407 



and sandstones may be said to characterize to a marked degree the beds 

 from the lowest Silurian to the summit of the Cretaceous. In the North- 

 west, the Cretaceous series have been separated into five well-marked 

 groups, made up mostly of sediments deposited in quiet waters, con- 

 taining in various localities an astonishing quantity as well as a great 

 variety of beautifully-preserved fossils. No portion of our present seas 

 can have a greater variety or abundance of molluscous forms than are 

 now exposed in long belts, or zones, across the Cretaceous area on the Up- 

 per Missouri; but, as we advance south wardfrom the Upper Missouri, the 

 typical characters of the Cretaceous divisions are gradually lost, so that 

 in Colorado they are not well defined. In the interior basin, they are 

 not well defined, and, had they been studied to any extent there, previ- 

 ous to their examination on the Upper Missouri, it is doubtful if any 

 other classification than Upper, Middle and Lower Cretaceous would 

 have been given. 



Between Eock Springs and Salt Wells stations, on the Union Pacific 

 Eailroad, there is an interesting anticlinal valley that may aid in throw- 

 ing some light on the structure of this region. This anticlinal was first 

 noticed and described by me in my Annual Eeport for 1870, page 171. 

 I have examined it several times since, but have had no opportunity of 

 following this anticlinal north and south of the railroad. In this way, 

 the problem could be solved ; for to the west of this anticlinal valley 

 there is a series of the coal group from the well-defined Cretaceous up 

 to the Green Eiver group, while to the east the series can be studied 

 to the summit of the Washakie group. In the anticlinal valley about 

 Salt Wells station, and west of it, the characteristic clays of No. 4 

 Cretaceous are well shown, and reach a thickness of at least three 

 hundred feet. Tracing this bed upward through the gradual changes 

 to the sands and laminated sandstones of No. 5, I found, immediately 

 beneath the laminated sandstone, numerous rusty calcareous con- 

 cretions, full of fossils, quite characteristic of Nos. 4 and 5, BacuUtes, 

 Sca2}Mtes, and Inoceranuis. Having thus tixed a horizon, I endeav- 

 ored to follow it up. Looking toward the west from Salt Wells sta- 

 tion, a long series of bluffs can be seen on either side, composed of the 

 shales of No. 4, holding so far as the eye can determine a horizontal po- 

 sition ; but facing to the east, on either hand far south and north, a series 

 of high, j)ointed bluffs extend, as far as the eye can reach, underlaid by 

 No. 4, but capped with No. 5, and inclining about northeast at an angle 

 of 5° to 15°, generally with an average dip of 10° to 12°. This inclina- 

 tion continues far eastward until the marine and brackish-water beds 

 pass beneath the fresh-water strata of the Washakie, near Bitter Creek 

 station. Above No. 4, there are about twelve hundred feet of laminated 

 sandstones and arenaceous clays; the layers of sandstone becoming 

 thicker and more massive toward the summit. This group undoubtedly 

 represents No. 5, greatly thickened and somewhat changed in its external 

 features. Above No. 5, and distinctly marked, is an extensive group 

 of irregular-bedded and somewhat variegated sandstones, with intervals 

 of sand and clay, mostly yellow, gray, and brown in color, extending to 

 Point of Eocks. The entire series I estimated at about one thousand 

 feet in thickness ; but it would have required a more extended examina- 

 tion north and south of the road to have determined this with precision. 

 Above and below Point of Eocks station, there is a thick bed of gray- 

 ish-brown sandstone, which at once arrests the eye from its remarkable 

 structure. It is shown along the railroad east and west of Point of 

 Eocks station for several miles, is very variable in thickness, sometimes 

 reaching 250 feet, and forming a massive though rugged bluff' wall. The 



