20 TT. Cross — Post- Laramie Deposits of Colorado. 



Eldridge and the writer.* In this Journal for April, 1S89, 

 the second of these articles " The Denver Tertiary Forma- 

 tion, " was published in revised form. During the last three 

 years further discoveries bearing upon this question have been 

 made in various fields by several observers, and it is desired to 

 present here a connected statement of these new facts, together 

 with a discussion of their significance. 



Description of the Deposits. 



Denver and Arapahoe Formations. — In the detailed exam- 

 ination of the Denver region above mentioned, it was found 

 that the continuous so-called " Laramie " section exposed near 

 Golden consisted of three divisions : 1°, a lower member, TOO 

 -800 feet thick, conformable with the Fox Hills, containing 

 productive coal measures and a flora and fauna characteristic 

 of the Laramie as commonly known ; 2°, a middle member, 

 800 feet thick, unconformable with the lower member, char- 

 acterized by a conglomerate which carries pebbles recognized 

 as coming from the Laramie, Fox Hills, Niobrara. Benton, and 

 Dakota, Cretaceous, the Jura, the Trias, and the Carbonifer- 

 ous ; 3°, an upper member, 1400 feet thick, unconformable 

 with the middle member, and characterized lithologically as 

 composed very largely of debris of andesitic lavas, none of 

 which appeared in the preceding deposits. To the middle 

 member of this series Mr. Eldridge gave the name " The 

 Arapahoe Beds ; " to the upper member was assigned the name 

 "The Denver Beds." It was found that the celebiated 

 fossil-leaf horizon of Table Mountain, at Golden, belonged to 

 the Denver beds and. consequently that the " Laramie " flora 

 of Golden, described by Lesquereux and Ward, belonged to 

 two distinguishable horizons, and chiefly to the upper one. 

 The Arapahoe and Denver beds were found to contain a ver- 

 tebrate fauna not known in the coal measures below, and to 

 this point reference will be made in a later section of this 

 article. 



The stratigraphical and lithological evidence above summa- 

 rized indicated clearly that the Arapahoe and Denver beds 

 were separated from the Laramie below by a long period, of 

 very important orographic disturbances, as attested by the 

 pebbles in the Arapahoe conglomerate. As the fossil flora and 

 fauna of the formations did not seem to be necessarily opposed, 

 the two formations in question were referred in, the publica- 

 tions cited to the early Eocene, and probably to a horizon 



* On some stratigraphical and structural features of the country about Denver, 

 Colorado; by George H. Eldridge. The Denver Tertiary Formation; by Whit- 

 man Cross. Proc. Col. Sci. Soc, vol. iii, part I, pp. 86-133. 



