32 Later Extinct Floras of JVbrth America^ 



argillaceous rook, of a light drab color, upon which the leaves 

 are delineated with a distinctness which renders them pleasant 

 objects of study, as well as attractive specimens for the cabinet. 

 They are usually detached with their petioles in such numbers 

 and form as indicate maturity and a common cause of fall, 

 such as an annual frost The mollusks associated with them 

 show thai they wore deposited in the sediment which accu- 

 mulated at the lioftom of fresh water, and they are generally 

 read out smoothly, and so entire, that if is evident that no 

 violence, not even the action of a rapid current, could have 

 been attendant upon their deposition. 



The explorations* of Dr. Hayden prove that this Miocene 

 lignite formation occupies the beds of extensive lakes which 

 formed basins on the surface of the continent when it had but 

 recently emerged from the Cretaceous sea. As has been re- 

 marked elsewhere, the lower members of the series contain a 

 few estuary shells, showing the a -alt water at that 



period, but during the deposition of by far the greater portion 

 of these beds the water of the ocean was entirely excluded 

 from the basins in which they accumulated. There is. there- 

 fore, every reason to believe thai the dele-is of ligneous plants 

 which compose this collection were derived from trees which 

 grew along the Bhores of the lakes and Btreams of the Tertiary 

 continent : that then, a- now. alternations of seasons prevailed, 

 by which the foliage of these trees was detached by an autum- 

 nal frost, and that falling into the water beneath or near them, 

 and sinking to the bottom, they were enveloped in mud, pre- 

 cisely a- leaves of our ,-\ camores, willows, oaks, &c, accumulate 

 at the bottoms ofour Btreams and lake- at the presenl day. 



In comparing the group of plants here presented to us with 

 those now living upon the surface of the earth, any one will 

 at once struck with the resemblance which they present 

 to the flora of the temperate /.one. and particularly to that 

 of our own country. In their Btudy, I have constantly 

 found that on making comparison! with the plants of remote. 



