LATER GEOLOGICAL HISTORY OF NORTH AMERICA 23 



Toward the close of the Eocene and in the early OHgocene the 

 climate seems to have reached its maximum of warmth — the forests 

 along the Gulf coast are sub-tropical, coral reefs are found in 

 Georgia and temperate forests covered Alaska, Greenland and even 

 the lands as far north as Spitzbergen. At this time there was again 

 a free interchange of Hfe, both animal and vegetable, between 

 North America, Europe and Asia. 



With the continued elevation of the Rocky Mountain region 

 great changes in cHmatic conditions were brought about in the West 

 and the plains type of country became developed over large areas. 

 This was important since it represents the beginning of that cleavage 

 between the modem Atlantic and the Pacific forest and general 

 floral provinces which exhibit so many striking contrasts. The 

 accompanying map (fig. 4) shows the extent of the OHgocene seas 

 and the area of the plains country which was called into existence 

 by the rising mountains which cut off the moisture laden winds 

 from the Pacific and caused the forests to be gradually replaced 

 by the prairies. 



During the succeeding Miocene and Pliocene periods many identi- 

 cal trees persisted in the East and the West but they were effectually 

 cut off from intercommunication except in the far North by the 

 central arid region and each area has henceforward had a history 

 entirely its own. 



Fossil floras of these later Tertiary times are rather rare and are 

 only found in local deposits in some favorably situated mountain 

 lake basin or river flood plain or in the chance burial of drifted 

 vegetable debris in the predominantly marine deposits of the con- 

 tinental borders. The most extensive of these local floras is that 

 preserved at Florissant, Colorado, where successive showers of 

 volcanic ashes have preserved remnants of a large lake and river- 

 side upland flora. The cHmate appears to have been somewhat 

 warmer than it is at the present time in the heart of the Rockies 

 and this flora includes species of figs, magnolias, soapberry, persim- 

 mon, ailanthus, beech, elm, maple, locust and oak. Libocedrus 

 or incense cedar is stiU present and the sequoia trunks which were 

 silicified at the same time that the foliage was preserved in the 



