235 



AVhat is the Aoe of the Aubkev Limestone of the IvOcky 



Mountains? 



By Albert B. Rea(4an. 



The Carboniferous rocks of the Rocky Mountains are divided litho- 

 logically and palaeontologically into two distinct groups: The Red Wall 

 and the Aubrey groups. The Red Wall is divided on palaeontologlcal 

 grounds into the Upper and Lower Red Wall, and the Aubrey on strati- 

 graphical and lithological grounds into the Upper and Lower Aubrey. 

 The Upper Aubrey is usually called the Aubrey Sandstone, the Lower 

 Aubrey the Aubrey Limestone. In this paper it is the writer's purpose to 

 establish the age of the last named group. 



This group of rocks rests conformably upon the Upper Red Wall and 

 shows conclusively by its position that it is Palaeozoic. Then as the 

 Upper Red Wall is Coal Measures in age (see paper on "The Fossils of 

 the Upper Red Wall Compared with those of the Kansas Coal Measures"], 

 the Aubrey Limestone must be either Upper Carboniferous or Permian. 

 Its position immediately above the Red AVall suggests the former; that is, 

 that it is Upper Carboniferous. This conclusion is attested by the fossils 

 identified from the group. They are: Seminida argeutia, Product its 

 punrt'tfii-'^, Prodiicfus semi-n'CtiailatKS, Pwdurtu)< coHntua (f), a Prodartii.'^ rlosehj 

 allied to if wit, P. portJochienu^, Spirifrr caineminx, BeJleropJion, Spirifcv liii- 

 eutu)<, Eaompihalas periiodosus, Arinculupcctrn orcidcnUdix Ariitculopccti'n, a Hcm- 

 ipmniti'H (Gilbert), Mekrlla stnata-contaia, etc. 



These fossils were all obtained in the first 100 feet of the Aubrey 

 Limestone. They are all Upper Carboniferous, not Permeo-Carboniferous, 

 in age. and therefore establish the age of the rocks in which they are 

 found to be Upper Carboniferous beyond a doubt. 



Note.— A few shells (Plevrophonis, Schlzodiis, and BalceveUu) found by 

 Mr. Gilbert (U. S. Geographical Surveys west of the 100th meridian, vol. 

 3. page 177) in the topmost layer of the Auljrey Limestone suggests the 

 Permeo-Carboniferous of the Mississippi Talley. This would seem to 

 imply that the Aubrey Sandstone Avhich is conformably superimposed on 

 the Aubrey Limestone is Permeo-Carboniferous in age. 



