564 BAILEY WILLIS 



extent to justify a distinct series of names for the lens and tiie 

 parts of the formation above and below it. One lens may receive 

 the formation name, but two or more must be named distinctly. 



Figure 7 presents the case of a formation which is a least 

 practicable lithologic individual for mapping, but which includes 

 fractional parts that are important in discussion. The fractional 

 character of the parts may be indicated by calling them mem- 

 bers, each member being given a distinctive geographic name. 



Lithologic ifidividual and thickness. — Thickness is not an essen- 

 tial character of a lithologic individual. Layers, strata, beds, 

 and their complexes vary so generally in this respect that indi- 

 viduality based upon constancy thereof must be lost in a 

 short distance. And if thickness be not constant for any one 

 individual, still less can volume be considered an element of the 

 definition of formations in general. Lithologic individuality 

 knows no such limits. Continuity of rock character is the 

 essential core of the definition, and this may extend through a 

 hundred or a thousand feet or more or less. 



The writer has avoided the vise of the phrase "lithologic 

 unit," because discussion developed the fact that, in the minds 

 of some, unit means a definite quantity, and these persons think 

 that a thick formation should be a group or series, because of its 

 magnitude. Not volume, but uniformity of constitution, defines 

 a lithologic individual or " formation." 



Lithologic individual and time. — Thickness of strata, at right 

 angles to bedding, is considered a measure of the epoch of 

 deposition, due account being taken of the estimated rate. Thus 

 for that particular place the lithologic unit is significant of defi- 

 nite lapse of time. It bears, however, no mark of date. If we 

 trace the formation a hundred or a thousand miles, and again 

 measure its thickness, another determination of an epoch is 

 obtained, again without date. Are the epochs identical ? The 

 common assumption is that they are, and one reads of the epoch 

 of the Dakota formation without reference to locality. Thus 

 used, the phrase should mean all the time during which the 

 Dakota sandstone was spread from its eastern to its western 



