CHAPTER III, 



CLASSIFICATION, DESCRIPTIOI^, AND DISCUSSION OF THE 

 GROUPS OF STEATA. 



The formations of this district present essentially an unbroken series 

 from the Weber Quartzite, which lies immediately beneath the Carbon- 

 iferous series, but is itself probably of older Paleozoic age, to the Uinta 

 Group, which is the latest of the fresh-water Tertiary deposits of that 

 region^ inclusive. 



The accompanying illustration. Fig. — , is intended to represent in 

 their order a general section of these groups of strata, as they are de- 

 veloped or exposed in this district, to which area alone the thickness 

 assigned to each is intended to apply. Following this illustration is a 

 brief description of each of the groups represented by it, which is in- 

 tended for more immediate reference in connection with the figure; but 

 more detailed descriptions of the groups will appear where tbey are 

 discussed on following pages. 



Explanation of the section. Fig. 1. 



No. 1. Uinta Group. — Fine and coarse-grained friable sandstones, with intermixed 

 gravel in some jjlaces, forming a conglomerate ; distinctly or obscurely stratified ; the 

 materials composing the strata in some places nearly or quite incoherent. General 

 aspect gray or dull brownish red. Unconformable by sequence upon the other Tertiary, 

 and several of the older groups in the district. Thickness about 1,200 feet. 



No. 2. Bridger Group, — Variegated ; reddish, grayish, greenish, and purple bad-laud 

 sandstones. Thickness exposed in this district only about 100 feet. 



No. 3. Green River Group. — Upper division consisting of irregularly-bedded sand- 

 stones, often concretionary, with occasional laminated carbonaceous layers. Lower 

 division consisting of laminated sandy shales or thin-bedded sandstones. Sometimes 

 finely laminated shales which .ire more or less caicareous, and occasional dark carbona- 

 ceous layers are interspersed. Maximum thickness of both divisions about 1,400 feet. 



No. 4. Wasatch Group. — Alternating toft and harder sandstones at top rnd base, be- 

 tween which the greater part of the group is made up of soft variegated bad-land sand- 

 stones. These bad-land sandstones are generally somewhat softer in the eastern than 

 in the western part of the district. Thickness a' out 2,000 feet. 



No. 5. Laramie Group. — Sandstones ; thinly bedded, or in some places more heavily 

 bedded, and forming hogbacks at many of the flexures of that formation in the district. 

 Color sometimes reddish-ferruginous, and sometimes of the usual dirty yellowish or 

 grayish hue ; variegated in places by carbonaceous layers, and beds of coal. Tnick- 

 ness about 3,500 feet, but in some places probably less. 



No. 6. Fox Hills Group. — Sandstones, lieavily or thinly bedded; or sandy shales. 

 Often weathering so soft at the base of the group as to obscure any line of demarca- 

 tion between it and the next beneath. Thickness about 1,800 feet. 



No. 7. Colorado Group. — Dark-colored shales, clayey or sometimes quite sandy ; with 

 occasional thin layers of sandstone. Thickness about 2,000 feet in the eastern part of 

 the district and much less in the western part. 



No. 8. Dakota Group. — Yellowish or brownish rough sjindstones above ; irregularly 

 bedded pebble conglomerate below; and between these there is usually a greater or 

 less thickness of variegated sandstone. Thickness about 500 feet. 



No. 9. Jurassic. — Variegated, soft, bad-land sandstones with some strata of more in- 

 durated sandstone, and usually from 10 to 15 feet in thickness of shaly, sandy, and 

 calcareous, fossiliferous rock near its base. Thickness about 600 feet. 



No. 10. Triassicf — Yellowish, obliquely-laminated firm sandstones above ; yellowish, 

 soft sandstones below, and between them a series of bright red or brownish red, more or 

 less firm sandstones. Thickness about 2,000 feet. 



No. 11. Uj}2)er Carhoniferous. — lixegularly bedded, light-yellowish sandstones with 



19 



