THE CAMBRIAN PERIOD. 245 



of North America. The Olenelli of Newfoundland and Europe were thought 

 to be so different from the Olenelli of most of North America (see Fig. 118, p. 

 281) as to be more properly referred to another genus (Holmia), and the Holmia 

 fauna, as a whole, was thought to be simpler than the Olenellus fauna, and there- 

 fore perhaps older. Furthermore, the distinct Paradoxides fauna is found only 

 in those regions where the Holmia fauna occurs; it is not in immediate asso- 

 ciation with the commoner types of Olenellus. The stratigraphic relations of 

 the types of Olenellus and Paradoxides faunas therefore is not established, if 

 the Holmia and Olenellus faunas be not one. Question has also been raised as 

 to the contemporaneity of the Middle Cambrian fauna of the larger part of 

 North America, and the Paradoxides fauna of Europe. 



In the Hudson-Champlain valley the Olenellus fauna appears to have lived 

 on until the advent of the Dikellocephalus fauna. No well-marked Middle 

 Cambrian fauna intervenes, though sedimentation seems to have been uninter- 

 rupted. 



The questions which have been raised may lead to some minor modification 

 of former views, but do not seem likely to change them fundamentally, or to 

 lead to very different opinion concerning the course of the physical history of 

 the continent in the Cambrian period. They seem to make two interpretations 

 possible. The one is that the Holmia fauna of Europe and Newfoundland was 

 contemporaneous with the Olenellus fauna of North America. According to 

 this view, they are one geologically (chronologically), but somewhat distinct 

 geographically. In this case it would not be strange that the succeeding faunas 

 were also somewhat unlike, the Paradoxides fauna succeeding the Holmia in one 

 region, and the Middle Cambrian, without the genus Paradoxides, succeeding 

 the Olenellus fauna elsewhere. The geographic diversity may have been as 

 great in the Middle as in the Early Cambrian. According to this view, the 

 Olenellus fauna of the Hudson-Champlain valley may have been contempora- 

 neous with both the Holmia and the Paradoxides fauna of the region farther 

 east. A Holmia has been found in Pennsylvania, but a partial barrier between 

 Newfoundland and the Hudson-Champlain valley, is supposed to have prevented 

 free mingling of the contemporaneous Holmia and Olenellus faunas. It must 

 be further supposed that the causes which brought about a change from the 

 Early to the Middle Cambrian faunas, both to the east and to the west, did not 

 produce like results in the Champlam valley, since the Lower Cambrian (Olenellus) 

 fauna lived on. None of these assumptions are in violation of principles which 

 are believed to have been in operation throughout geological history. 



The alternative interpretation is that the Holmia fauna is really older than 

 the Olenellus proper, and that the Olenellus of the west and of the Hudson- 

 Champlam valley may have been contemporaneous with the Paradoxides of 

 the east. According to this interpretation, the Paradoxides fauna of Newfound- 

 land may have been contemporaneous, at least in part, with the Olenellus fauna 

 of New York and with the Olenellus and Middle Cambrian faunas of the west. 

 Like the last, this interpretation does not violate known principles of life develop- 

 ment, for two distinct faunas in one region may have been contemporaneous 



