formations of the Middle Cretaceous. 315 



teristic species, but the first two being equally abundant in the 

 formation below. Fish integuments also are equally plentiful 

 in either series of beds. Both the Fort Benton and Niobrara 

 are, however, largely dependent for their paleontological dis- 

 tinction upon species of much more unusual occurrence than 

 those which are everywhere met with. 



The Fort Pierre. — In marked contrast with both of the 

 above formations is that of the Fort Pierre. The latter is, in 

 the main, a great body of leaden-gray clays, carrying from 

 bottom to top lenticular bodies of impure limestone and, in 

 several localities midway in the series of beds, a zone of 

 yellowish, quartzose, and more or less calcareous sandstone, of 

 a thickness varying between one and two hundred feet. Though 

 the formation attains the exceptional thickness of over seven 

 thousand feet in the vicinity of Denver, Colorado, it generally 

 falls far below this figure, rarely attaining fifteen hundred feet, 

 and frequently only seven or eight hundred. 



The clays are of a remarkably uniform texture ; often show 

 a tendency to concretionary structure, though always clearly 

 and evenly stratified ; are plastic in a noticeable degree ; and 

 have a general distribution of lime, gypsum, and alkali salts 

 throughout. Their capacity for moisture is such, that, upon its' 

 evaporation under the rays of the hot western sun and the dry 

 atmosphere of the prairies, the formation — especially if in an 

 approximately horizontal position— becomes most character- 

 istically reticulated with deep and gaping contraction cracks, a 

 condition which renders the country both scant of vegetation 

 and unattractive to the eye. 



Equally characteristic with the above are the lenticular 

 bodies of gray limestone, which occur promiscuously through 

 the formation and carry the bulk of its fossils. The dimen- 

 sions of these bodies very between two and six feet in the 

 direction parallel to the bedding planes of the clays, and be- 

 tween six inches and two feet in the direction normal to these 

 planes. Their composition lies between that of a clay with 

 very little carbonate of lime and a very pure limestone, gen- 

 erally inclining to the more calcareous variety ; they are fre- 

 quently reticulated with narrow calcite seams, which, under a 

 blow from the hammer, cause the bodies to break into sharp, 

 angular fragments ; in many of the limestone bodies fine par 

 tides of carbonized vegetable tissues also abound. 



The sandy zone of the Fort Pierre derives special interest 

 from its frequent, close resemblance to the heavy beds of sand- 

 stone at the base of the Laramie, from which, however, it is 

 deadily distinguishable both by its fossils and its stratigraphical 

 rosition. 



The life of the formation is abundant, but its especially 



