46 DESCRIPTIVE GEOIiOGY. 



horizontal, covering the greater part of the valley, and resting directly 

 against the granite. The chief point of interest in the formation centres in 

 the immense deposits of gypsum, which appears to underlie the entire valley, 

 just below the surface, cropping out everywhere and in many places, over 

 wide areas, actually forming the surface itself. There would seem to be sev- 

 eral beds, varying in thickness from a few feet up to ] 5 feet. It is mostly 

 a clear white pure gypsum, and, if required for market, could be mined on 

 a most extensive scale. On the south side of this projecting mass of gran- 

 ite, and north of Reschaud Creek, the Palaeozoic rocks crop out for a long 

 distance, dipping to the southward at a highly inclined but uniform angle. 

 In places, the Mesozoic beds may be found overlying them, but the Tertiary 

 beds lie high up on the range, concealing the greater part of the strata. 

 South of Reschaud Creek, the Palaeozoic limestones, standing at an angle of 

 70° to 75°, extend in a nearly north and south direction for 3 or 4 miles, 

 in an unbroken line, until reaching a small tributary of the Chugwater. In 

 the region of the Chugwater, the outlines of the Archaean body are very 

 irregular, and the sedimentary beds circle around the mass, conforming 

 closely to all the lesser details of the older rocks. 



Just north of where the Chugwater leaves the Laramie Hills, the 

 Archaean mass projects to the eastward, and the later beds, influenced by 

 the underlying body, turn in like manner in the same direction, present- 

 ing a great variety in structural features, and marked by abrupt changes 

 in strike and dip. Within 7 miles, in an east and west line, the entire 

 series of ridges, from the base of the Palaeozoic to the top of the Colorado 

 group, describe a complete letter Z. Immediately to the south of the 

 Palaeozoic beds, already mentioned as lying to the southward of Reschaud 

 Creek, the entire series of beds occur, standing at a very high angle, and 

 striking nearly due north and south, until just before reaching the Chug- 

 water they turn off slightly to the southeast, curving around the Archaean 

 mass. Here the Palaeozoic strata appear very prominently, rising nearly 

 vertically, and reaching to the summit of the Archaean body. The Triassic 

 and Jurassic strata, although inclined at a high angle, do not extend high 

 up on the flanks ; but the Dakota sandstone, on the contrary, rises in a very 

 bold manner, forming a high wall, dipping at an angle of 85° from the 



