DISCUSSION OF AGE OF FORMATIONS 495 



Permian faunas the world over their characteristic expression, tliere is 

 lacking in the Moenkopi fauna the common Permian aspect. 



In place of the above genera we find in the Moenkopi such typical 

 Mesozoic forms as the crinoid Fentacrinus, Triassic ammonites, and the 

 genera Nucula, Lima, and small Turritellas. When present in abundance, 

 as here, Nucula and Lima are much more characteristic of the Mesozoic 

 than of the Paleozoic; while the small Turntellas, in contradistinction 

 to the large species, similarl}' characterize the Triassic. Triassic 

 ammonites, though not present in our collections, have been gathered in 

 various sections from Arizona to Idaho in strata of similar age. For 

 example, in western Utah, in Beaver Canyon, 4 miles east of Minersville,- * 

 these ammonites are accompanied by a fauna similar to that of the lower 

 ]\Ioenkopi in our Arizona sections. The strata containing this fauna 

 are limestones similarly intercalated between red shales, which in turn 

 rest upon a more massive limestone faunally similar to the Ivaibab of 

 Arizona. These Utah red beds Girty^^ correlates with Walcott's Permian 

 of Kanab Creek in northern Arizona, the Permo-Carboniferous of the 

 Wasatch mountains in northern Utah, and the Lower Triassic (called 

 Meekoceras beds from the Triassic ammonites of this genus present) of 

 southeastern Idaho. ^^ 



In brief, in the absence of the most typical Permian genera, in the 

 characteristic Triassic reduction of brachiopods and increase of pelecypods. 

 and in the presence of typical Triassic forms, a Triassic age for the 

 Moenkopi appears to be indicated. The persistence of many Permian 

 species tends to place it in the Lower Triassic. If this be true, we had 

 in western North America a region where Permian species persisted until 

 after the ushering in of Mesozoic times. The most typical Permian species 

 had died off with their confreres in other parts of the world, but a few re- 

 tained sufficient energy to give forth replicas of themselves (though with 

 slight modifications) into Triassic time. The normal open sea of the Lower 

 Triassic, present in California, Oregon, and Idaho, was separated from 

 the lands in the Eocky Mountain region by a lagoon, or lagoons, occupying 

 at times the present areas of northwestern x4rizona and western Utah. 

 Into these lagoons came at intervals representatives of the normal sea 

 fauna to the west. The presence of these visitors in these faunallv re- 

 tarded areas is one of the indications of the existence of Mesozoic time 

 in the world at large. 



Dr. Girty has for some time tentatively held the Triassic aire-^ of tliis 



2*U. S. Geol. Survey, BnU. 316, 1907, p. 363. 



25 Op. cit, p. 364. 



26 U. S. Geol. Survey, Bull. 436, 1910, p. 7; also idem.. Prof. Paper 93, 1917. p. 31. 



27 U. S. Geol. Survey, Bull. 436, 1910, p. 7. 



