466 CROSS AND HOWE — RED BEDS OF SOUTHWESTERN COLORADO 



Dolores River section of the Cutler formation — Continued 

 Top Feet 



5. Shale, probably calcareous ; contains nodules of limestone ; color, red.. 5 



4. Sandstone, micaceous ; color, dark purplish red 5 



3. Calcareous sandstone, irregularly nodular and flaggy; contains gray 



nodules of limestone, but no well denned limestone ; color, red 25 



2. Arkose sandstone, micaceous, thin conglomeratic, cross-bedded ; near 



the base there is a thin black shale which is quite variable; color, 



white, with bluish and green zones 15 



1. Calcareous sandstone, with a gnarly gray limestone at the top ; color, red. 10 



Total 1,508 



Age of Cutler beds. — The Cutler beds are manifestly the uppermost 

 portion of the Paleozoic section now preserved in southwestern Colorado. 

 Since they rest on beds which contain a fauna intermediate in character 

 between those of the Pennsylvanian and the so-called Permian of the 

 Mississippi valley, though more closely related to the former, it is natural 

 to refer the Cutler provisionally to the Permian series. If it does not 

 belong to the Permian it is transitional (Permo-Pennsylvanian) or be- 

 longs to the uppermost Pennsylvanian. Such is the simple reasoning 

 when the relations of the Hermosa and Rico faunas to the eastern Car- 

 boniferous are considered. But the question is not so simple when the 

 relations of the Colorado formations to those of the Plateau province are 

 reviewed. The Hermosa and Aubrey faunas are both regarded as Penn- 

 sylvanian, but Mr Girty informs us that the Hermosa has no species 

 in common with the typical Aubrey of the Grand Canyon section, as far 

 as known. Mr Girty further states that the lower Aubrey fauna from 

 beds at the junction of the Grand and Green rivers, comprising a large 

 part of the Aubrey fauna described by White in Powell's Uinta report 

 (36), is markedly different from the Aubrey of the Grand canyon, as it 

 also is from the Hermosa fauna of Colorado. These faunal differences 

 must seemingly be explained in one of three ways : (1) By a rapid 

 gradation of forms within a comparatively narrow zone; (2) by the as- 

 sumption of an effective barrier between the Aubrey and Hermosa seas, 

 extending for hundreds of miles from eastern New Mexico west and 

 northwest across New Mexico and Utah ; or (3) by assuming one of the 

 formations to be younger than the other, and that the Pennsylvanian sec- 

 tion is incomplete both in Colorado and in the Plateau country. Studies 

 of the fossiliferous Carboniferous about the La Sal mountains, which are 

 in contemplation, should throw light on this subject, and further discus- 

 sion seems unnecessary at this time. 



If the Aubrey and Hermosa are practically equivalent, as the strati- 

 graphic relations suggest, the Cutler beds occupy a position correspond- 

 ing to that of the Permian of the Kanab valley in Utah and the formation 



