THE TRIASSIC OE EED BED FORMATION. 131 



SECTION VI. 

 THE RED BEDS. 



(Tkiassic ?) 



Resting conformably upon the Carboniferous rocks is a series three or 

 four hundred feet in thickness of deep red, sandy, and gj 7 psiferous clays, 

 with an intercalated stratum of limestone. These beds yield no trace of 

 organic remains, but they underlie conformably a series of lighter colored 

 clays, marls, limestones and sandstones with abundant and characteristic 

 Jurassic fossils. Beds of a similar brilliant red color are a conspicuous 

 element in the structure of the entire Rocky Mountain region, and though 

 they bear in some localities a few fossils, it is chiefly from their position 

 between well determined strata of Carboniferous and Jurassic age that they 

 are usually considered to be Triassic. On account of their prevailing red 

 color they are also commonly known as the "Red Beds," and that name is 

 here preferred, for the reason that it implies nothing in regard to their cor- 

 relation with fossiliferous strata elsewhere. If the name Trias is occasionally 

 used in these pages it is always with a reservation, for certainly in the Hills 

 there is nothing to prove its applicability. 



The difficulty of ascertaining the position of the base of the series has 

 already been described in speaking of the Carboniferous. There is no such 

 doubt about its separation from the Jura. At the top of the Red Beds there 

 is an unequivocal change in the character of the sediments and immediately 

 above it are Jurassic fossils in abundance. The Red Beds consist of the 

 following members in descending order : 



Feet. 

 3. Deep red clay, sometimes becoming soft, shaly, argillaceous sandstone ; 



containing large quantities of gypsum ... 200-250 



2. Piukisli or purple argillaceous limestone 15-40 



1. Deep red clay, sometimes becoming a soft, argillaceous sandstone ; contain- 

 ing but little gypsum 75-100 



Dr. Hayden, in his general section of the geological formations of the 

 Hills, includes in the lower portion of the Red Bed series all the strata 

 between the purple limestone (2 of the above section) and the Carbonifer- 

 ous limestones — " brick-red material verv similar to the bed D [3 of above 



