244 STEPHENSON: TONGUE, A NEW TERM 



Sedimentation began in this assumed area with the deposition 

 of sand, formation A ; later clay, the base of formation B, began 

 to be deposited on the left and as far to the right as a, while 

 sand was still accumulating on the extreme right; as time 

 proceeded the sand-forming conditions shifted to the left and 

 upward as far as b, followed by a shifting of the clay forming 

 conditions to the right entirely across the area, the latter con- 

 tinuing until all of formation B had been produced. The shift- 

 ing of the two types of sedimentation resulted in the formation 



Fig. 1. Diagram showing the relation of tongues to formations. 



of a body of clay x projecting into the sand formation A, and 

 a corresponding body of sand y projecting into the clay forma- 

 tion B. 



So long as such features are relatively small, that is, less than 

 4 or 5 feet thick and less than a mile long, they can, as a rule, 

 be disregarded in geologic mapping without seriously distorting 

 the facts. There is also no difficulty in their treatment when the 

 area to be mapped does not include the places of junction of 

 the projections with the main formations, for then the features 

 can be classed as lentils, members, or even formations, without 

 apparent inconsistency. But when the projections attain 25 to 

 200 feet or more in thickness and from a few to many miles in 

 length, and the areas under consideration are large enough to 

 show the connection of the projections with the main formations, 

 the usual methods of mapping compel an arbitrary or even 

 false representation of the facts. 



