CALIFORNIAN PALAEOGEOGRAPHY 
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. Luring 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. 347—348. 
t Arnold, Ralph, “ Tertiary Pectens of California.” 
f Suess, E., “ Antlitz d. Erde,” III 2 , p. 487. 
