80 DESCRIPTIVE GEOLOGY. 



series of argillaceous sandstones, with thin layers of fine red clay and beds 

 of gypsum, the latter varying in thickness from two or three inches up to 

 several feet ; the thickest deposit of pure solid gj^psum, measuring 22 feet 

 in width, lying between two beds of hard red sandstone. Nowhere in the 

 region of the Eocky Mountains, within the belt of this exploration, are the 

 gypsiferous deposits so well developed, at the same time so easily accessible 

 from the railroad. Overlying the gypsum series, again occur the red sand- 

 stones, passing up into yellowsh-red friable beds, shading off into white sand- 

 stones and marls, which may represent the base of the Jurassic series. 

 Above the white sandstone occurs a bed of fine bluish-gray cherty limestone, 

 which crops out prominently, and probably represents the stratum which 

 is so persistent east of the Colorado Eange; this, in turn, is overlaid by 

 white and cream-colored sandstones and marls, passing into brownish 

 beds, with layers of variegated clays and marls, and again overlaid by fri- 

 able white sandstone, which appears to form, in many places, the summit 

 of the plateau. 



The following section represents more clearly the succession of strata, 

 with estimated thicknesses, beginning with the Jurassic on the summit of 

 the plateau, and descending to the base of the Triassic, which rests on 



Archsean rocks : 



Jurassic. 



1. Friable white sandstone "^ 



2. Eeddish brown sandstone, with layers of variesrated clavs and I 



^ ^ y 100 feet, 



marls 



3. Cream-colored sandstones and marls i 



4. Bluish-gray cherty limestone 25 feet. 



5. Grayish- white sandstone - 75 feet. 



Triassic. 

 G. Yellowish-red sandstone \ 



7. Fine deep-red sandstone ...j 



8. Argillaceous sands and shales, with iuterstratified layers of fine cloy 



and gypsum, including one bed of gypsum 22 feet thick 150 feet. 



9. Eed compact sandstone ,. 250 leet. 



10. Reddish-gray sandstone ^ 



11. Thin bed of gray cherty limestone >■ 225 feet. 



12. Coarse, friable, ash -colored sandstone, with pebbles y 



1, 200 feet. 



