260 EXPLORATIONS ACROSS THE GREAT BASIN OF UTAH. 



high perpendicular walls, and is worn into a maze of intricate ravines, forming a pec 

 and frequently highly picturesque scenery. It attains ;i considerable thickness; si 

 exposures are 200 and more feet high. 1 had observed the same formation or 

 Republican River, from the mouth of Frenchman's Fork upward, and along Ariel 

 Fork to Rock Creek. Dr. Havden has given a section of the Tertiary strata of \> 

 and Niobrara Rivers, in a preliminary report on Lieutenant Warren's expeditio 

 Nebraska and Dakota (Annual Report of Captain Humphreys, Office of Explora 

 and Surveys, December, l-SfxS, p. 119); but there the strata seem to be devel 

 somewhat differently. Those deposits, which 1 have designated as Loess, correspoi 

 the Post-Pliocene deposits of that section, the descriptor 

 corresponds to the strata above Fort Kearney on the Pla 

 due to local influences, and the latter strata, perhaps, h 

 of Dr. Havden's Pliocene bed, F. 



■h, 



how( 



^ver. 



s< 



■nrcelv 



ho 



differ 

 uppe 



ence 



n 



my be 



(ortion 



Near the junction of the 



Xoi 



rth and Soi 



ith Forks of Platte 



) River, the first roc 



strata were observed. The}' i 



.•out 



inue along 



the South Fork, ci 



•opping out at interv 



at one or the other side of the 



> riv 



er, and wei 



•e found most deve 



doped in Ash Hoik 



where thev attain a thickness 



of I 



>ver 250 fe 



et. This series is 



composed of an alt 



nation of loose, finely sandy, 



and 



[ of harder 



rocky strata, the 1 



atter consisting of f 



or coarse drift-sand, generally 



cen 



aented by 



carbonate of lime, 



forming more or 1 



calcareous sandstones, and g 



rittv 



-, very imp 



ure limestones. ] 



Partly they are coa 



sandy, partly finely earthy oi 



r e'\ 





fracture, and a f 



ew are subcrystalli 



Their age is,' probably, the*Pli< 



>cen 



e-Tertiary; 



but I have no pal 



eontological proof ol 



They have evidently been deposited before the last great continental up 

 they present such an unfinished and recent appearance, that I am incliin 

 them as among the latest formations of the Tertiary period. Moreover, 1 

 answ r er the description given by Dr. Hayden in his above-named s< 

 Pliocene strata, F 3. I can, however, not recognize other portions of hii 

 formations which I have observed on that portion of Platte River. 



There is no strongly-marked line between these deposits and the ne 

 are probably Miocene. 



The first rock, at the forks of Platte River, is composed of drift-sai 

 carbonate of lime, and partly porous and not much indurated, partly ce 

 overlaid by the loosely arenaceous deposits described before. The por 

 found to contain — 



Carbonate of lime 45 p< 



Sand, silica, and some alumina o5 p< 



In Ash Hollow these strata vary much in appearance: some are 

 subcrystalline, and somewhat chalky, irregularly intermixed with loose, s, 

 in the purer pieces the sand is fine, and can only be recognized by disso 

 in acid. Others are buff-colored, of a fine grit, coarse grit, compa 

 cemented ; a few are even conglomeratic. 



In the most calcareous of such recks, from various localities, I four 



