STRATIGRAPHIC NOMENCLATURE iiii 



m shale. While a difference of opinion exists as to whether or not 

 the name m should be applied in the above case to the middle mem- 

 ber, it is generally agreed that, when the shale formation m 

 breaks up into a number of units, as in diagram IV, none of which 

 can absolutely be identified with the original mass m, each of the 

 smaller members should receive a distinct name, while collectively 

 they may be called the fii group, being the exact equivalent of the 

 m shale. If lenses of sandstone or conglomerate of importance 

 are present in a formation these should receive distinct names, as 

 n and p lenses in m shale. (Diagram VI.) If only one lens is 

 present, however, this may be known by the same name as the 

 enclosing formation, though it may be better to give even a single 

 lens a distinct name. Thus, in the Cattaraugus formation of south- 

 western New York and adjacent areas in Pennsylvania, three con- 

 glomerate lentils occur, the Wolf Creek, near the base, the Sala- 

 manca higher up, and the Kilbuck still higher up. In some locali- 

 ties only one of the upper two lentils is present : in others both are 

 absent. The desirability of distinct names, even where only one of 

 these lentils occurs, is apparent. 



Where the main mass is of uniform character, but contains 

 thin beds of another character, the whole may be classed as one 

 formation (in), while the minor strata are spoken of as distinct 

 members. (Diagram VII.) Thus the Waldon sandstone forma- 

 tion of the southern Appalachians contains the Sewanee coal mem- 

 ber besides shale and other coal members and conglomerate lenses. 



Names of sedimentary formations are derived from localities 

 where the formation is best developed or where it was first studied. 

 "The most desirable names are binomial, the first part being geo- 

 graphic and the other lithologic (e. g., Dakota sandstone, Trenton 

 limestone, etc.) The geographic term should be the name of a 

 river, town, or other natural or artificial feature at or near which 

 the formation is typically developed. Names consisting of two 

 words should be avoided. Names taken from natural features are 

 generally preferable, because less changeable than those of towns or 

 political divisions. When the formation consists of beds differing 

 in character, so that no single lithologic term is applicable, the 

 word "formation" should l)e substituted for the lithologic term 

 {e. g., Rockivood formation)." (18:24.) 



Selection of Names for Systems, Series and Stages 

 (Groups). These divisions, as already noted, are of much wider 

 distribution than formations. The names of systems are mostly 

 uniform throughout the world, as Devonic, Triassic, Cretacic, etc. 

 American terms have in some cases been proposed where the origi- 



