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



around the depression is symmetrical. A feature seen in drip impressions 

 made after a rain in Dubois County, Indiana, on July 17, 1920, the drops 

 falling from an elm tree, was the presence in the center of many depres- 

 sions of a small cone about equal in height to the depth of the depression. 

 This feature the writer has not seen in rain-drop impressions, nor has he 

 been able to produce it in drip impressions made in the laboratory. It is 



thought that it may have been due 

 to the drops falling as disks and 

 not as spheres, with the disks being 

 of less thickness in the middle than 

 on the peripheries. These impres- 

 sions had ragged raised margins, 

 and in every respect, save for the 

 presence of the central cone, were 

 similar to rain-drop impressions. 

 They varied in diameter from 

 about a millimeter to more than 

 half an inch, with more impres- 

 sions approximating the larger di- 

 mensions than the smaller. Drip 

 impressions are shown in figure 4. 

 It is obvious that impressions of 

 this origin generally require the 

 presence of large vegetation, and 

 hence could not have been exten- 

 sively developed before the evolu- 

 tion of such. Their abundant oc- 

 currence should, therefore, not be 

 expected in strata earlier than the 

 Devonian or possibly late Silurian ; 

 but impressions made by drops falling from overhanging rocks and cliffs 

 must have been made throughout geologic time. 







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Figure 4. — Drip Impressions in soft Mud 



Spray and splash Impressions 



Spray and splash impressions are produced where wind drives the 

 spray from the waves against muddy or sandy surfaces or where water is 

 splashed by falling objects. The former may be seen on the muddy shores 

 of almost any body of water, while the latter are readily produced by 

 throwing rocks into bodies of water which have muddy shores. The im- 



