PERMIAN. 485 



beneath. This has suffered extensive denudation by erosion and is now found only in patches 

 from 10 to 20 feet in thickness over limited areas, the sandstone filling the spaces between. 

 Recent erosion has cut channels down through each to the formation beneath, which is a member 

 of the Upper Aubrey group. A slight plane of erosion with an entire change in the character 

 of the rock separates the two groups. 



The Permo-Carboniferous of Mr. G. K. Gilbert" is the same as my lower division of the 

 Permian. It is placed as a subdivision of the group, now that the beds above are known to be 

 of Permian age. 



The stratigraphy of the section shows a group separable into two divisions, defined above 

 and below by planes of unconformity by erosion and a decided change in the character of the 

 beds from those of the subjacent and superjacent formations. There is no physical break in the 

 beds above the Permian limestone of the upper division before the conglomerate is reached. 

 This stratigraphical arrangement is sustained by the evidence of the fauna found in the limestones 

 and associated arenaceous layers in the upper division. 



The genera Myalina, Schizodus, Nucula, Aviculopecten, Murchisonia, Naticopsis and 

 Goniatites are represented in the lower chocolate-colored limestone. The faima is distinct in 

 specific character from that of the Carboniferous groups beneath, and more intimately related 

 to that of the fossUiferous beds of the Upper Permian division. Mr. G. K. Gilbert obtained from 

 this same horizon Pleiurophorus, Schizodus, and Bakevellia, a group of shells, as he states, 

 suggesting the Permo-Carboniferous of the Mississippi Valley.* 



Twenty-three genera represented by thirty-fom- species comprise the fauna of the upper 



division. Of these, the following have strong Paleozoic relations: Scolithus ?, Lingula 



mytiloides, Discina nitida, Orthis ?, Khynchonella uta, Terebratula ?, Nucxda, two 



species, Aviculopecten, three species, Myalina, four species, Naticopsis, two species, Pleuroto- 

 maria ?, Macrocheilus ?, Cyrtoceras ?, Goniatites ?, and Nautilus ?. 



The Permian character of the faiona is more marked by the presence of Plem-ophorus, 



three species, Schizodus ?, three species of Bakevelha including B. parva, Pteria ?, 



Mytilus ?, Rissoa ?, and the still more typical Mesozoic genera Pentacrinus and 



Pileolus ?. 



The Pentacrinus plates were discovered by Mr. Edwin E. Howell below the Shinarump 

 conglomerate in southwestern Utah. They belong to a species distinct from P. asteriscus of 

 the Jurassic. Three species are now known to pass from the lower division to the limestone of 

 the upper division. 



The Permian character of the fauna, taken with the evidence afforded by the stratigraphy, 

 clearly establishes the Permian as a well-defined and distinct group in the Colorado Valley. 

 It occm^ at the same horizon as the Permian determined by Mr. Clarence King in northern Utah, 

 western Colorado, and southern Wyoming, fully corroborating the views advanced by him of 

 the age of the beds resting on the "Bellerophon bed" of the Upper Carboniferous.'' 



Walcott's section is as follows: 



Permian (855 feet): 



Plane of unconformity by erosion. 



Upper Permian (710 feet) : Gypsiferous and arenaceous shales and marls, with impure shaly 

 limestones at the base. 

 Plane of unconformity by erosion. 



Lower Permian (145 feet): Same as upper division with more massive limestone at the base. 

 ~ Plane of unconformity by erosion. 

 Upper Aubrey (835 feet): Massive cherty limestone with arenaceous gypsiferous bed passing down 

 into calciferous sandrock. 



G. H. Girty" comments on the above section as follows: 



There is hardly room for doubt that Walcott's Permian is equivalent to the "Permo- 

 Carboniferous" of the Wasatch Mountains and to the Permian(?) of the section studied by 



oU. S. Geog. Surveys W. 100th Mer., vol. 3, 1875, p. 177. See also Arch. R. Marvine's report in same, p. 213. 



6 Idem, p. 177. 



cU. S. Geol. Expl. 40th Par., vol. 1, pp. 245-246 and atlas. 



<* Comment on manuscript. 



