150 GEOLOGY OF THE BLACK HILLS. 



range the Red Beds overlap greatly the Carboniferous rocks, they must be 

 quite unconformable also to the Carboniferous.* 



In central and northern Colorado the Red Beds have been carefully 

 studied by the late Mr. Marvine, Dr. Peale, and others. They have there 

 the normal red sandstone character, varying in thickness from 400 to 1,600 

 or 2,000 feet, and underlie Jurassic strata, but owing to a general absence 

 or concealment of the Carboniferous they are found (almost as far north as 

 the Union Pacific Railroad) to rest directly upon Archaean rocks. Mr. Mar- 

 vine mentions that in the Middle Park region of Colorado the " develop-, 

 ment of the Carboniferous is far more marked, and consists principally of a 

 second series of red beds of deeper purple hue than those referred to the 

 Triassic,"f and that in their upper part Dr. Peale found leaves regarded by 

 Professor Lesquereux as Permian. Northward the Red Beds are found on 

 the flanks of the Laramie, Wind River, and Bighorn Mountains from Colo- 

 rado to the British Possessions. North of the Platte they have not yet 

 been critically examined, but it is known that a red arenaceous series 

 underlies very fossiliferous Jurassic strata, and rests upon rocks of Car- 

 boniferous age. 



In the Wasatch Mountains Mr. King, of the Fortieth Parallel Survey, 

 finds the Trias to be composed of quartzites and silicious and dolomitic 

 limestones capped by the typical red sandstones, and entirely un fossiliferous. 



Mr. G. M. Dawson, of the British North American Boundary Survey, 

 finds on the eastern flanks of the Rocky Mountains, on the 49th parallel, 

 300 feet of characteristically colored sandstones, which, by comparison with 

 similar beds in the United States, he regards as of Triassic age.J 



The interposition of the Red Beds between Jurassic and Carbonifer- 

 ous strata cannot be taken as positive proof of their Triassic age ; but in the 

 absence of evidence to the contrary it fully warrants their provisional assign- 

 ment, and the case is strengthened by the fact that in certain parts of the 

 Rocky Mountains an unconformity is found on the one hand between the 



"Surveys West of the 100th Meridian, Vol. Ill, p. 499. 

 t United States Geological Survey of the Territories, 1873, p. 105. 



t Report on the Geology and Resources of the Region in the Vicinity of the Forty-ninth Parallel, B. 

 N. A. B. Com., p. 71. 



