18 GEOLOGICAL SURVEY OF THE TERRITORIES. 



served, rocks of this age do not occur again until we reach Colorado 

 Springs, south of the railroad. The brick-red group rests on the Meta- 

 morphic rocks, and, starting from the granite nucleus, we -pass across 

 the upturned edges of the sedimentary beds, as they incline from the 

 €ast slope of the mountains at various angles. We have been in the 

 habit of calling the brick-red beds Triassic; but it is by no means 

 proven, and as there are red beds of similar mineral character in the 

 well-marked Carboniferous group below, and the Jurassic above, I have 

 sometimes been disposed to refer them to one or the other, or to both, 

 and regarding the Triassic as wanting. The thickness of the red group 

 as exposed at different localities varies considerably, and it is oftentimes 

 difficult to decide whether the difference is due to original deposition, or 

 whether the beds have been crushed together, or concealed by newer form- 

 ations. If the Triassic group is wanting in this region I cannot point out 

 any locality where there is any marked unconformability between the 

 Jurassic and the Carboniferous, and this fact might be used to favor 

 the belief that the red group is Triassic. Above the red group is a 

 series of variegated beds, which seem never to be absent alon g the 

 xQargins or flanks of the eastern ranges of mountains from the north line 

 of our territory to Mexico. Is^orth of the railroad the Jurassic marls are 

 often filled with characteristic fossils, but south of that point they disap- 

 pear, and they have not yielded any positive paleontological proof of their 

 age to the numerous explorations as far south as Santa Fe. The litho- 

 logical characters of the group, however, are well preserved, although 

 from the line of the railroad far south to New Mexico the group is thinly 

 represented. Above the Jurassic is a fair representation of the Creta- 

 ceous series. No. 1, or the Dakota group, is well shown and is always 

 characteristic, though seldom containing any organic remains, but the 

 other divisions, which are so well defined farther north, are here very 

 obscure. The geologist studying these beds in their southern extension 

 first, would hardly think of separating them into the five well charac- 

 terized divisions of the Northwest. To one who has carefully studied 

 the divisions along the Misssouri Eiver the Cretaceous beds in Colorado 

 and New Mexico may be separated into the five groups without much 

 difficulty. No. 3 is represented by a thin bed of impure gray limestone and 

 thin calcareous shale, with Ostrea congesta and a species of Inoceramus. 

 The fossils are about the same as those occurring on the Missouri, but 

 the rocks have little of the chalky texture, as observed in the Northwest 

 and in Kansas. Nos. 2 and 4 are black shaly clays, and do not 

 differ materially from the same groups occurring in other localities to 

 the northward. No. 5 contains a great abundance of well-marked Cre- 

 taceous fossils, many of the species identical with those found on the 

 Missouri Eiver. This group passes up into the Lignite strata, appar- 

 ently without any marked unconformability. In i^assing upward in 

 number 5, one by one the mollusca of purely marine character disappear 

 until only some varieties of oysters remain with the plants peculiar to 

 the Lignitic group. I may say here that it is quite possible that a 

 more thorough examination of the strata intermediate between those 

 with well-defined Cretaceous fossils and the Lignitic beds would show 

 at least an uncomformability of sequence. In the Laramie Plains there 

 is a group holding this intermediate position of several hundred feet in 

 thickness, which I have called beds of passage. 



There is an interesting fact which may be stated just here, that there 

 are no important flexures in the sedimentary group, whatever there may 

 have been in the Metamorphic rocks, but the difference in the inclina- 

 tion of the former is very great at different localities. Sometimes the , 

 uplifted zone is ten to fifteen miles in width and composed of a great 



