IMPRESSIONS OF FLOATING BUBBLES 367 



floodplain. The photograph of this type of impression also shows im- 

 pressions of the type described under the next heading. 



An interesting result arising from the experimental work on this and 

 the succeeding type of bubbles is shown in figure 6. In these experiments 

 the preliminary work was done and the material was left to settle and 

 clear. In nearly every case trails were observed on the surface of the 

 mud. Search was made for worms, but none could be found. It was 

 subsequently observed by one of the students in the course in sedimenta- 

 tion, Mr. C. G. Carlson, that the trails were made either by floating 

 bubbles or small particles of floating matter. Wherever these touched 

 bottom, a trail was made in the soft mud, and many of the bubbles before 

 their stranding did quite a bit of wandering. It thus developed that 

 "worm trails" may also be developed by bubbles. 



Impressions made by Bubbles developed from the expelled Air of 

 overflowed surfaces 



Impressions formed by bubbles arising from air expelled from dry sur- 

 faces flooded by sediment-loaded waters are like those of type 4, where 

 the bubbles remain attached to the mud until it has solidified sufficiently 

 so as not to flow. It has been found that when dry earth is flooded air is 

 expelled in large quantities and impressions arising from bubbles of this 

 origin must be formed on all tidal flats at the times of flooding and on 

 all floodplains. The writer has observed impressions of either type 4 or 5 

 in many parts of {he country and for a long while considered them to 

 have been made by rain. * 



Impressions made by Bubbles floating in Waters which are 

 gently elsing and falling 



Bubbles floating in very shallow waters, which are gently rising and 

 falling a few millimeters because of wave activity, produce pits in finely 

 divided muds. If the bubbles be changing position while they are thus 

 rising and falling, rows of pits will be developed. These pits are approxi- 

 mately the same dimensions as the bubbles which produced them. They 

 differ from the pits made by bubbles of the origin already described, in 

 that the surfaces of the impressions are not smootli, but irregular, the 

 margins are not sliarp, and the depressions are shallow. Furthermore, 

 the motion of the water tends to make them shallower and ultimately to 

 obliterate them. 



