanos. " The medanos," 

 he observes, " are hillock-hke elevations of sand, some having a 

 firm, others a loose base. The former [latter], which are always 

 crescent-shaped, are 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 smaller and hghter ones move quickly forward, before the 

 larger ; but the latter soon overtake and crush them, whilst they 

 are themselves shivered by the collision. These medanos assume 

 all sorts of extraordinary figures, and sometimes move along the 



plain in rows forming most intricate labyrinths A plain 



often appears to be covered with a row of medanos, and some 

 days afterwards it is again restored to its level and uniform as- 

 pect 



" The medanos with immovable bases are formed on the blocks 

 of rocks which are scattered about the plain. The sand is driven 

 against them by the wind, and as soon as it reaches the top point, 

 it descends on the other side until that is hkewise covered ; thus 

 gradually arises a conical-formed hill.f Entire hillock chains with 

 acute crests are formed in a similar manner On their south- 

 ern dechvities are found vast masses of sand, drifted thither by 

 the mid-day gales. The northern declivity, though not steeper 

 than the southern, is only sparingly covered with sand. If a hill- 

 ock chain somewhat distant from the sea extends in a line paral- 

 lel with the Andes, namely, from S.S.E. to N.N.W., the western 



same localities as those examined by Mr. Blake, and tlie difference in their 

 character may be due to a difference of origin or of age. 



In New Mexico, sixty miles south of Fort Stanton, there are inland dunes 

 composed of finely granulated gypsum. — American Naturalist, Jan. 1871, p. 

 695. 



* The dimes of the plains between Bokhara and the Oxus are all horseshoe 

 shaped, convex towards the north, from which the prevailing wind blows. On 

 this side they are sloping, inside precipitous, and from fifteen to twenty feet 

 high. — BtTRNES, Journal in Bokhara, ii., pp. 1, 2. 



+ The sand-hills observed by Desor in the Algerian desert were fixed, chang- 

 ing their form only on the surface as sand was blown to and from them. — 

 Sahara und Atlas, 1865, p. 21. 

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