550 STUDIES FOR STUDENTS 



CONCLUSIONS ON GEOLOGICAL SIGNIFICANCE OF MUD- 

 CRACKS 



From the preceding analysis of the origin of mud-cracks, as 

 observed at the present time and the conditions under which they 

 would be geologically preserved, it would seem that next to coal 

 beds formed in situ^ or abundance of land fossils belonging to the 

 animal kinsjdom, that mud-cracks form one of the surest indications 

 of the continental origin of argillaceous deposits. The structure 

 is also seen most commonly to originate under climatic conditions 

 where the other tests are apt to fail. 



It may be considered, therefore, that mud-cracked shales pre- 

 dominantly indicate former flood-plain deposits, usually on delta 

 surfaces which have displaced shallow seas. Removed from the 

 vicinity of the sea and occurring in continental basins with older 

 rock rims, they may have originated as playa deposits and indicate, 

 a formerly truly desert climate. 



More rarely mud-cracked shales may be found as transitional belts 

 separating unlike formations and indicative of the sun dried margins 

 of former lakes or seas. In any case the associated characteristics 

 which have been pointed out should assist in arriving at a conclusion 

 in regard to the particular conditions of origin. 



PREVAILING INTERPRETATIONS 



The prevailing views upon a subject are exemphfied by the 

 statements of the current textbooks and manuals. These not only 

 guide the formation of views of the younger students of the subject, 

 but represent the longer-established and verified opinions of the 

 older body of scientists. While frequently speciahsts in various 

 lines would regard the presentation of their departments even in 

 the better textbooks as not strictly up to date, this is, on the whole, 

 not without its benefits, since it is necessary for new knowledge to 

 become seasoned with time before its exact place and importance 

 can be assigned among the body of well-estabhshed principles. 



It will be desirable in summing up the present subject to compare 

 the conclusions just arrived at with those statements concerning 

 the significance of mud-cracks which are given in the standard texts, 

 and which it is beheved have been largely influenced by the habit 



