38 DESCRIPTIVE GEOLOGY. 



mination have been obtained from Jurassic rocks east of the Colorado 

 Range; but as characteristic strata have been easily recognized, which are 

 very persistent wherever the Jurassic occurs, and beloAV which, on the 

 Laramie Plains not far above the Red Beds, well-defined Jurassic fossils 

 have been identified, it seems, for the present at least, desirable to place 

 the line of separation at the top of the compact red sandstones. 



In thickness the rocks, which have been referred to the Jurassic, vary 

 from an extreme development of 250 feet, down to less than 75 feet. They 

 attain their greatest thickness in Colorado in the region of the Big Thomp- 

 son Creek. To the northward, their average width may be taken at 200 

 feet. In Wyoming, along Lodge Pole and Horse Creeks, they appear less 

 clearly defined, being in part obscured by loose soil, and in part resembling 

 the Triassic beds. Here they are probably represented in some places by 

 only 75 feet of strata. Still farther to the northward, they expand again 

 to at least 150 feet, showing most of the characteristic beds to be found in 

 Colorado and Western Wyoming. The lowest beds in the Jurassic series 

 are usually reddish-yellow sandstones and shales, passing into lighter- 

 colored beds, partaking more and more of a marly nature, with inter- 

 stratified beds of hard sandstone and some limestones. The upper beds 

 are friable sandstones interstratified in compact gray and cream-colored 

 marls, with varying proportions of lime. In several localities, these cal- 

 careous marls would seem to be represented by well-defined beds of lime- 

 stone. 



At Box Elder Creek, near where the stream leaves the mountains, the 

 following section was made across the Jurassic beds: 



1. Fine friable sandstone. 



2. Gray marls and clays. 



3. White marls. 



4. Yellowish calcareous sandstone. 



5. Cherty limestone. 



6. Orange sandstone with light-colored clays. 



7. Gray marl with purple and reddish-brown bands of clay and thin layers of sand. 



8. Gray arenaceous marl. 



9. Reddish-yellow friable sandstone- 



It represents a section of rock strata of from 200 to 250 feet in thickness. 



