HI9T0KICAL REVIEW OF THE LARAMIE PROBLEM. 



57 



made it necessary to modify greatly the deter- 

 mination of the age relations of the principal 

 coal-bearing portion of the section, until then 

 accepted as Laramie. It was referred to the 

 Mesaverde formation and was described by Lee 

 as follows: 



The lower part of it [Mesaverde] was called "Fox Hills" 

 and the upper part "Laramie" by the geologists of the 

 Hayden Survey, and other geologists have followed them 

 in referring the coal to the Laramie. However, a study of 

 the fossils, in connection with the stratigraphy of western 

 Colorado, proves that it is much older than the La;amie, 

 and the entire se.ies of cjal bearing roc';s in this field is 

 now refe.-red to the Mesaverde formation, originally named 

 by Holmes in southwestern Colorado. Where the full 

 section is exposed the Mesaverde is separated from the 

 Laramie by a marine formation known as the Lewis shale, 

 but in the Grand Mesa field there is no representative of the 

 Lewis or Laramie beds. The sedimentary rocks of Ter- 

 tiary age rest iinconformably upon the Mesaverde. 



Lee^ continued and extended the woi'k in this 

 field in 1909. On further and more critical 

 study it was found possible to separate the 

 Mesaverde into several members, as follows: 

 The Rollins sandstone member, at the base, is a 

 white clifT-making sandstone containhig fucoids 

 and marine invertebrates. Above this is the 

 Bowie shale member, with a maximum thick- 

 ness of about 425 feet, composed of dark- 

 colored shale and gray sandstone. It con- 

 tains marine and brackish-water invertebrates 

 and important coal deposits; it is not present 

 in all parts of the field. Unconformably above 

 this is the Paonia shale member, 400 feet thick, 

 made up of sandstone and shale, in j)laces 

 carbonaceous, with plant remains, mainly 

 fresh-water invertebrates and coal deposits. 

 Above the Paonia is what is called the undif- 

 ferentiated part of the Mesaverde, a series of 

 gray quartzose sandstones and shales some 

 2,000 feet thick, containing plants and fresh- 

 water invertebrates. 



The Paonia shale member and the overlying 

 undifferentiated Mesaverde constitute the beds 

 which had previously been called Laramie and 

 which, ac(;ording to Lee, "may prove to be 

 Ijaramie or younger." The age significance 

 of the mvertebrates from these beds was inter- 

 preted by Stant/on as follows: 



Although this nonmarine fauna contains many Lararrde 

 elements, on the whole it agrees better vnih the fauna of 

 the Mesaverde as we are now beginning tx) know it, and the 

 stratigraphic and structural evidence of the entire region, 



s Lee, W. T., Coal fields of Grand Mesa and the West Elk Mountains, 

 Colo.; U. S. Geol. Survey Bull. 510, 1912. 



including the Durango, Grand Hogback, and Yampa 

 fields, is strongly in favor of referring all the upper coal- 

 bearing beds of the Grand Mesa field to the Mesaverde. 



The plants were critically reviewed by me 

 as follows: 



In conclusion I feel justified in stating that in my 

 opinion the beds containing the plants here under dis- 

 cussion arc of post-Montana age. The facts upon which 

 this conclusion is based are (1) the apparently satisfactory 

 demonstration of an unconformity between the upper and 

 lower coal-bearing rocks, (2) the fact that the marine 

 invertebrates are confined to the beds below the uncon- 

 formity, (3) the fact that the plants are confined to the 

 beds above the unconformity, (4) the marked difference 

 in the quality of the coals in the lower and upper beds— 

 that is, below and above the unconformity, (5) the marked 

 resemblance between the plants of the Grand Mesa field 

 and those of Black Buttes, Wyo., which is only a short 

 distance north, and (G) when the first report was given, 

 comparisons of the plants were made with areas then 

 tentatively regarded as Montana but which subsecjuent 

 investigation has shown beyond much fiuestion to be of 

 post-Montana age. 



From the above exposition it appears that 

 the upper coal-bearing beds in the Grand Mesa 

 region that were formerly regarded as of 

 Laramie age are now tentativejy classed as 

 Mesaverde, with the possibility that subsequent 

 study may prove that they are Laramie or 

 younger. In any event, their reference to the 

 Laramie is not now accepted. 



GLENWOOD SPRINGS AREA, COLORADO. 



The Glenwood Springs area is directly north 

 of the Grand Mesa area and is in stratigraphic 

 connection with it. It has been studied by 

 A. L. Beekly, of the United States Geological 

 Survej', but the report is not yet published. 

 A brief description of this field was given by 

 Gale," who showed that it also merges with the 

 Grand Hogback field and has the same struc- 

 tural relations. The section of coal-bearing 

 rocks is essentially the same as that of Grand 

 Mesa and like it was referred to the Mesaverde, 

 and both Lewis and Ijaramie were believed to 

 be absent. In considering the paleobotanic 

 evidence for the possible "Laramie or later" 

 age of the beds in the Grand Mesa area above 

 the Paonia shale, I wrote as follows' concerning 

 similar evidence in the Glenwood Springs field: 



If the discussions and comparisons made above had 

 included the adjacent Glenwood Springs area, with which 



6 Gale, H. S., Coal fields of northwestern Colorado and northeastern 

 Utah: U. S. Geol. Survey Bull. 415, pp. 128-136, 1910. 



' Knowlton, F. H., in Lee, W. T., Coal fields of Grand Mesa and the 

 West Elk Mountains, Colo.: U. S. Geol. Survey Bull. 510, p. 47, 1912. 



