I102 PRINCIPLES OF STRATIGRAPHY 



tions of New York, which were formed under local and in part 

 continental conditions, and cannot, therefore, represent a standard 

 by which the more widespread marine conditions existing elsewhere 

 can be measured. Wherever possible, such local formations should 

 be taken out of the standard scale of strata and replaced by forma- 

 tions of purely marine origin. These may, of course, not exist 

 within the limits of the territory for which the scale is made, in 

 which case the old terms, perforce, have to be retained. 



The best example of a truly representative classification of the 

 divisions of a larger formation, which has yet been devised, is that 

 of the Triassic system. In no one region of the world is there a 

 complete representation of marine Triassic strata; in fact, the best 

 known divisions of this system are to a large extent non-marine. 

 But, by a careful study of all the widely dissociated marine mem- 

 bers and their relation to each other, a standard classification, more 

 nearly perfect than that of most other similar formations, has been 

 devised. By its use the various dissociated marine members of 

 each region, as well as the non-marine members, may be measured 

 and the time relation of each to the others and to all may be 

 ascertained. 



Time Scale and P ormation Scale. 



While the time scale is thus of primary importance as a stand- 

 ard of comparison, a formation scale is also needed. A formation 

 is a stratigra])hic unit, composed in general of similar or closely 

 related strata and characterized by a particular assemblage of organ- 

 isms (fauna or flora). Sometimes a formation may consist of a 

 single stratum — more frequently it comprises many strata. The 

 rules recently promulgated by the United States Geological Survey 

 for the government geologists in the preparation of the geologic 

 folios of the United States (iSii*?) make the formation the carto- 

 graphic unit, and define it among sedimentary rocks as follows: 

 "Each formation shall contain between its upper and lower limits 

 either rocks of uniform character or rocks more or less uniformly 

 varied in character, as, for example, a rapid alternation of shale 

 and limestone." It is further suggested that, "As uniform con- 

 ditions of deposition were local as well as temporary, it is to be 

 assumed that each formation is limited in horizontal extent. 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 lith- 

 ologic character, its stratigraphic association, and its contained 

 fossils." 



