758 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



that this portion of the formation is of Oligocene age. The lower member may be Ohgocene, or 

 perhaps Wasatch or Bridger, in age. 



In southern Colorado just east of the mountains is the Huerfano basin, described 

 by Hills **^' "® and studied by Osborn.®^^ The latter summarizes Hills's conclusions 

 and his own: 



In this third paper (1891) Prof. Hills positively identified the "upper beds" as equivalent 

 to the Bridger group, and restricted the term Huerfano to these beds, applying the terms 

 Cuchara to the middle division and Poison Canyon to the lower division. These divisions, 

 correlated with the measurements previously assigned them, would then occur as follows: 



{Huerfano beds Bridger group 3,300 

 Cuchara beds 1 Lower Eocene (Green River, Wasatch f 300 

 Poison Canyon beds. . . J and Puerco) (3,500 



WMle these studies were in progress a large collection of fragmentary bones was made by 

 Mr. Milligan, of Gardner, which is now preserved in the museum of the Colorado Scientific 

 Society and which Prof. HiUs kindly enabled the writer to carefully examine. 



The essential features of Prof. Hills's conclusions may be summarized as follows : 



1. The identification of the total Huerfano series of 3,300 feet with the Bridger or middle 

 Eocene, and the provisional identification of the Cuchara and Poison Canyon series with the 

 lower Eocene, in the absence of fossils, upon stratigraphical evidence. 



2. The post-Laramie formation of a great anticlinal axis, as the eastern border of the 

 Huerfano Lake to the east and southeast of the Wet Mountain Range and Spanish Peaks, and 

 the subsequent removal of this axis by erosion. 



3. The eruption of the laccolithic Silver Mountain and Spanish Peaks subsequent to the 

 deposition of upper lake deposits of Bridger age. 



4. The drainage of the Huerfano Lake to the north through the Wet Mountain Valley. 

 It should be stated here that these opinions were expressed at a time when several geologists 



had identified Laramie deposits east of the Rockies as Tertiary, owing partly to Prof. Marsh's 

 identification of a Laramie dinosaur with Bison alticornis. 



For the sake of clearness of contrast it may be well to summarize at this point the geological 

 conclusions formed by the writer and Dr. Wortman during their brief reconnaissance of this 

 region : 



1. That west of the Huerfano Canyon the variegated marls, clays, soft shales, and sands 

 aggregate only 800 to 1,000 feet in thickness, and are nearly horizontal in position. They 

 may be positively divided into Upper beds, equivalent to the Bridger, and Lower beds, equivalent 

 to the Wind River, or Upper Wasatch. These constitute the only true Huerfano Lake deposits. 



2. That the Cuchara and Poison Canyon beds are unconformable with the Huerfano beds 

 and older than the Eocene, probably marine Cretaceous, as partly determined by the presence 

 of a species of Baculites in the yellow sandstone of the typical Poison Canyon section. 



3. That the present canyon of the Huerfano River cuts through the base of the main 

 anticlinal axis of post-Laramie origin, which formed the eastern boundary of the lake. This 

 axis extended to the south so as to include the base of SUver Mountain toward the Cuchara 

 divide; but it lies from 3 to 7 miles west of the anticlinal axis described by Prof. HiUs. 



4. That the Huerfano Lake deposition did not extend as far to the east or south as the 

 Spanish Peaks, and that the variegated beds observed there are of older origin. This would 

 materially effect the geological age of the prominent neighboring laccoliths. 



The geological features of these conclusions can hardly be dignified by the term "a theory 

 of the Huerfano Lake," for thev were formed during a hasty reconnaissance of this basin, while 

 Prof. Hills's theory certainly deserves the deliberate consideration of a prolonged survey. In 

 fact, this basin, with its volcanic disturbances and eruptions, presents in compact form a 

 fascinating problem in the geology of Tertiary times. 



