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



being less than a hemisphere. There are no appreciable raised rims, the 

 surface of the mud being flat from the depressions outward. The edges 

 of the depressions are sharply defined and regular. The surfaces of the 

 pits are smooth to the eye, but are irregular in microscopic detail (fig- 

 ure 5). 



In the writer's experiments, bubbles were formed by having a spray of 

 water fall on an artificial upland, the falling waters making the bubbles, 

 which then floated into a tank in which the water was kept at a shallow 



Figure 6. — Imptessions made hy floating Bubbles 



In the lower left-hand corner of the photograph are impressions made by exploding 

 bubbles in very liquid mud. 



level over clay and fine sand. After many bubbles had become anchored 

 the mud was permitted to dry. Wherever bubbles remained until the 

 mud had settled to an extent sufficient not to flow, impressions remained. 

 Bubbles were also formed by mixing water and mud in a pan, the agita- 

 tion creating the bubbles, which were anchored by the suspended mud 

 settling on them. It was found that impressions of this kind were formed 

 whenever shallow muddy waters had bubbles formed in them. They must 

 be formed in large numbers on every tidal mud fiat and on every river 



