16 



LARA.MIK FI^OHA OF THK DENVER BASIN. 



whereas the adjacent beds of Table Mount aiii 

 were practically horizontal. Some geologists 

 invoked the presence of a hypothetical fault 

 to account for this obvious discordance, but 

 it is now known to he due to a siinrp fold and 

 not to a fault. This is sliown by the fact 

 that at Green Mountain, 3 or 4 miles south of 

 Golden, the Arapahoe and the lower part of 

 the Denver, as well as the Laramie, are verti- 

 cal. Both the lower and the upper beds are 

 abundantly plant-bearing, and both hatl fur- 

 nished extensive collections, but, as it later ap- 

 peared, no distinction was made in the speci- 

 mens or in tiie records concerning them, all 

 being called simply " Laramie," with the result 

 that when the horizons were proved to be dis- 

 tinct, the greatest confusion and difficulty 

 arose in the effort to sepai'ate tiicm. 



As early as the summer of LS.Sl, when Whit- 

 man Cross " began his studies in the Denver 

 J Basin, he "first observed," to use his own 

 language, "that the Table Mountain strata 

 possessed characteristics proving them to be- 

 long to a series distinct from the normal Lara- 

 mie." No published announcement of this 

 discovery was made at the time, and field 

 work was continued in the region for a number 

 of succeeding years, during which G. H. Eld- 

 ridge ascertained that another distinct Tertiary 

 formation occurred between that discovered by 

 Cross and the normal f^aramie. A preliminary 

 statement of tlic most important results of this 

 investigation was made by Eldridge and Cross 

 in two papers read before the Colorado Scien- 

 tific Society July 2, 1888. Jn the first of these 

 papers"'" Eldridge named and described the 

 Arapahoe foirnation," which, he stated, was 

 "the formation next succeeding the Laramie in 

 geological order and unconformablv resting on 

 it." It was characterized as follows: 



It is composed of a basal member of conglomerate or 

 gritty ,santl,stone, according to its di.stance from the foot- 

 hill.s, with an overlying zone of gray argillaceous or 

 arenaceous shales, containing lenticular ma.s.ses of hard, 

 quartzose sand.stone, with an occasional ironstone; when 

 confined between under- and overlying groups it has a 

 thickues.s varying between 600 and 1,200 feet. 



"Cross, Whitman, The Tertiary Denver formation: Am. Jour. Sci., 

 3d ser., vol. 37, p. 282, 18)i9. 



" Eldridge, G. H., On some stratigraphieal and structnral features of 

 the country about Denver, Colo.: Colorado Sci. Soc. Proc, vol. 3, pp. 

 jjC-UH, 1888. 



" This was flrst named the " Willow Creek beds," but on the ground of 

 preoccupation the name was changed in a footnote (p. !I7) to Arapahoe. 



In the succeeding paper Cross ^* nameil and 

 described the Denver Tertiary formation, wliirli 

 he found to be unconformablv overlying tiie 

 Arapahoe. Lithologically it was ft)und to be 

 composed almost entirely of tmdesitic volcanic 

 material : its thickness was given as 80f) to 1 ,200 

 feet. 



As already indicated, both Arapahoe and Den- 

 ver were regarded originally as of Tertiary age 

 and both were found to contain fairly abundant 

 vertebrate remains belonging to turtles, croco- 

 diles, dinosaurs, and, it was at first supposed, 

 mammals. These remains were studied by 

 O. C. Marsh, and at first the study resulted in 

 great confusion. The dinosaurs from the Den- 

 ver were pronounced to be "typical Jurassic 

 dinosaurs of both lu'ibivorous and carnivorous 

 types." ^^ Though found in the same beils, 

 what was described as a l)ison was referred to 

 latest Pliocene time. Later these dinosaurs 

 and the supposed bison were found to belong 

 to the Ceratopsidae, a group of horned dino- 

 saurs especially abundant in Converse County, 

 Wyo., to which consideration will be given in 

 subsequent pages. This determination, accord- 

 ing to the vertebrate paleontologists, fixed 

 their age as Cretaceous. The invertebrates 

 consisted of a few rather poorly preserved 

 fresh-water types that did not prove of much 

 value in fixing the age. 



Fossil plants were abundant, especially in 

 the Denver beds, but, as already stated, the 

 specimens collected were not labeled with the 

 names of specific localities, and all were re- 

 garded as of Laramie age. Before they could 

 be utilized in the light of this newer informa- 

 tion as regards their stratigraphic relations, it 

 was necessary to separate them on the basis 

 of the matrix. They were fortunately pre- 

 serveil in the United States National Museum, 

 and a study of the matrix undertaken by Cross 

 resulted in showing that they were abundantly 

 distinct from those of the underl\-ing Laramie - 

 in fact, out of about 98 species in the Laramie 

 and some 140 in the .Vrapahoe and Denver, 

 only about 15 nominal species were found in 

 common. Subsequent studies of the Artipahoe 



" Cross, Whitman, The Denver Tertiary formation: Colorado Set. Soo. 

 Proc., vol. 3, pp. 119-133, lW«i. Recast and republished under the same 

 title in Am. Jour. Sci., 3d ser., vol. 27, pp. 2fil-2S2, 1889. 



"Cannon, G. L., On the Tertiary Dinosauria found in Denver beds: 

 Colorado Sci. Soc. Proc., vol. 3, p. 143, 1SS8. 



