ge:neral summary 371 



From what has been stated on foregoing pages, the writer is of the 

 opinion that impressions which have been ascribed to falling rain are of 

 such origin in only a small percentage of cases. If this view be correct, 

 it follows that conclusions based on the occurrence of rain impressions 

 can not be considered valid unless data are given proving that the im- 

 pressions were really so made. As a criterion for the determination of 

 deposits formed under conditions of occasional exposure to the atmos- 

 phere, the occurrence of small depressions is valuable, but must be used 

 with great care, and the determination should always be made that -the 

 depressions are margined by elevated rims and are not continued down- 

 ward as tubes. 



Wherever bubbles or, for that matter, any floating objects come in con- 

 tact with soft muds, trails are made which resemble worm trails and may 

 be mistaken for such. As a general fact, the trails are very shallow. 

 Bubbles rising through mud of favorable consistency form vertical tubes 

 which may be mistaken for worm tubes. 



Impressions of the types described are made on the upper surfaces of 

 strata and there occur as depressions. Their obverse is on the under side 

 of the overlying strata, where they occur as wart-like mounds. It hence 

 follows that where impressions made by rain, bubbles, etcetera, are pres- 

 ent, one is enabled to tell which is the top or younger side of a section, 

 and this may give the key to the unraveling of a problem in complicated 

 structure. 



