235 



What is the Age of the Aubrey LtxMkstone of the Uocky 



Mountains? 



By Albert B. Rea(4an. 



The Carboniferous rocks of the Rocky Mountains are divided litlio- 

 logically and palaeontologically into two distinct groups: Tlie Red Wall 

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

 grounds into the tpper 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 

 Aulirey the Aubrey Limestone. In this paper it is the Avriter'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 Avith those of the Kansas Coal Measures"), 

 the Aubrey Limestone must be either LTpper Carboniferous or Permian. 

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

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

 identified from the group. They are: Sciii'uiiila argentia, Productiis 

 jmiv-t'itds, Prod net u.'i semi-recticulatus , Product ux costutitx (f), a Productus closely 

 allied fo if not, P. porflockienus, Spirifcr ramendnx, Bellerophon, Spirifer liii- 

 ecdiiK, Eii<H)iph(diix jienwdoxus, Avinculopectcn uc<-ident(dl^ Ariiuudoprcfc)}, a I fern - 

 ipr(initi'>< (Gilbert), Melrlla striata-costata, 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 rocl^s in which they are 

 found to be Upper Carboniferous beyond a doubt. 



Note.— A few shells {Pleurophoriis, Scliizodus, and BalceveUa) found by 

 :Mr. Gilbert (IT. S. Geographical Surveys west of the lOOth meridian, vol. 

 3, page 177) in the topmost layer of the Aubrey Limestone suggests the 

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

 imply tliat the Aubrey Sandstone which is conformably superimposed on 

 the Aubrev Limestone is Permeo-Carboniferous in age. 



