362 W. H. TWENHOFEL IMPRESSIONS BY BUBBLES AND RAIN-DROPS 



or even less (figure 1). After a rain of a few minutes a mud surface 

 becomes thorouglily sculptured through the presence of a multiplicity of 

 coalescing pits (figure 1). Further rain covers the surface with muddy 

 water, previous impressions are destroyed, and there is no possibility of 

 their being made until the water leaves the surface. 



Figure 3. — i*/(p/e*-s/o«s made ty artificial Hail 



Eain-drop impressions are also made in sand. If wet sand be struck 

 by rain-drops, impressions are made and continue to be made until the 

 sand becomes covered w^ith water. These depressions are similar to those 

 made in mud, except that the raised rims are not so sharp or so well de- 

 fined (figure 2). Pits are also made in dry sand and dust by rain-drops, 

 but the margins are less sharp than where the sand or mud is wet. Dry- 

 ing out of the sands tends to make the depressions and the marginal ele- 

 vations less pronounced, and a little wind will altogether obliterate them. 



