1 134 PRINCIPLES OF STRATIGRAPHY 



Among the pseudoplankton the shells of Ammonites should 

 be especially noted as being included, at least to some extent, in sedi- 

 ments of varied character over wide areas.* The genera and spe- 

 cies of ammonites as a rule were short-lived cephalopods, different 

 species characterizing different zones. Besides having their shells 

 widely distributed after death, by flotation, living ammonites also 

 seem to have spread rapidly and widely, probably during a mero- 

 planktonic stage. 



Holoplanktonic organisms may also suffer a wide distribution 

 and, if they contain parts capable of preservation, these may be 

 entombed in sediments of widely different character. Such cases 

 are seen in the pteropod oozes of various geologic horizons. 



The wide distribution of the Ordovicic graptolites was probably 

 due to epiplanktonic as well as holoplanktonic dispersion. Grapto- 

 lite species were short-lived, hence successive zones are characterized 

 by distinct types over a wide area. Plants whose seeds are widely 

 distributed by winds or other agents produce good index fossils 

 for continental deposits. Here, however, the climatic factor exer- 

 cises a limiting influence, since plants will only grow where climatic 

 conditions are favorable. 



(b) Grade of index fossil. The grade of the index fossil com- 

 monly has a very direct relation to the magnitude of the strati- 

 graphic divisions to be correlated. Thus, while the class of trilo- 

 bites as a whole may serve for the recognition of Palseozoic rocks 

 the world over — none having as yet been found outside of the 

 Palseozoic — smaller subdivisions must be used for the correlation 

 of more restricted stratigraphic divisions. Thus the family Cono- 

 corvphidcv among the trilobites is characteristic of the Cambric, 

 and any member of that family will serve to determine the Cambric 

 age of the strata in which it occurs. The family Olenidcc is princi- 

 pally restricted to the Cambric, though some members occur in 

 the Ordovicic. The most characteristic types, nevertheless, serve to 

 correlate the Cambric formations in all their occurrences. While any 

 of the more characteristic genera of this family (Olenidse) will 

 thus serve in correlating Cambric formations as a whole, certain 

 genera of this family serve as indices of the three principal sub- 



* The pseudoplanktonic dispersal of the shells of Ammonites is strongly- 

 advocated by Walther, but questioned by Ortmann (34), Tornquist (46), J. P. 

 Smith and others. Tornquist has urged against such interpretation the observa- 

 tion that in the Jurassic and Crefacic the Ammonites are distributed according 

 to climatic zones. Examples of dispersal by flotation are, however, known, for 

 as shown by Clarke (Naples fauna) the Goniatite fauna of the Styliolina or Ge- 

 nundewah limestone of western New York ("prenuncial intumescens fauna") 

 must be regarded as derived in this manner. (See also Chapter XXIX.) 



