246 STEPHENSON: TONGUE, A NEW TERM 



provided the main formation of which it forms a part is rela- 

 tively much larger. It would seem desirable, however, that the 

 term should be restricted to projections less than half as thick as 

 the main formation, for any mass that is of greater thickness 

 could reasonably be regarded as the continuation of the forma- 

 tion itself, and as such would require no additional name. In 

 the particular examples in Mississippi, to be described on sub- 

 sequent pages, each of the two tongues, the Mooreville tongue 

 of the Selma chalk and the Tupelo tongue of the Coffee sand 

 member (see pp. 247-8), are about 20 miles long, one is approxi- 

 mately 200 feet thick and .the other 100 feet thick. The Oktib- 

 beha tongue of the Selma chalk (p. 249) is between 50 and 60 

 miles long and only 25 to 50 feet thick; this seems like a rather 

 extreme length for a subordinate geologic feature, but when 

 this tongue is compared with the main formation, the Selma 

 chalk, which is at least 350 miles long, and where most fully 

 developed nearly a thousand feet thick, the tongue is a relatively 

 small feature. 



The distance to which a tongue maintains its distinguishing 

 lithologic character down the dip away from the area of out- 

 crop need scarcely be considered in determining what features 

 shall be classed as tongues, any more than the same dimension 

 is considered with reference to lentils, members, or formations ; 

 for although changes in conditions of sedimentation oceanward 

 from a shoreline are always interesting in connection with prob- 

 lems relating to geologic processes and geologic history, such 

 factors need not as a rule be taken into account in areal map- 

 ping and geologic nomenclature. 



The method here proposed for treating tongue-like projec- 

 tions of formations has already met the approval of the geol- 

 ogists with whom the writer has consulted, for those who have 

 had extensive experience in geologic mapping recognize that 

 such features exist, and appreciate the difficulties attendant 

 upon their treatment according to the usual methods of 

 presentation. 



It seems desirable to present several concrete examples of the 

 features under consideration to serve in a sense as types for 



