2 NORTH AMERICAN INDEX EOSSILS. 



For example we may assume twenty species of organisms rang- 

 ing through five formations, A to E, in the type section, as shown 

 in the annexed diagram. It is evident that species 12 to 16 are 

 the special indices of the formations E to A, respectively. We 

 will now assume that a formation in another, not too far re- 

 moved, region, contains none of these but has the following species 

 generally abundantly represented: 2, 3, 6, 8, 17, 18, 19, while 

 more rarely represented are: 1, 5 and 10. It is clear that this as- 

 semblage or fauna is most like that of formation C of the type sec- 

 tion, even though species 14, the special index species of that for. 

 mation, is wanting. 



E 





















D 



























C 























1 



B 





















T" 



A 













1 





1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 



Influence of the Character of the Bottom and Sediment In the 



above illustration, the application is most reliable if the character 

 of the sediment is similar to that of the type section, indicating 

 similar conditions of existence. It is well known that with the 

 change in sedimentation occurs a change in fauna, whole classes 

 of organisms disappearing and new ones appearing. Thus nothing 

 can be more distinct than the calcareous beds of the Beekmantown 

 horizon of some parts of the Champlain Valley, with their trilobites, 

 cephalopods and gastropods, and the black shales of the same age 

 in the Hudson Valley, which contain only graptolites. It thus 

 becomes necessary that the fossils of the formations of the type 

 section should be determined for the different phases of occurrence. 

 This is necessary because each formation has normally an off-shore 

 or pure water (generally calcareous) facies and a shore facies con- 

 sisting commonly of quartz sands or even conglomerates, while 

 between these two are various transition facies. * Furthermore 

 a marine formation may in another region be largely represented 



* The principles here touched upon are fully treated in the forthcoming " Principles 

 of Stratigraphy ' ' by the senior author. 



