EAELIEE TERTIARY (EOCENE AND OLIGOCENE). 759 



Hills *"^ describes the Poison Canyon, Cuchara, and Huerfano formations as 

 they occur in the Spanish Peaks quadrangle. The two older formations (Poison 

 Canyon and Cuchara) are placed doubtfully in the Eocene, the Poison Canyon being 

 regarded as possibly equivalent to the Arapahoe of the Denver basin. Osborn's 

 determination (1897) of the Eocene (Bridger and Wind River) age of the Huerfano 

 formation is sustained by later studies of the fauna by Wortman and Matthew.^^^® 



J 13. EAST-CENTBAL COLORADO. 



The Florissant lake beds, discovered by Hayden and described by Peale and 

 Scudder,^^® occupy an area too small to be shown on the map, about latitude 39°, 

 longitude 105° 20', in the pre-Cambrian mass of the Colorado Front Range. The 

 beds consist of volcanic ash, from showers and washed volcanic detritus, more 

 than 50 feet thick. The deposit is noted for the rich harvest of fossil insects it 

 has yielded. For descriptions of these fossils Scudder''^"'''^''''^-'^^^ and Cockerell '®^' '''^ 

 should be consulted. 



J-K 12-13. UINTA VALLEY, ROAN OR BOOK PLATEAU, AND GRAND MESA. 



South of the Uinta Range in Utah lies the Uinta Valley, which Emmons ^^^ 

 examined cursorily in course of his work for the Fortieth Parallel Survey. The strata 

 extend into northwestern Colorado and were there studied by White, ^^* who dis- 

 tinguished the Wasatch, Green River, Bridger, and Uinta groups, applying the 

 term Uinta to the rocks described by Powell as the Browns Park group. The 

 Wasatch, Green River, and Bridger are described as conformable, and the Uinta as 

 resting unconformably upon the Bridger and older formations around the east end 

 of the Uinta Range. ' According to the recent work of Gale^®' in this area, the Browns 

 Park formation is of later age than the true Uinta of King. It underlies the Bishop 

 conglomerate (Bishop Mountain of Powell), which is the same as the Wyoming con- 

 glomerate of the Fortieth Parallel Survey. In Gale's preliminary report ^^* the 

 Tertiary formations are described as follows : 



[The "Browns Park" formation] consists of loose or slightly consolidated sandy material 

 with local harder sandstone beds and some beds of gravel. Contains much calcareous material 

 in the form of cement or filling between the quartz sand grains. Its color is everywhere chalky 

 or limy white. The thickness is not determined. 



[The Green River formation is] composed of shale, sandstone, and beds of oolitic rock. 

 The shaly beds predominate and are very compact and firmly bedded. They are generally 

 exposed in escarpments and high bluffs, in which the weathered beds have a very characteristic 

 chalky-white aspect. The shales are, however, of various shades of gray, drab, and light brown 

 and are in many places hard and thin bedded. In some districts the lower part contains much 

 massive white sandstone. Measured sections exceed 2,400 feet; upper limit not reached. 



[The Wasatch formation is] composed chiefly of clay or soft clay shale; commonly variegated, 

 but various shades of red and drab predominate. It also contains beds of pebbles or conglom- 

 erate of very perfectly rounded siliceous material, such as jasper, colored vein quartz, chert, 

 or flint. Sandstones in places very massive and hard. From 4,000 feet in thickness on the 

 eastern side of the Uinta Basin to about 2,500 feet near the Utah line. 



[The Fort Union or earlier beds are] not readily distinguished from the Wasatch in the 

 Uinta Basin, but are apparently more clearly differentiated in the Yampa field. As devel- 

 oped in the latter region they consist of massive white or light-colored sandstones and shales 



