664 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



group and are elsewhere mapped undivided as the Niobrara formation. The Niobrara beds are 

 distinguishable by their hmy character from all other Cretaceous formations, except the Green- 

 horn limestone of the Benton group, from which, however, the limestone of the former is readily 

 distinguished by its color and fracture. 



The Timpas formation comprises about 200 feet of strata, of which the lower 45 to 50 feet 

 is limestone and the remaining 150 feet calcareo-argillaceous shale interrupted by thin limestone 

 bands, which become prominent toward the top. The limestone at the base is made up of 

 layers, usually less than 12 inches thick, which are separated from one another by very much 

 thumer layers of calcareous shale. The fracture is conchoidal and rudely parallel with the 

 bedding planes, and the grayish- white weathered surfaces break off naturally into thin conchoidal 

 flakes. As the Greenhorn limestone is dove colored and has a cross fracture, the two are easily 

 distinguished. The only fossil at all noticeable is the large oval, concentrically ridged shell 

 Inoceramus deformis, characterized by the excessive bulge of the ventral valve. The hmestone 

 itself, however, consists largely of the skeletons of foraminiferal organisms, readily seen in thin, 

 transparent sections under the microscope. The middle and upper portions of the formation 

 consist mainly of bluish or dove-colored shale, with thin bands of limestone at intervals. There 

 are three of these bands from 1 to 3 feet thick in the upper 40 feet of the section, and one of these 

 marks the top of the formation. * * * 



The total thickness of the Apishapa approaches 500 feet. The lower portion for about 

 40 feet consists of dark-gray or blue-gray shales, followed by rotten shales of paper-hke lamiua- 

 tion about 90 feet thick, which grade through blue sandy shale into calcareo-arenaceous shales. 

 The latter become coarser and flaghke toward the middle of the formation and constitute 

 about one-third of it. The upper 100 feet resembles the basal portion, but iacludes two and 

 at times three thin layers of hmestone and usually lens-shaped concretions of similar but more 

 impure material. The middle zone is always more or less bituminous and fairly constant in 

 character. The remainder of the formation varies considerably except in its general shaly 

 features and the presence of limestone strata near the top. The fossil remains consist of fish 

 scales, which are generally abundant in the shales and sometimes in the coarser flaghke layers. 

 In the sandy shales at the base of the middle zone the tracks of what was probably a small 

 crustacean are characteristic. These tracks appear as a double row of short, straight hnes, 

 those on one side incHned toward those on the other. 



The Pierre shale succeeds the Apishapa formation of the Niobrara group, and 

 of it Hills says : 



The beds of the Pierre epoch consist of argillaceous shales throughout. The thickness is 

 estimated at from 1,300 feet at the southeast extremity of the outcrop to 1,500 feet at the north- 

 west extremity, the changing dip and small number of exposures makiag accurate measurement 

 very difficult, if not impossible. The shales vary much in appearance. The basal and upper 

 zones weather to a yellowish-green color; the middle zone is dark gray to lead-gray, occasionally 

 almost black. The latter zone contains an abundance of hme-iron concretions that break 

 up readily and impart a rusty tint to the surface. They are always arranged paraUel with the 

 bedding. 



The Trinidad sandstone follows the Pierre shale. According to Hills it rep- 

 resents — 



some part or possibly the whole of the Fox Hills group, but on account of its relation to 

 the Laramie and the thickening of the lower zone of the formation northward it is presumably 

 the upper portion only. In the Spanish Peaks quadrangle the total thickness is about 150 feet 

 in the vicinity of Trinidad and 170 feet toward the north boundary. As elsewhere the forma- 

 tion consists of a basal zone of thin-bedded, fine-grained dark-gray sandstone layers, separated 

 from one another by thinner partings of shale; and an upper zone of massive hght-gray sandstone, 

 capped by a few feet of brown sandstone in contact with the overlying Laramie. * * * 



The marine Cretaceous ended with the Trinidad. The conditions of the succeeding Laramie 

 epoch were shaUow waters connected with the ocean. * * * 



