360 AV. H. TWENHOFEL IMPRESSIONS BY BUBBLES AND RAIN-DROPS 



recent and fossil rain-drop impressions and gives some criteria by which 

 they may be recognized. Some data are also given on impressions from 

 the Triassic rocks of the Newark series, which are ascribed to hail. 

 Mennier has shown how rain-drop impressions may be artificially de- 

 veloped.* 



At different times attempts have been made in the Sedimentation 

 Laboratory of the University of Wisconsin to find out in hoAV many ways 

 impressions could be made which resemble rain-drop impressions and 

 might be mistaken for such. It has been learned that, in addition to pits 

 made by organisms, these develop in nature in at least nine different 

 ways, namely: (1) impressions made by rain-drops, (2) impressions 

 made by hail, (3) drip impressions, (4) spray and splash impressions, 



(5) impressions made by bubbles floating in shallow water and becoming 

 anchored to the bottom because of mud settling on their outer surfaces, 



(6) impressions made by bubbles formed on the surface of an area over- 

 flooded, the bubbles develof)ing from the air pushed out of the earth being 

 submerged and held to the bottom by the settling sediment, (7) impres- 

 sions made by the rising and falling of bubbles over very shallow waters, 

 (8) impressions made by bubbles arising from the decay of organic mat- 

 ter contained in mud and making their way to its surface, where they 

 either burst or remain attached, and (9) the small pit and mound struc- 

 tures described by Kindle.^ Each of these nine types of impressions will 

 be described in detail. 



Impressions made by Rain-drops 



In the writer's experiments rain-drop impressions were made artifi- 

 cially by throwing water from a spray to a height of about ten feet above 

 the tray in which was placed the mud intended to receive the impressions. 

 The mud used consisted of mixed lake marl and glacial clay and was 

 made of a consistency to receive and retain impressions. The marks de- 

 veloped are identical in appearance with those made by actual rain. The 

 impressions consist of circular to elliptical pits surrounded by slightly 

 elevated and somewhat ragged rims. The surfaces of the depressions are 

 rough, with the small elevations of sufficient magnitude to be readily 

 seen Avith the eye. The rims are slightly more elevated and the depres- 

 sions somewhat deeper on the sides toward which the falling drops were 

 directed. The depth of the dejDressions vary with the dimensions of the 



* Stanislas Meunier : Geologie Experimentale, Bibliotheque Scientiflque Internationale, 

 1899, p. 38. 



^ E. M. Kindle : Small pit and mound structures developed during sedimentation. Geol. 

 Mag., December 6, 1916, vol, iii, pp. 542-547. 



