1 126 PRINCIPLES OF STRATIGRAPHY 



Suez barrier by tbe canal has iDerniitted a certain commingling of 

 faunas, a phenomenon predictable for the faunas on opposite sides 

 of the Isthmus of Panama on tlie completion of the canal. 



Contemporaneous faunas existed in North America at various 

 times in its geologic history. The most noted case is that of the 

 Upper Devonic. An indigenous fauna, the Ithaca fauna, derived 

 largely by development from the earlier Hamilton fauna, occupied 

 the eastern area in New York and Pennsylvania, while an immigrant 

 fauna, the Naples fauna, occupied the region to the west of this. 

 At first the two faunas were separated by a land barrier, but this 

 was subsequently submerged. Nevertheless, the two faunas con- 

 tinued in their essential integrity through Portage time, though the 

 area of occupancy of each varied from time to time, but within 

 comparatively narrow limits. 



Prenuncial Faunas, Colonies. Prenuncial faunas are the ad- 

 vance invaders of a new territory of members of a foreign host, 

 which subsequently occupies the territory. Such have been noted 

 in some cases, especially in that of the Styliolina limestone of the 

 Upper Devonic of New York, which marks the first invasion of 

 the Naples fauna into this Upper Devonic province. 



The term "Colonies" was employed by Barrande to designate 

 the appearance of a fauna normal to a later geological horizon, 

 during a period when an earlier fauna still flourished. Though the 

 examples cited by Barrande have proved to be mostly inadequate 

 to establish his theory, the fact remains that faunas in their en- 

 semble suggestive of a much later period may appear in deposits 

 otherwise marked by the normal fauna of that period. Thus, during 

 the Upper Siluric (Upper Monroan) time, a fauna in large measure 

 suggestive of Middle Devonic time flourished in Michigan, Ohio, 

 and Ontario (Anderdon fauna). In this fauna something over 

 twenty species have exact specific, or closely similar, representatives 

 in the Onondaga or Schoharie formation. This similarity is largely 

 found in the coral, brachiopod, pelecypod, and trilobite elements of 

 the fauna, while the cephalopods and gastropods are of typical 

 Siluric species. The whole fauna is succeeded by a normal Siluric 

 fauna, and is separated by an extensive hiatus from the overlying 

 limestones of Middle Devonic age. The explanation of this com- 

 mingling of faunas is found in the fact that two faunas, with dis- 

 tinctive characters, existed simultaneously, one of which furnished 

 the faunal elements with Devonic affinities, but did not make much 

 headway against the resident normal Siluric fauna. Continuing to 

 modify in its own center of distribution, the more specialized fauna 

 finally evolved into the normal Middle Devonic fauna of that region. 



