358 Sc/tuc/wrt — Carboniferous of Grand Canyon of Arizona. 



may have been formed under brackish water, but else- 

 where the Supai is a typical series of continental Hood- 

 plain deposits accumulated on a subsiding area under a 

 semi-arid climate. Whatever organisms were entombed 

 were completely oxidized away. This is shown in the 

 ferric condition of the included iron and the complete 

 absence of actual fossils. The Supai formation, when 

 compared with a typical intermontane continental deposit 

 like the Newark formation of the Connecticut Valley,i of 

 Triassic age, is seen to be devoid of conglomerates, is 

 more regularly bedded, and has a better assorting of the 

 rock materials. From this the writer concludes that the 

 Supai was laid down under greater and longer enduring 

 sheets of seasonal waters, causing the materials to be 

 more equally distributed than would be the case in mean- 

 dering rivers of a narrower flood-plain. During the dry 

 seasons the muds and sands were relieved of all organic 

 materials by capillary action and the atmosphere. 



Age of Lower Supai. — The Lower Supai rests upon 

 the Redwall limestones. This contact may be studied 

 just beneath the sign post "Cathedral Stairs," that 

 stands on Supai strata. It is of the disconf ormable type 

 and the Redwall is more or less eroded. Where the con- 

 tact was studied this limestone-dolomite series terminates 

 in thin-bedded deposits with the beds varying in thick- 

 ness from 12 to 20 inches. The Supai begins at once 

 with its characteristic sandy and rain-pitted shales and 

 muddy sandstones. This disconformity has far greater 

 significance than the physical phenomena as seen on the 

 Hermit trail would seem to indicate, for the fossils 

 of the Redwall are here all of early Mississippian age. 

 Nothing of later Mississippian time is present here 

 or elsewhere in Arizona, and the land interval preceding 

 Supai deposition was certainly longer than all of Red- 

 wall time. 



To fully understand this disconformity we must go 

 farther northwest in the Grand Canyon, where Noble has 

 fully studied the contact. In a letter to the writer dated 

 May 10, 1916, he says : 



"In the Kaibab division of the canyon there is evidence of a 

 disconformity just at the top of the massive Redwall cliff, and it 

 is possible that further study may show the break between the 

 Pennsylvanian and Mississippian to be here, although it is com- 

 monly believed that the upper part of the Redwall is Pennsylva- 



