86 THE ATMOSPHERE— WIND 



trary, high winds sweep along quantities of sand and fine gravel, 

 which are hurled against any obstacle and gradually cut it away. 



Very hard rocks yield but slowly to the cutting action of wind- 

 driven sand, and in them the chief effect to be observed is a 

 scratching and polishing of the surface. The same principle is 

 employed in the sand-blast, which is a jet of sand, driven at a 

 high velocity and used to engrave glass, polish granite, and do 

 other work of the kind. Soft rocks are quite rapidly abraded and 

 cut down by the drifting sand, and go to increase the mass of 

 cutting material. The softer parts are cut away first, leaving the 

 harder layers, streaks, or patches standing in relief. In this way 

 very fantastic forms of rocks are frequently shaped out : pot-holes 

 and caverns are excavated by the eddying drift, and archways 

 cut through projecting masses. 



As the wind does not lift the harder and heavier particles to 

 any great height, the principal effect is produced near the level 

 of the ground, and thus masses of rock are gradually undermined 

 and fall in ruins, which in their turn are slowly abraded. Isolated 

 blocks are sometimes so symmetrically cut away on the under side, 

 that they come to rest upon a very small area and form rocking- 

 stones, which, in spite of their size and weight, may be swung by 

 the hand. 



The fine particles abraded from the rocks by drifting sand have 

 undergone no chemical change, the process being entirely me- 

 chanical. 



The abrading effects of wind-driven sand may be observed in 

 any desert region where naked rocks are exposed, as for example 

 in the arid parts of Utah and Arizona. One very characteristic 

 effect of this natural sand-blast is found in the appearance of the 

 pebbles shaped by it. Pebbles of very hard and homogeneous 

 materials, such as quartz or chalcedony, are highly polished. 

 Those made from igneous rocks have the softer minerals worn 

 away, leaving the harder to stand in relief in curious patterns, 

 while limestone is carved into beautiful arabesques. 



We have seen that the rain is slowly shifting the soil seaward, 

 and in dry countries the wind acts in similar fashion. Strong 



