CHAPTER VI 



STRATIGRAPHY— MESOZOIC FORMATIONS. 

 The Mesozoic formations in our district are divided about as follows: 



Thickness in feet. 



Triassic 1, 000 to 1,500 



Jurassic 400 to 900 



Cretaceous 4,000 to 4,700 



-5, 400 to 7, 100 



In the Triassic beds the arenaceous element seems to predominate, a 

 few bands of limestone appearing in the Jurassic. Sandstones and marls 

 prevail in the Dakota group. In the rest of the Cretaceous, shales form 

 the largest portion of the strata, alternating with sandstones and thin 

 bands of bluish limestone. 



The red sandstones of the Trias (?), in the eastern part of the district 

 have a more laminated structure than wa§ noticed either in the Front 

 range in 1873, or in the Gunni«son this year. Cretaceous strata cover, 

 larger areas than any of the underlying strata, as will be noticed as we 

 proceed. 



TRIASSIC. 



Although the red sandstones which are referred to the Triassic form a 

 well-defined lithological series, and are prominently exposed over the 

 Rocky Mountains and at widely-separated localities, with very little 

 change, less is, perhaps, definitely known in regard to their age than of 

 any of the sedimentary formations of the West. 



The correctness of the assumption of Triassic age for them depends 

 entirely upon their position. Up to the present time no fossils have 

 been found in them. The character of the sandstones is not favorable 

 to the preservation of organic remains. 



They have been referred to the Triassic by Marcou, Newberry, Hay- 

 den, and others who have studied them in the West. 



In the Black Hills and at Red Buttes, on the North Platte, in Wy- 

 oming Territory, they underlie well-defined Jurassic layers, as deter- 

 mined by Dr. Hayden. In Colorado, also, they are beneath Jurassic 

 layers, along the eastern edge of the Front range. Near Pleasant Park, 

 in 1873, I found Carboniferous fossils* in a series of red limestones and 

 calcareous sandstones. These beds were beneath the red sandstones. 

 Again, as already mentioned in the preceding chapter, I found fossils of 

 Permian age below the Red Beds, so that they must be referred either to 

 the Permian or to the Jurassic, leaving an unoccupied gap between. 

 Therefore, until fossils are found by which their age can be definitely 

 settled, I think it best to refer them to the Triassic. 



The line between the Triassic and the Jurassic is indefinite, and I 



* Report United States Geological Survey, 1873, page 198. 



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