S7RATIGRAPH1C CLASSIFICATION 563 



A very similar case is that of the Shenandoah limestone, 

 which carries Cambrian fossils in its lower portion and Calcifer- 

 ous fossils near the top. But, though it thus corresponds to 

 parts of two great periods, as we usually classify geologic time, 

 it is mapped as one formation because it is a lithologic unit. 



Formation najnes. — "The formation should be recognized and 

 should be called by the same name as. far as it can be traced 

 and identified by means of its lithologic characters, aided by 

 its stratigraphic association and contained fossils." Following 

 this rule {Tenth Annual Report, Part I, p. 64), various cases may 

 arise, some of which are illustrated in the accompanying dia- 

 grams. 



Figure i shows the 7n shale passing into a limestone which 

 retains identical stratigraphic associations. Being exactly con- 

 tinuous stratigraphic units, they should retain the same geo- 

 graphic name on grounds of convenience and simplicity. 



Figure 2 shows the 7n shale grading into a limestone with 

 prolonged overlap, so that the two rocks must be discriminated 

 in one area. Not only are they lithologically different but they 

 have different stratigraphic associations, and they should receive 

 distinct geographic names : ?n shale and ;/ limestone. 



Figure 3 shows the ni shale continuing as an individual with 

 reduced thickness into new stratigraphic associations. Individu- 

 ality is not dependent on thickness nor on stratigraphic associa- 

 tion only; it is determined by continuity, and the formation may 

 retain its name, ni shale. But the group pms cannot then be 

 called the nt group, because rn would have two meanings, one 

 for a simple part, and one for a complex whole. 



Figure 4 shows the ni shale replaced by five formations, two 

 of which, B and D, are shales like it. Individuality is here lost 

 in multiplicity. Neither B nor D can be distinguished as repre- 

 senting m, which must, accordinly, give way where they divide 

 into two. The complex ABODE may be called the m group, 

 since it has equivalent stratigraphic associations with //z. 



Figures 5 and 6 illustrate the occurrence and naming of local 

 lenses in a formation where such divisions are not of sufficient 



