30 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull. 100. 



It should be noted that this list contains 13 of the 14 species collected 

 from the upper part of the undisturbed Fort Benton shales in the 

 Arkansas valley. 



The stratigraphic evidence that the Pugnellus sandstone is near the 

 top of the lower division of the Colorado formation is equally conclu 

 sive. A section measured on Muddy creek gives the following suc- 

 cession of strata in ascending order: 



Dakota: Hard brown and gray sandstone with plant remains, not fully exposed. 



Feet, 

 Colorado : 



Dark clay shales 100 



Gray limestone in thin bands alternating with shale. Inoceramus labia- 



tun abundant 30 



Dark shale 300 



Rather^coarso gray and yellowish sandstone, becoming shaly above, 

 with occasional very fossiliferous bands and lenticular masses con- 

 taining the species listed above 40 to 50 



Brown calcareous sandstone with Ostrea conges la, Prion oc yd us ivyo- 

 mingensis, and shark's teeth 4 



Light drab limestone with Inoceramus deformis, 1. labial us, and Ostrea 

 congesta passing up into calcareous shales 75 



Total thickness of Colorado exposed 550 



Other localities in the park show that the upper bed of limestone is 

 overlain by 300 to 400 feet of light gray or buff calcareous shales, suc- 

 ceeded by the drab clays of the Montana formation, with a probable 

 thickness of 3,000 feet. The latter have yielded specimens of Inocera- 

 mus proximus, Lucina occidentalism Baculites ovatus. Ptychoccras, and a 

 few other characteristic forms. 



The southern continuation of the Arkansas valley Cretaceous area 

 in Colorado and northeastern New Mexico has been described by Dr. 

 J. S. Newberry, 1 Prof. J. J. Stevenson, 2 and others. 3 From these de- 

 scriptions it is evident that the Colorado formation there retains about 

 the same lithologic and paleontologic features as in the region above 

 described, excepting that the Pugnellus sandstone with its shallow- 

 water fauna has not been noticed, and there seems to be a greater pro- 

 portion of limestone throughout the formation. 



Dr. Newberry states that on the banks of the Canadian river 4 the 

 " Middle Cretaceous" exposures are from 800 to 1,000 feet thick, con- 

 sisting mostly of blue limestone interstratified with dark blue and 



1 Macomb's exploring expedition from Santa Fe, N. Mex., to junction of Grand and Green rivers. 

 Geol. Rept., Washington, 1876, pp. 32-35. 



2 U. S. Geog. and Geol. Surveys west of the One hundredth meridian, vol. ill. Supplement, pp. 88-158. 

 and Am. Geologist, vol. in, pp. 391-397. 



3 Prof. Jules Marcou visited this region in 1853 in connection with the Pacific railroad surveys, and 

 his observations were published in vol. m of the reports of that survey, in the Geology of North A nier- 

 ica, Zurich, 1858, and elsewhere. In an article on "the Mesozoic scries of New Mexico" (Am. Geolo- 

 gist, vol. IV, 1889, p. 155-165 and 216-229) the same author gives titles of most of the geological reports 

 relating to New Moxico. 



4 Op.cit.,p. 33. 



