WALCOTT.] WYOMING. 211 



WYOMING. 



As geologist to Capt. Reynold's expedition to the headwaters of the 

 Missouri and Yellowstone Rivers, Dr. F. V. Hayden identified, iu 1861, 

 the fossil iferous " Potsdam " sandstone of the Black Hills, and consid- 

 ered a sandstone holding the same relative position and exhibiting tbe 

 same lithologic characters on the slopes of the Big Horn Eange as the 

 same formation. It is further stated that " a few thin layers of fine 

 calcareous sandstone were observed filled with fossils characteristic of 

 this period." 1 



From the Big Horn Range he crossed to the Laramie Eange. At the 

 head of the La BontC Creek he noticed abed resting discordantly upon 

 Azoic slates 50 to 100 feet in thickness, holding the same position and 

 possessing the same lithologic characters it reveals at other localities. 



I could discover no fossils in it at this point, but I am confident that this bed rep- 

 resents the Potsdam sandstone. This rock (the Potsdam) is more or less changed by 

 heat from beneath, but I was able to trace it continuously from the source of the 

 Chugwater Creek to the source of Cache la Poudre, a distance of over 100 miles. It 

 was also seen along the eastern slope of the Wind River Mountains, but did not con- 

 tain any organic remains. 3 



A more extended account of the character of the Primordial sand- 

 stone of the Rocky Mountains was published by Dr. Hayden in 1862. 3 

 It is accompanied by a description of the species of fossils found iu the 

 Black Hills and the Big Horn Mountains and further reference to it 

 will be found in the account of the Cambrian rocks of Wyoming. 



All of the essential part of Dr. Hayden's article of 1861 was reprinted 

 in an article on the Geology and Natural History of the Upper Mis- 

 souri, which appeared iu 1863. 4 This essay is, as stated by the author, 

 the substance of a geological report made by him to Capt. Raynolds on 

 the exploration of the Yellowstone and Missouri Rivers. The latter 

 report was issued iu 1869, accompanied by a large map, dated 1859-'60, 

 -upon which the geographic distribution of the formations identified by 

 Dr. Hayden are delineated. Within the present area of Wyoming a 

 narrow belt of the u Potsdam n sandstone is represented as entirely sur- 

 rounding the Big Horn Mountains, and as forming a continuous band 

 with that along the eastern side of the Wind River Range and an in- 

 terrupted belt on the western side of the latter range. The two moun- 

 tains now known as the Wyoming Mountains also had a narrow band 

 of " Potsdam n sandstone about them. To the north this band is repre- 

 sented as continuing westward across the Big Horn River, then north 

 on the western side of the Yellowstone Lake region as far north as Bea- 

 ver River, where it is interrupted. Several mountains directly north 



l Sketoh of the geology of the country about the headwaters of the Missouri and Yellowstone Rivers 

 Aiu. Jour. Sci., 2d ser., vol. 31, 1861, p. 234. 



2 Op. cit,,p.234. 



3 Tho Primordial Sandstone of the Rocky Mountains in the Northwestern Territories of the United 

 States. Am. Jour. Sci. 2d ser., vol. 33, 18o2, pp. 08-79. Canadian Jour., new ser., vol. 7, 18G2, pp. 149-151. 



4 On tho Geology and Natural History of the Upper Missouri. Am. Philos. Soc. Trans., vol. 12, new 

 Ber.,1863, pp. 1-218. 



