211 



to the wearing out of the softer parts along the lines of the bedding. 

 The dip is to the east about 5°. 



In Plate VIII, "Profile rocks, Pleasant Park," we have a group of sand- 

 stones, at least as old as the Jurassic, and probably of Carboniferous age. 

 These are located much farther to the north, near the sources of Plum 

 Creek, a branch of the South Platte. We have here a pretty full 

 series of the sedimentary beds exposed by the upheaval of the mount- 

 ains. Although no fossils could be found, it is quite possible that the 

 variegated sandstones resting on the granites are Lower Silurian, while 

 the deep purple sandstones shown in the picture are Carboniferous, and 

 close to them, extending off to the eastward, are the brick-red sandstones, 

 doubtfully of Triassic age. Triassic beds may occur along the east 

 slope of the Eocky Mountains, but as yet we have no proof of their ex- 

 is :ence, and, therefore, when we refer to them it is with doubt. From 

 the sources of Plum Creek to the South Platte Canon, a distance of 

 twenty miles, there is a belt of country about half a mile in width, in 

 which these eroded sandstones present great variety of forms, some of 

 wliich are shown in the plate. The rocks vary much in texture, from a 

 >grained sandstone to a rather coarse conglomerate. The irregular 

 linae of deposition are quite remarkable, showing that the entire 

 ,'Up of sediments was deposited in moving waters. The strata them- 

 ^es vary much in texture within a few feet, and on this account the 

 nms are much varied. Deep cavities are worn out of the sides where 

 materials were softer than the surrounding parts. In the back- 

 ground the dim outline of the granite foot-hills of the Colorado range 

 is well shown. These sandstones form a portion of what are called the 

 " .^'(Og-backs," in the West. They form one of the inner ridges which 

 are ,80 common all along the immediate base or foothills of the mount- 

 ain^. The scenery along the base of the Colorado range, of which Plate 

 YlYi illustrates a small portion, will always be regarded with the greatest 

 i'jterest, not only by the tourist but also by the geologist. It has its 

 parallel in no other country of which we have any knowledge, either 

 from personal examination or from the descriptions of other explorers. 



