82 DESCKIPTIVE GEOLOGY. 



brownish, rusty sandstone, with occasional concretions of ferruginous mate- 

 rial. All of these beds di]] at low angles, varying from 8° to 15°. 



On the north side of Sheep Mountain, where the Big Laramie River 

 leaves the Medicine Bow Range, running out on to the plain, the entire 

 series of sedimentary rocks again appears resting upon the Archaean body. 

 The Carboniferous beds, here a light blue arenaceous limestone, stand 

 nearly vertical, close up against the metamorphic rocks, and overlaid by 

 the bright-colored Red Beds, with the fine gray marls of the Jurassic indi- 

 cated by their characteristic soil and peculiar topography, lying between two 

 formations of massive sandstone. Every formation, from the Triassic to the 

 Niobrara division of the Colorado group, appears well developed along the 

 south bank of the river, inclined at 16° to 18°. Above the bank, the Fort 

 Benton clays are somewhat concealed by overlying soil ; but along the 

 bluffs, where the river cuts the beds at right angles to their strike, the dark, 

 almost black clays, passing up into slate-colored marls, are well exposed. 

 The bluish-gray marls, which characterize the junction between the Fort 

 Benton and Niobrara divisions, and form a prominent feature east of the 

 Colorado Range in the region of Big Thompson Creek, are shown here, 

 carrying the well-known Tnoceramus prohlematicus. On the north side of 

 the river, outcrops of the light-colored marls stand out jDrominently, but 

 would seem to be quite thinly developed ; at least, they rapidly pass into 

 coarse, friable sandstones, which, in long, gentle, bench-like ridges, extend 

 out upon the plains. 



Between the Big and Little Laramie Rivers, east of Bellevue Peak, the 

 Colorado beds are again found resting on the Archaean, as in the case of 

 Sheep Mountain, but are still more hidden by overlying Quaternary 

 deposits. At the northern end of Bellevue Peak, the lower sedimentary 

 rocks again reach the surface, and adhere so closely to the Archeean out- 

 line as to form a semicircular wall, with a diameter of less than 4 miles, 

 showing all the formations, from the Carboniferous to the top of the Colo- 

 rado group, curving around the older rocks. The Carboniferous beds lie 

 1,000 feet or more up on the sides of the mountain, inclined at varying 

 angles, having, where they first appear on the east slope, a dip of 20° to 25° 

 to the northward. These Carboniferous limestones possess a saccharoidal 



