IMPRESSIONS FROM BUBBLES FROM ORGANIC MATTER 369 



from different portions of the geologic column are of this origin. Im- 

 pressions of this origin should be associated with sediments rich in 

 organic matter. 



In the writer's experimental work, impressions of this kind were made 

 by using lake muds and placing yeast for making bread beneath the mud, 

 and also by submerging caked dry clay beneath in the muds. 



Pit and mound Structures developed in rapidly precipitated Mud 



The pit and mound structures in Kindle's experiments were formed in 

 clay precipitated in salt water, the settling of the clay being accompanied 

 by small upward currents in parts of the mud. The experiments were 

 made in quart milk bottles of the constricted neck type, and the upward 

 currents made their appearance at the beginning of the constriction. In 

 vessels with no constriction, similar structures were not formed. The pit 

 and mound structures were developed on the surface of the settling clay 

 at the places where the upward currents discharged, and consisted of 

 small mounds of 3 to 10 millimeters in diameter, each with a small crater- 

 like depression on its summit, the depression having a diameter the size 

 of a pin-head or smaller. Similar structures were not observed to develop 

 in sediment deposited in fresh water, and Kindle ascribed them to the 

 rapid settling of the mud induced by the salt in the water. In the writer's 

 experiments, mud and water, both fresh and salt, were thoroughly mixed, 

 so as to make a thick soupy-like fluid. This was then poured into tanks 

 and permitted to settle. Over the surface many small currents of water 

 were observed to be rising from the mud, and these developed small 

 mounds. The water appeared to have been squeezed from the settling of 

 the mud. The small currents could be observed with the eye, but not 

 well. Under the microscope they were seen to be bringing up small quan- 

 tities of suspended material and piling this about the margins of the small 

 openings. It was also found that the shape of the vessel is not a con- 

 trolling factor. No such phenomena were observed in muds that were 

 permitted to settle gradually. In the present state of knowledge, it thus 

 appears that structures of this type develop wherever large quantities of 

 sediments are precipitated, irrespective of whether the waters are fresh or 

 salt; but, as this is more likely to occur in salt than in fresh water, it 

 should follow that these structures should be more characteristic of de- 

 posits of the former than the latter. 



Impressions made by organisms consist of pits at the openings made by 

 burrowing animals, the pits made by floating plants touching the bottom 



