496 H. W. SHIMER PERMO-TRIASSIC OF XORTHWESTERX ARIZONA 



I'ormation. More lately this belief lias become a conviction. In a letter 

 in 1915 to Gregory-^ he says: 



"There no longer seems substantial reason to doubt that Walcott's Permian 

 is the Lower Triassic (MeeJcoceras zone) of Idaho and the Termocarbonifer- 

 ous' of Utah." 



An alternative to the Lower Triassic age of the Moenkopi is that these 

 red sediments were deposited dnring the Upper Permian^ and at this 

 time there had already evolved some forms which later became typical of 

 the Triassic. In this paper, however, we incline to the belief that a period 

 begins with the first distinct appearance of any portion of tlie fauna 

 typical of it. Just as old leaves may persist upon a tree long after the 

 bursting buds herald the presence of a new period of growth, so a fauna 

 may persist in some favored regions long after the ushering in elsewhere 

 of a new fauna declares that a new period is here. 



Shinarump conglomerate.-^ — A gray cross-bedded conglomerate and 

 sandstone. This conglomerate is separated from the Moenkopi below by 

 a strongly expressed disconformit}^ Gullies in the ^loenkopi filled with 

 Shinarump pebbles and fragments of wood are common and wide-spread. 

 Some of its pebbles inclose Permian Fusulinidag. ^*The fossil wood and 

 fragments of bones in the conglomerate are believed to have Upper 

 Triassic affinities.'^ ^° 



CHINLE FORMATIOX^ 



The Chinle formation of Gregory (=AVard"s upper part of Lithoden- 

 dron member, his Leroux, and the lowest 100 feet of his Painted Desert 

 formation) consists of variegated, red and purple, thin-bedded gyp- 

 siferous marls, shales, sandstones, and concretionary limestones. 



There is apparently a slight disconformity at the base of this forma- 

 tion.^^ Yertebrate fossils described some years ago by F. A. Lucas^^ are 

 called by him Upper Triassic in age. These are here "the same combina- 

 tion of belodont and labyrinthodont as in the Keuper" of Europe. The 

 ostracod crustaceans^* point to the same conclusion. 



28 Op. cit., p. 31. 



29 Gregory : Op. cit, p. 37. 



so Gregory : U. S. Geol. Survey, Prof. Paper 93, 1917, p. 41. 



31 Gregory : Op. cit., p. 42. 



32 Gregory : Op. cit., p. 39. 



33 Science (n. s.), vol. 14, 1901, p. 376; U. S. Nat. Mus. Proc, vol. 27, 1904, pp. 193- 

 195. See also : Gregory, op. cit., p. 46. 



3* W. Cross : .Tour. Geology, vol. 16, 1908, pp. 107, 108. 



