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



as they rise and fall with the waves, and Doctor Woodworth'' has called 

 the writer's attention to the fact that certain jellyfish may make pits 

 under the same conditions. Some of these structures of organic origin 

 might be mistaken for imj^ressions made by other agents. These struc- 

 tures are not described in this paper, as their manifold varieties constitute 

 a large task, requiring observations extending over a long period of time. 



General Summary 



Excepting the pit and mound structures and those made by organisms, 

 the impressions described on previous pages are of two general types: 

 impressions made by falling substances and impressions made by bubbles. 

 The former can be developed only on exposed surfaces of mud or sand, 

 and, as those made on the latter are apt to disappear when the sand dries 

 out, it follows that the preservation of these impressions should generally 

 occur in deposits that contain sufficient mud to hold the material together 

 after drying has occurred. The bubble impressions can be made on sur- 

 faces that are covered with water from extreme shallowness to great 

 depths. Types 5, 6, and 7 are favored by shallow water; type 8 may 

 occur in waters of any depth. Types 5, 6, and 7 have nearly the same 

 significance as impressions made by drip, rain, and hail, but they can not 

 be told from some of these formed in the manner described for type 8. 

 The bubbles developed in muds rich in organic matter, which explode on 

 reaching the surface of the mud, make shallow depressions which are 

 much like those made by rain-drop, drip, hail, and spray and splash, but 

 such impressions are ape to be continued downward by tubes. Since 

 impressions of this origin should occur most generally in deposits rich in 

 organic matter, while the other types of impressions may be associated 

 with any type of mud, this fact may enable one to distinguish sediments 

 which were subject to exposure from those made under water. If im- 

 pressions occur in sediments ^joor in organic matter and have considerable 

 distribution, it is probable that they were formed in shallow waters whose 

 bottoms were at times exposed. If the impressions of such occurrence 

 are margined by an elevated rim, they were probably made by hail, rain, 

 drip, spray, or splash. If the impressions are in deposits which are rich 

 in organic matter, one should be careful in deciding as to the origin of 

 the deposits. The raised rim of the rain-drop, hail, drip, spray, and 

 splash impressions may not always be preserved, but as there are always 

 many of such impressions, careful search may prove the presence of the 

 marginal elevations. 



" Personal communication, March 21, 1921. 



