PRINCIPLES OF PALEONTOLOGIC CORRELATION 685 



common. These differences Roemer ascribed to climate, noting 

 that then, as now, the isothermal lines came much further south 

 in eastern America than in Europe. 



It has been shown that at the beginning of .Cretaceous time 

 the faunal relations of the west coast of North America were 

 still with the Boreal region, as in the Upper Jurassic. But this 

 did not last long, for even before the end of the Knoxville 

 epoch this fauna had died out, and was replaced by immigrants 

 from another region. At first there were only a few stragglers, 

 but soon the rich fauna of the Horsetown stage or Gault made 

 its appearance, of a type precisely like that of southern India, 

 and eastern Africa. A similar association of genera and species 

 is also known in the European region, where it seems to have 

 been endemic, and from which it probably reached the rest of 

 the world by migration. This incursion of exotic faunas marks 

 the last great period of readjustment of the geologic column in 

 various parts of the world, and is therefore of the utmost impor- 

 tance in correlation. The kinship of the western American 

 faunas to the Indian was stronger than that to the eastern 

 American almost until the end of the Cretaceous, when a simi- 

 larity to the interior province began to show itself. This change 

 culminated in Eocene time, in the zone of Venericardia planicosta, 

 when the barrier between the western and the interior Cretaceous 

 provinces was temporarily removed, and through the Atlantic 

 there was direct connection with the European waters. This is 

 the last interregional zone, but it marked an era of retrogression 

 of the sea, rather than of transgression, and since that time the 

 marine provinces and regions correspond closely with the exist- 

 ing boundaries of temperature and shore lines. 



Dispersion of marine animals in past a?id present. — Theoretically, 

 pelagic faunas would be the best means of correlating distant 

 regions ; but in all probability we have no fossil pelagic faunas. 

 J. Walther suggests that in the widely dispersed species of 

 Mesozoic ammonites we have virtually a preservation of pelagic 

 animals, or at least that their shells floated after death, and 

 were distributed all over the earth by marine currents. This 



