CALIFOBNIAN PALAEOGEOGEAPHY 199 



west at that time, since no late Mesozoic deposits are known 

 from the western parts of that country. If we supposed that 

 western Mexico had then been connected with some other 

 land surface, a faunistic interchange could have taken place 

 between the latter and western North America. 



In early Tertiary times the central sea, which formed the 

 eastern boundary of the western belt of land referred to, had 

 almost disappeared from the interior of America, but large 

 tracts of western California were still under water (see 

 Fig. 14). Professor Smith* argues that a temporary con- 

 nection must have existed during the Eocene Period between 

 the Atlantic and the Pacific Ocean, because the cha- 

 racteristic Atlantic shell Venericardia planicosta had been 

 met with in the Eocene deposits of California and Oregon. 

 In Oligocene and Miocene times the whole of the eastern 

 borders of Mexico were submerged, while the sea was at 

 first retreating from western California and then again 

 invading it. During the latter part of the Miocene Period 

 the sea even encroached on western Mexico.f All the 

 same, certain parts of the coast ranges in western Cali- 

 fornia never seem to have been entirely submerged during 

 Tertiary times and probably formed part of the Pacific land 

 belt which has now almost entirely vanished. I think 

 the alternative union and disruption of these western Cali- 

 fornian land-masses with the mainland of North America 

 must have played an important role in the origin and 

 development of the American fauna. It seems as if Mexico 

 had at first formed the stepping-stone to North America for 

 new immigrants, and later on western California. I have 

 endeavoured to represent this idea on two maps (Figs. 14 and 

 16), but how the changes were actually brought about has not 

 been made quite clear to us through geological research. 



It has been suggested by Messrs. Ordonez and Aquilera that 

 the Cape portion of lower California really forms the western 

 continuation of the Mexican Sierra del Sur.J But the very 

 important question now arises from a zoogeographical point 



* Smith, Perrin, " Geological History of California," pp. 347348. 

 t Arnold, Balph, "Tertiary Pectens of California." 

 J Suess, E., " Antlitz d. Erde," IIP, p. 487. 



