THE TERTIARY LAKE BASIN OF FLORISSANT, 



COLORADO 



By Junius Henderson 



During the summer of 1905 an expedition, under the auspices of 

 the University of Colorado, visited the ancient lake basin at Floris- 

 sant, Colo., about 40 miles west of Colorado Springs and 20 miles 

 from Cripple Creek. Large and valuable collections of both fossil 

 and recent plants and animals were secured. As a result, some hitherto 

 unnoticed species have already been described from that region, and 

 it is hoped that further information of importance may be given to the 

 scientific world as work progresses. 



The literature of the Florissant formation is scattered through 

 numerous monographs, reports, and other publications which are not 

 much read except by those engaged in special geological or biological 

 studies. Consequently this interesting region is not well known to 

 the general public. It is therefore deemed best to pubUsh this account 

 setting forth in a general way some of the principal facts observed 

 by us or hitherto published by others, with a few comments thereon, 

 together with hints as to unsettled questions and a bibhography of the 

 literature of the subject, so far as known to the writer. 



The Tertiary lake beds of Florissant have long been known to 

 geologists, and particularly to paleontologists, because of the great 

 number and variety of fossil plants and insects they contain, besides 

 some fossil fish, mollusks, and birds. The basin is one of the most 

 important in the world for fossil insects, and fossil leaf impressions 

 may be easily collected by thousands. 



These beds have generally been assigned to the OHgocene division 

 of the Tertiary age, but it is perhaps more in accordance with modern 

 American usage to consider the OHgocene merely the upper part of the 

 Eocene, and hence to call the Florissant formation Eocene. The 

 determination of the age of this formation necessarily depends entirely 

 upon paleontological methods. As the deposits were formed in a 



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