330 GEOLOGICAL SUEVEY OF THE TERKITOEIES. 



bituminous clay 15 to 20 feet thick, as exposed to the level of the creek. 

 The difference in the compounds of these conglomerate strata is merely 

 in the larger size of the i)ebbles, vrhich at Cheyenne vary from the size 

 of a pea to that of the head, and in the presence of thin beds of fresh- 

 water limestone, resembling tufa, which are seen here and there, of very 

 limited extent, similar in composition to the calcareous clay-beds at the 

 base of the Green Eiver group. I found in these conglomerates two 

 pieces of fossil-bones, the only traces of fossil-remains which I was able 

 to recognize in this peculiar formation. Though I cannot positively say 

 as yet if it closes the Lignitic period, and is therefore a member of this 

 formation, its i^eculiar identity of position and of composition is* worth 

 remarking. A boring of a few hundred feet at Cheyenne would settle 

 the question and show if, as I believe it, the Lignitic beds may be found 

 there at a depth of 200 to 300 feet. Anyhow, all what we have seen as 

 yet tends to confirm the statement already made by Dr. Haydeu in 

 1868, (Sillimau's Journal,) ^'that all the lignite Tertiary beds of the West 

 are hut fragments of one great basin, interrupted here and there hy upJieaval 

 of mountain chains, or concealed by the deposition of newer formations^ 



§ 5. Cheyenne to Cakbon Station. 



Along the Union Pacific Railroad, from Cheyenne up the Laramie 

 Plains, the country has been explored by Dr. Hayden, who, in his 

 report, (1870,) describes the passage from the primitive rocks, and marks 

 further upon the plateau the re-apjiearance of the Cretaceous, its con- 

 tinuity and its limits to the West. A number of specimens sent to me 

 in former years for examination, being labeled as from Eock Creek and 

 Medicine Bow, I expected to find at these localities, at least some 

 isolated basins of the Lignitic. This indication was a mistake ; for, 

 indeed, the Cretaceous strata are continuous along the railroad to four 

 miles west of Medicine Bow, where they are seen abruptly passing under 

 thick strata of the Lignitic barren sandstone. Though I did not find 

 atiy fossil-plants at Eock Creek, my visit there afforded me a good 

 opportunity of studying in the country around, the upper groups of the 

 Cretaceous, and therefore of remarking at short distances the essential 

 differences in the characters of both the upper Cretaceous sandstone 

 and tlie Lignitic sandstone over it. Besides, the great quantity of 

 remains of invertebrate animals, especially abundant in the upper Cre- 

 taceous, all representing deep marine species characteristic of the forma- 

 tion — Ammonites, Bcaphytes, and Baculites, &c. — the matter itself, a 

 kind of sandy calcareous shale, appears at first sight far different from 

 that of the fucoidal sandstone. The color of the first is dark-brown or 

 yellowish-brown, the texture finer-grained, mixed in some places with 

 calcareous infiltrations; the banks, when exj)osed along the creeks 

 or cut by erosion, do not weather in round, concretionary forms. They 

 break in large cubic i)ieces, or separate in shaly layers, forming, by 

 disintegration, heaps of broken fossil-shells, of angular fragments 

 of rocks and of dust rather than sand. At Medicine Bow, the line of 

 connection of both formations is perhaps more diiffcult to fix than at 

 other localities, the fucoidal sandstone here being mostly barren of 

 remains of marine plants. But from its base to its top, in a thickness of 

 i:)erhaps 200 feet, it is barren, too, of any remains of animals, while here 

 and there, branches of fucoids appear, as thrown by the waves, being 

 generally mixed with fragments of wood and stems of dicotyledonous 

 l)lants. From the cut of the railroad west of Medicine Bow, where this 

 sandstone is seen overlying the Cretaceous, and where two, fine mineral 



