1902.] AND ANCIENT GEOGRAPHY. 363 



■united with Brazil, to which it belongs tectonically, while, i?i the 

 Upper Cretaceous, Guiana was united with Central America, and 

 was separated from Brazil by the transgression of the Atlantic 

 Ocean in the Amazonas valley. 



This latter strait thus formed a continuation of the South Atlantic 

 Ocean, which came into existence, as we have seen above (p. 350), 

 at about the middle of the Cretaceous. 



The Upper Cretaceous conditions were generally preserved in this 

 region during the beginning of the Tertiary, and the Eocene and 

 Oligocene sea extended, in the Amazonas transgression, far to the 

 west (brackish Oligocene deposits are known near Pebas, Peru). But 

 during this time (older Tertiary), the elevation of the Cordilleras 

 must have become evident l in the western parts of this interoceanic 

 connection, since older Tertiary deposits are wanting in this region. 

 Thus those parts which now comprise Colombia, Ecuador, Peru 

 and Bolivia became land, and the Amazonas Strait was shut off from 

 the Pacific Ocean, being transformed into a deep bay, which occu- 

 pied the Amazonas valley as far as the foothills of this new eleva- 

 tion (Cordilleras). Therefore, this interoceanic connection was 

 interrupted in the beginning of the Tertiary, the main part of South 

 America, or the old " Archiplata " of von Ihering, becoming united 

 with northern South America (Venezuela and Guiana). But we 

 have seen above (p. 344) that at the same time (earlier Tertiary or 

 uppermost Cretaceous) another interoceanic connection had formed 

 in the isthmian region, and this replaced the Amazonas connection 

 of the Upper Cretaceous era. 



The old connection of the Brazilian mass with Africa continued 

 in part as we have seen (p. 350) during the Upper Cretaceous, for 

 its northern portion, Guiana. That is to say, an intermigration 

 of the faunas of Guiana and Africa was yet possible in the Upper 

 Cretaceous. The fact that during this time (and in the beginning 

 of the Tertiary) a strait or bay extended along the region of the 

 Amazonas river as far as the Pacific Ocean (or as far as the 

 Cordilleras), furnishes the explanation for the zoogeographical fact 

 that animals immigrating from Africa into Guiana during the 

 Upper Cretaceous could reach Central America and the West 

 Indies, but not those parts of Brazil which are to the south of this 

 old Amazonas Strait : this seems to apply to our Potamonidcz, and 



1 The first signs of an elevation belong to the Upper Cretaceous. 



