190 



POPULAR SCIENCE MONTHLY 



Andes, the Pacific, and the rivers Vitor 

 and Tambo. Its length and breadth 

 are about equal, perhaps fifty miles 

 in extent. The mean elevation of the 

 pampa is about four thousand feet, 

 increasing toward the north. It is a 

 great plain with occasional low hills, 

 almost devoid of animal and vegetable 

 life, except among the low hills facing 

 the sea. It appears to have been 

 formerly the bed of the ocean. The 

 surface is composed of sand, sprinkled 



the wind, and the cusps lie in the 

 direction of motion. Their size varies 

 between rather wide limits. They are 

 in general from one hundred to two 

 hundred feet broad, and from ten to 

 twenty feet high. They are composed 

 entirely of a fine gray sand, and are 

 moved along by the wind so perfectly 

 that not only is the crescent form pre- 

 served, but none of the sand is left 

 behind to mark the passage. A casual 

 glance at the surface of the pampa 



A Sand-dune on the Pe?ert of Islav. 



over with stones and small boulders, 

 and ail occasional outcrop of rock. 

 Scattered over the pampa, especially 

 in its northern portion, are hundreds 

 of crescent-shaped sand-dunes. Their 

 form is always the same, approxi- 

 mately that of the new moon, unless 

 some unusual object is encountered by 

 the dunes in their journey across the 

 desert. Their motion seems to be 

 always toward the north or northwest, 

 in the same direction as that of the 

 prevailing south and southeast wind. 

 The convex surface is directed toward 



detects little if any of the sand which 

 enters into the composition of the 

 1 dunes. The same variety of sand is 

 found, however, by digging beneath the 

 surface. It appears that all the avail- 

 able surface sand has already been 

 collected by the wind into these sym- 

 metrical heaps, and that, unless the 

 surface is disturbed by some convul- 

 sion of nature, the dunes may all 

 finally disappear among the hills on 

 the north of the desert. This theory 

 seems to be confirmed by the abun- 

 dance of dunes in the northern part of 



