NOTES ON SPECIES 491 



our specimens may belong to this species, though the cross-section is 

 more broadly deltoid than in the type. 



Warthia aniei-icana Girty. — A single specimen from the Sawyers Tank 

 section is identified with this Guadalupian species. Tlie dorsum is 

 helmet-shaped when young, more roimded later at aperture; umbilicus 

 closed: no trace of slit-band or surface ornamentation. It is slightly 

 larger than the S23ecimen illustrated in the Professional Paper 58, plate 

 23, figure 15. 



Warthia sp. — Our collections contain a considerable number of ratlier 

 poorly preserved shells presenting the following characters : length and 

 breadth subequal (about 17 millimeters), umbilicus closed, dorsum 

 broadly rounded, surface smooth. 



Discussion of Age of FoFtMATioNs 



IN GENERAL 



In the discussion of the age of the Kaibab and the Moenkopi formations 

 it seems well to begin with the Eedwall and end with the Chinle. In 

 this way their relation to the formation above and below will appear. 



REDWALL LIMESTONE 



The Eedwall limestone contains typical lower Mississippian fossils. It 

 is correlated with the Escabrosa of southern Arizona, the Madison of 

 Wyoming and Montana, and with the lower Banff shale and lower portion 

 of the upper Banff limestone of Alberta. 



SUPAI (LOWER AND MIDDLE) 



What appears to be in places the upper portion of the Eedwall contains 

 a Pennsylvanian fauna. ^"^ These massive limestone beds in the western 

 areas of the Grand Canyon region correspond, however, according to 

 L. F. Noble,^^ to the lower Supai red shales and sandstones of the more 

 easterly areas, a transition occurring between these areas. The shales 

 and sandstones of the eastern Kaibab Plateau division of the Grand 

 Canyon area become toward the west more calcareous, with intercalated 

 limestone beds. These in turn give place, in the Kanab Plateau and 

 farther west, to a massive limestone like the Eedwall. The reo'ion of the 

 Kaibab Plateau was thus apparently, during lower Supai time, the 

 boundary between the open sea to the west and the land to tlie east. 



The fauna of this lower Supai (= uppermost Eedwall of western areas ) 



" W. T. Lee : U. S. Geol. Survey, BuU. 352, 1908, p. 15. 



^'^ Charles Schuchert : Am. .Jour. Sci.. 4th ser., vol. 45. 1918, p. 358. 



