60 GEOLOGY OP THE DENVEE BASIN. 



In the San Juan area of southwestern Colorado, Mr. R. C. Hills 1 found 

 in the upper part of the red sandstones lying between the purple Permo- 

 ( larboniferous (?) sandstones below and Jurassic limestones above \ ertebrate 

 and plant remains indicative of a Triassic age. 



Newberry 2 and Cope 3 have separately referred certain plant and 

 vertebrate remains discovered in the red sandstones "I New Mexico to the 

 Upper Trias. In the present volume Prof. 0. C. Marsh also mentions the 

 presence in the red sandstones of New Mexico and Arizona of dinosauriau 

 and crocodilian remains of Triassic types. 



Inasmuch, then, as there seems to be reasonable ground for the 

 assumption thai a portion, at least, of the Red Beds horizon of the Rocky 

 Mountain region is Triassic in age, and since the beds treated in this report 



occur immediately beneath well-defined Jurassic strata and very probably 

 represent the upper part of the great series of Red Beds, it seems proper 

 to designate them, provisionally at least, Triassic. 



The dividing line between this series of strata and the succeeding, the 

 Jurassic, must, in the absence of paleontological evidence, he somewhat 

 arbitrarily taken: in this case it has been determined mainly on structural 

 grounds — an undulating line, as of unconformity or at least of interrupted 

 deposition — though stratigraphy and a lithological character allied to the 

 underlying Red Beds have had somewhat to do with the division. 



JURA. 

 MOKKISON FORM VI'lON. 



Throughout the Denver field and for much of the distance along the 

 eastern base of the Rocky Mountains iii Colorado the Jura is essentially a 

 formation of fresh-water marls, of an average thickness of about 200 feet. 

 Its upper limit is sharply defined l>v the Dakota sandstone, while the brown 

 and pink sandstone closing the Trias as clearly marks its lower limit. To 

 this formation ha- been assigned the name "Morrison," from the town near 



which it i- typically developed. 



\.'t, .hi the occurrence of fossils in the Triassic and Jurassic beds near San Miguel, in Colo- 

 rado, Am. Jour, Soi, ■ . \ ol. XIX, p, 190, 1880; Jura-Trias of southwestern Colorado, ibid., Vol. 

 XXIII. p, 243, 1882. 



Mon. 1". s. Geol. Survey No, \l\ . pp. 8-15, 1888. 



\ m Philos, Soo., Proc, Vol. XXIV, p.227, 1887. 



