NATURAL SCIENCES OP PHILADELPHIA. 



433 



General Section of the Tertiary rocks of Nebraska. 



The Fort Union, or Great Lignite Group, occupies extensive areas of 

 country in Nebraska, and has been seen beneath the White River Group at 

 several distant localities. It was evidently deposited in large bodies of water, 

 which were at first brackish, and then gradually became fresh. The great 

 numbers of fossil leaves, and numerous beds of lignite contained in it, clearly 

 show that the shores of these ancient estuaries, lakes, &c, in which this for- 

 mation was deposited, supported dense forests of large trees, and a growth of 

 other vegetation, far exceeding in luxuriance anything now met with in 

 these latitudes. Indeed, the presence of true Fan Palms, of large size, and 



1861.] 



:9 



