666 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



The Benton shales have a thickness of more than 500 feet at the north end of the field. 

 They are somewhat thinner at the south end and taper uniformly from both ends toward Boulder 

 Creek, where the formation almost disappears. The great body of the shale is dark. There are 

 frequent layers a few inches thick which are strongly impregnated with iron. The formation is 

 calcareous in varying degree. At many places some beds are composed of a black limestone 

 showing a crystalline surface when broken and having a strong bituminous odor. * * * As 

 the summit is approached the blackness disappears and the last 75 feet (ohserved near the north 

 end of the field) show light-colored limestone, shale, and sandstone. North of Sixnule Canyon 

 these beds, which may be regarded as transitional to the Niobrara, show the following section: 



Section of upper part of Benton formation north of Sixmile Canyon. 



Feet. 



Greenish sandstone, calcareous in upper third 15 



Shales, blue to yellow, noncalcareous 15 



Sandstone, noncalcareous, firm, rather purplish 10 



Shale, iron stained at intervals 10 



Shale, calcareous (or calcareous at top). 



Below these beds is a limestone of variable thickness strongly resembling the basal Niobrara. 

 It occurs in lenses only. The greenish sandstone at the top is found below the basal Niobrara 

 wherever the latter is exposed in this district. It is generally 10 or 15 feet thick and is not 

 calcareous except in the upper portion. As it is often very much fractured and the cracks 

 are abundantly filled with carbonate of lime, this calcareous character may be due to infiltration 

 from the hmestone above. The blue and yellow shales below are also seen wherever the base 

 of the greenish sandstone is exposed. 



The Benton is succeeded by the Niobrara, whose prominent characteristic is its calcareous 

 nature. It is composed in small part of true limestones, but the greater mass is made up of 

 calcareous shales, while considerable portions are of intermediate character. Its thickness at 

 Fourmile Canyon is a little more than 400 feet. The basal stratum of compact hmestone rests 

 upon the greenish sandstone which is at the top of the Benton. Occasionally a foot or more 

 of light-colored marl intervenes. Below the Niobrara, as pointed out above, are occasional 

 beds of similar limestone and calcareous shales interbedded with noncalcareous sandstones. 



Above the Niobrara are the Pierre shales, which are more than 5,000 feet thick. They are 

 slate colored, leaden gray, d ark brown, and sometimes nearly black. * * * While in general 

 noncalcareous, the Pierre has local limy beds. At places these form continuous strata, as, for 

 example, 4 miles north of Boulder, one-half mile east of the contact with the Niobrara. Here, 

 for a thickness of nearly 40 feet, strong limestone beds are so closely grouped as to give the 

 outcrop the appearance of the basal Niobrara. At other places the limestone beds are smaller 

 and more isolated or are divided into concretionary masses often containing fossils. Less 

 prominent calcareous masses may be found at any horizon, either in beds or in more or less 

 perfect concretions. Concentrations of iron occur in similar but less massive forms, ranging 

 from clear-cut beds to weU-formed nodules. The lime and the iron may or may not occur in 

 the same concretionary mass. Many of the calcareous nodules mentioned contain much iron 

 carbonate, which, in progressive oxidation toward the center, gives rise to sharply marked 

 concentric shells differing in color. Sandy beds may occur at any place in the section. The 

 most prominent and persistent of these is about one-third way up from the base, or a little 

 higher. * * * From pure sands at one extreme to pure clay shales at the other, the Pierre 

 shows all gradations in composition. The sandy layers are generally firm and gritty, almost as 

 dark colored as the shales themselves, and not very porous. In rare instances light-colored, 

 friable sands are encountered in drilling oil wells. Tha thickness of the sandy beds is as variable 

 as their constitution, while the lateral extent of such beds, as indicated by occasional outcrops 

 and by the records of weUs, is, in a large majority of cases, a small fraction of a mile. Some of 

 these beds may be lenses, but doubtless the more common mode of lateral limitation of the 

 sands is by a gradual change in composition of the beds into the shales. 



