BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 32, pp. 359-372 DECEMBER 1, 1921 



IMPEESSIONS MADE BY BUBBLES, EAIN-DEOPS, AND 

 OTHER AGENCIES ^ 



BY W. H. TWENHOFEL 



{Presented before the Society December 28, 1920) 



CONTENTS 



Page 

 Introduction ,359 



Impressions made by rain-drops 360 



Hail impressions 36.3 



Drip impressions 363 



Spray and splasli impressions 364 



Impressions made by anchored bubbles 365 



Impressions made by bubbles developed from the expelled air of over- 

 flowed surfaces 367 



Impressions made by bubbles floating in waters wliicli are gently rising 



and falling 367 



Impressions made by bubbles produced from decay of organic matter 368 



Pit and mound structures developed in rapidly precipitated mud 369 



General summary 370 



Introduction 



Geologic literature abounds with statements relating to the occurrence 

 of rain-drop impressions, but in few instances is information given as to 

 the appearance of the marks which are so designated. When pitlike 

 structures have been found, they have generally been ascribed to rain- 

 drops, and it has been concluded that the containing deposits were those 

 of a region over which at times there was no water. Notes of caution 

 have occasionally appeared, but they do not appear to have been widely 

 read.- 



The best paper on the subject of rain-drop impressions with which the 

 writer is familiar is that of Lyell, in which are described "rain-marks of 

 the Recent, Triassic, and Carboniferous periods." ^ This paper describes 



1 Revised manuscript received by tlie Secretary of the Society May 3, 1021, 



2 Desor : Edinbourgli New Pliil. .Jour, for 18.50, p. 240, quoted by Lyell. 

 » Charles Lyell: Jour. Geol. Soc. London, vol. vii, 1851, pp. 238-247. 



XXVI— Bull. Geol. Soc. A.m., Vol. 32, 1920 (359) 



