NORTH AMERICAN INDEX FOSSILS. 



Introduction. 



Fossils are the remains of animals or plants, or the direct record 

 of their presence, preserved in the rocks of the earth's crust from 

 the earliest to the present time. They are the only reliable means 

 by which the age of any geologic formation can be determined, 

 and, as such, are of the utmost importance to all geologists deal- 

 ing with the non-igneous rocks. Not all fossils are equally good 

 indices of the age of the formation in which they occur, for some 

 are of very great vertical, and others of very limited horizontal dis- 

 tribution. It is evident that neither will serve as a good index 

 type. The best index fossils are those which combine a wide hor- 

 izontal with a limited vertical distribution, such as the graptolites 

 or the ammonites. 



In general it may be said that the more precise the required 

 identification of a horizon the more limited must be the range of 

 the fossil or fossils which are relied upon to indicate that age. 

 Thus while trilobites as a class may be relied upon as indicators of 

 Palaeozoic age, being unknown above this, a certain group of trilo- 

 bites alone will serve to indicate Cambric age, while a genus (Olen- 

 e/his, or Paradoxides) serves to indicate the lower or middle Cam- 

 bric respectively. Furthermore, a certain group of species of Para- 

 doxides, as for example the species of the P. eteminicus type, serve 

 to indicate a certain horizon in the Middle Cambric. 



It often happens that the fossils of a certain formation in a given 

 region include no species of a restricted type, such as would indi- 

 cate the exact equivalency of this formation with the ascertained 

 horizon characterized by such a species in the type section. 

 Under such circumstances it is necessary to determine the position 

 of the formation in question by its assemblage of animal remains 

 ox fauna or the assemblage of plant remains ox flora. 



