124 



ACTION OF WINDS DUNES. 



mass of giavel in rounded hills, gullied by the rain in all directions. 

 Frequently we find these granites on the surface of the soil, :n 

 great rounded blocks, piled up one on the other (fg. 206), in the 

 strangest manner, sometimes in unstable equilibrium, and suscep. 



Fig. 206. Degradation of granite as seen in different places. 



tible of oscillating from the slightest effort ; these are termed rock- 

 ing stones, in some localities. 



In mountains where the granite is easily decomposed, we often remark 

 that the mass, more or less cut, is in a sort of horizontal stories, divided by 

 vertical fissures, so as to present a kind of agglomeration of irregular paral- 

 le'llipipeds. It is supposed that, in consequence of atmospheric influences, 

 these angular blocks are altered on their faces and angles ; that the disag- 

 gregated parts are successively detached, producing rounded masses, piled 

 on each other like cheeses, as we now see, sometimes, isolated on the surface 

 of the soil. 



3. Action of winds dunes. Although winds act but very 

 ieebly on solid mineral masses, they exert an important influence 

 on deposits of fine movable sands. We know that in the deserts 

 df Africa and Arabia, the winds raise immense clouds of burning 

 sands, conveying them from place to place, and suddenly produc- 

 ing vast hills, sometimes quite high, which a new gale again de- 

 stroys. All sandy sea-coasts are exposed to similar effects ; the 

 least gale sets the sands in motion, and produces, on the previously 

 uniform surface, a multitude of wrinkles or ridges, parallel to each 

 other, separated by a greater or less interval, and each presenting 

 a gentle slope towards the wind, and a more abrupt declivity on the 

 opposite side, as represented (Jig' 207); the next gust of wind sets 

 all these ridges in motion, and each one is soon found to occupy the 

 space which separated it from the preceding ridge. This pheno- 

 menon of dunes, or downs, is seen in miniature on the sea-beaches; 

 and they sometimes invade immense tracts on adjacent planes. 

 These hills, placed one behind the other, in a direction perpendicu- 

 lar to that of the prevailing winds, are constantly in motion, and 

 constantly advance towards the interior of the land ; the wind from 



3. What are dunes ? How are they formed? 

 At what rate do dunes advance ? 



What is meant by ta lus 1 



