„.T,v.Tx-pi GEOLOGY EAST OF FEOXT RANGE CRETACEOUS. 105 



spun River they always retain enongli of their typical character, so that I have never 

 been at a loss to detect their presence at once, although after leaving the Missouri 

 River we do not find any well-defined lines of separation, either lithologically or pale- 

 ontologically. 



In Colorado tbe want of fossils in No. 1 has made its position the 

 most uncertain of any of the five -, but Dr. Peale's discovery in these beds 

 of the fragments of leaves in his section near the South Platte seems to 

 fix this horizon with quite a degree of certainty. 



Professor Lesquereux, in writing to Dr. Peale in relation to these 

 leaves, says that these are fragments of a Proteoides, very near Proteoides 

 acuta, (Heer,) if not a small form of the same. As yet no leaf of this 

 genus has been recognized in our American measures higher than the 

 Dakota group, or ISTo. 1. These sandstones, then, probably lie at the 

 base of the Cretaceous. 



In the so-called Jurassic and Triassic, I believe that not a single fossil 

 has rewarded this summer's search. In the Upper Missouri, not only 

 does well-defined Jurassic underlie No. 1, so that we might expect to 

 find it beneath No. 1 in Colorado, but Dr. Hayden has traced it also 

 directly to Lake Como, near the Union Pacific Eailroad, where it con- 

 tains well defined Jurassic fossils. 



In tracing it further south along the base of the mountains the fossils 

 disappear ; but in a section exposed on the Box Elder Creek, north of 

 the Big Thompson, Dr. Hayden found some beds which he described 

 (report, 1869, p. 119) as " undoubtedly the usual Jurassic beds, with all 

 the lithological characters as seen near Lake Como," and containing a 

 species of ostrea and fragments of Pentacrinus asteriscns, a characteristic 

 Jurassic fossil. These beds are the same that occur near and above the 

 cherty limestone in the sections which I have described as Jurassic, and 

 seem to settle the correctness of their assumed age as decisively as 

 possible without the direct finding of fossils in the very region under 

 discussion. 



THE TRIASSIC BEDS. 



The correctness of the assumption of this age for the red beds depends 

 only upon their position in relation to other beds of known age, for no- 

 where that I know of have fossils ever been found in these beds. 



Somewhat north of my district, and between the Cache a la Poudre 

 and Cheyenne, there appears between the red beds and the ArchjBan 

 rocks, outcrops of Carbomfeix)us rocks, so that the red beds here lie be- 

 tween Carboniferous and Jurassic rocks. West of the Middle Park, 

 where all the upper beds retain their characters as east of the mount- 

 ains, the development of the Carboniferous beneath the red beds is far 

 more marked, and consists principally of a second series of red beds of 

 deeper purple hue than those referred to the Triassic. 



Leaves gathered from the upper measures of these by Dr. Peale are 

 referred by Professor Lesquereux to the Permian. Unless the red beds, 

 then, are referred to the Triassic, there will be an unoccupied gap be- 

 tween the Permian and Jurassic ages; nor would we know to w^hich of 

 these ages the beds in question could be more properly referred. Until 

 fossils are found, therefore, to definitely settle their age, it seems most 

 reasonable to refer them to the Triassic. 



I have already spoken of the indefiniteness of the line between the 

 Triassic and Jurassic, and have suggested the convenience of placing 

 this arbitrary line above the more massive portions of the red beds, and 

 above the massive sandstones which usually cap them, and not far be- 

 low- tlie clierty limestone. 



