iioo PRINCIPLES OF STRATIGRAPHY 



Tertiary, but they met at first with httle favor. Paleeozoic was the 

 first to be adopted, while Secondary and Tertiary were still re- 

 tained. Later Mesozoic gradually replaced Secondary, but Tertiary 

 has still retained its hold in geologic literature to the present day. 

 To it the Quaternary ''' has been added, which comprises the forma- 

 tions designated by Lyell as Post-Pliocene, together with his Later 

 Pliocene or Pleistocene. These have frequently been included with 

 the Tertiary under the term Cxnozoic ( = Kainozoic), but they have 

 also been separated under the term Psychozoic, introduced by Le 

 Conte, but limited by him to the most recent formations, which in- 

 include abundant remains of man. 



It is thus seen that the classification at first proposed as a rock 

 classification became a chronologic one, as geologists began to 

 perceive that all kinds of rock may be formed during each period 

 of the earth's history. When the fossils of each of these four divi- 

 sions became better known, it was found that each was character- 

 ized by its peculiar assemblage of organisms. It was further 

 found that in most regions the strata of each of these larger sub- 

 divisions were separated from those above or below by a marked 

 unconformity, so that records of disturbances of widespread occur- 

 rence were looked upon as generally marking the dividing lines 

 between the greater subdivisions of the earth's history. The use 

 of unconformities in defining limits of geologic formations was also 

 extended to the further subdivision of the larger units, and, in 

 fact, such breaks have frequently been advocated as the best avail- 

 able criterion. But geologists have pretty generally recognized 

 the fact that a classification based on unconformities is an incom- 

 plete one, and that a complete record of geologic time can be ex- 

 pected only in a series resulting from continuous deposition. Such 

 a series is, however, nowhere obtainable, since in no known region 

 of the earth has there been continuous and uniform deposition. 

 Stratigraphers are thus compelled to construct their typical section 

 from fragments of overlapping sections from all parts of the world. 

 Each fragment thus used in the building up of the typical scale 

 must be complete in itself, and its relationship to the next adjoining 

 fragments of the scale must be determined. 



Selection of the Type Section. 



What, then, are the criteria which must guide us in the selection 

 of our typical section? First and foremost, the section must show 



* Proposed by Morlot in 1854. 



