LATER GEOLOGICAL HISTORY OF NORTH AMERICA 21 



US the choice of two areas as the place of origin of the flowering 

 plants. These are the Arctic region, or the vast and geologically^ 

 unknown Asiatic region. 



As Upper Cretaceous times drew toward their close the conti- 

 nental seas became more restricted and the shores withdrew from 

 positions indicated by the solid lines indicated on the accompany- 

 ing map (fig. 2). At this time the Rocky Mountain region within 

 the lined area on the map became a region of delta and coastal swamp 

 deposits, Vv^hich, as the shallow seas silted up or were drained, 

 extended eastward until the whole of the present plains region was 

 one of scattered swamps. This is why we now find the widespread 

 and valuable lignitic coals throughout this area. These events 

 brought to a close the last of the many marine submergences of 

 the interior of North America. Henceforward the subsequent 

 marine invasions of North America were strictly marginal and of 

 relatively small and progressively decreasing extent. 



With the geological changes which marked the close of the 

 Cretaceous in North America and with the gradual elevation of the 

 Rocky Mountains during the Tertiary a very different flora came 

 to occupy the site of the old lowland warm temperate forest. This 

 flora during the early Eocene was largely temperate in its character 

 and appears to have invaded western North America from the 

 north. Figs, magnohas and palms were still present in that region 

 but in dwindhng proportions and the forests contained very many 

 species of cottonwood, sycamore, hazel, sheep-berry, etc. The 

 coniferous trees Glyptostrobus, sequoia and the bald cypress 

 (Taxodium) were present in abundance, and a curious feature was _ 

 the presence of Ginkgo of which only a single Asiatic species sur- 

 vives in the modern flora. 



Southeastern North America had, at this time, a much enlarged 

 Gulf of Mexico as shown on the accompanying sketch map of the 

 Eocene geography (fig. 3). The low shores of this primitive Gulf 

 of Mexico were covered with a warm climate strand flora and con- 

 tained but few representatives of our familiar trees. A great 

 variety of tropical types had invaded that region from the south, 

 many of which like the bread-fruit and Nipa palm are no longer 

 found anywhere in the Western Hemisphere. 



