STEPHENSON: TONGUE, A NEW TERM 245 



In the hypothetical case represented by figure 1, the boundary 

 between formations A and B would have to be represented on a 

 geological map in part by drawing an arbitrary fine from the 

 outcrop of the point a to some such point as c on the outcrop of 

 the contact between the main bodies of sand and clay, treating 

 y as a sand member or lentil in formation B; or by drawing an 

 arbitrary line from b to d and treating x as a clay member or 

 lentil in formation A. In either case the true relations would 

 be inadequately represented. 



It is therefore proposed that such features as x and y in figure 

 1 be designated "tongues" and that, for convenience of treat- 

 ment they be given geographic names correlative in rank in 

 geologic nomenclature with such terms as "member" and "len- 

 til." In geologic mapping it is proposed to extend the color or 

 pattern representing the main formation to include the area of 

 outcrop of the tongue. 



Especial emphasis should be laid on the fact that a tongue is 

 not a member nor a lentil, either one of which differs lithologi- 

 cally from the typical material composing the formation of 

 which it forms a part, but is lithologically identical with, or 

 closely similar to, the formation. The term carries with it the 

 implication of connection at one end with the main formation, 

 though the tongue is underlain and overlain by materials litho- 

 logically different, and belonging to another or other formations. 

 Theoretically the tongue may end distally in a point ; more often 

 perhaps the terminus presents a series of minor tongues; or the 

 material composing the tongue may merge more or less gradually 

 into the kind of material composing the formation into which 

 the tongue projects. 



In the present stage of development of the subject it is not 

 possible to set definite dimensional limits to serve as a guide in 

 determining the features to which the term tongue shall be ap- 

 plied. Large-scale maps admit of representing relatively small 

 tongues, that is tongues only a few miles long and a few feet thick. 

 On the other hand a tongue-like projection as large or even 

 larger than some of the units to which the terms member and 

 formation are now applied, may be appropriately called a tongue, 



