10 OUTLINE OF PLANT GEOGRAPHY 



are associated with a flora composed for the most part of ferns, 

 cycads and conifers. The earliest fossils supposed to be angio- 

 sperms are of somwehat doubtful nature, and their relationships 

 are obscure; but even in the lower Cretaceous there are found 

 remains of such modern trees as oaks and willows, and in the 

 upper Cretaceous a great many existing genera are met with. 

 By this time most of the existing genera of conifers — pines, firs, 

 cedars, etc. — were well established, and often much more wide- 

 spread than at present. And such familiar angiospermous trees 

 as oaks, poplars, sycamores, sweet gum (Liquidambar), magnolias, 

 laurels and many others were found in many parts of the world. 



During much of the Cretaceous there are evidences of extensive 

 invasions of the land by the ocean, so that the distribution of the 

 principal land masses was very different from that now existing. 



In North America the Gulf of Mexico was connected with the 

 Arctic Ocean, completely separating the western portion of this 

 continent from the Atlantic area. Much of South America was 

 submerged, and extensive invasions of the sea took place in 

 Europe and Asia, and in Australia there are indications that an 

 ancient West Australian continent was separated from the north- 

 eastern part, then probably united with New Guinea. 



The end of the Cretaceous was a period of mountain building, 

 especially in western America, and with the land elevation, 

 North America assumed much of its present configuration. 



In Europe and Asia the Tertiary was a period of mountain 

 building, and the present conformation was attained at. a later 

 period than in America. 



While the northern continents had assumed very much their 

 existing condition before the end of the Tertiary, there were at 

 various periods direct connections between Eurasia and North 

 America. Remains of the land bridge still exist between Alaska 

 and Kamtchatka, and the mainland of Siberia is separated from 

 Alaska only by the narrow Behring's Strait. 



The connections between Europe and northeastern America are 

 much less obvious, but there is ample evidence that such connec- 

 tions did occur during the Tertiary. 



With these changes there is evidence from the fossil plants that 

 there was a general lowering of temperature during the Tertiary 

 in the northern hemisphere. During the earlier period (Eocene, 



