56 



LARAMIK KI.ORA OF TlIK DENVER BASIN. 



predominates. They contain scant fossil remains, and 

 sufficient information reT:ar(ling; their age has not been 

 obtained to correlate them with similar well-known beds 

 in other parts of the country. The entire section is with- 

 out doubt Cretaceous, and the principal coal-bearing 

 strata are within the Laramie formation. 



The following year (1906) the eastern part of 

 the Book Cliffs field was studied by G. B. 

 Richardson, and a preliminary report was 

 issued in 1907." In this report was recorded 

 a complete change of front regarding the age 

 of the coal-bearing rocks. Richardson said: 



Fossils have been found at several horizons between 200 

 feet from the base and 250 feet from the top of the forma- 

 tion. They consist of land plants and fresh- and brackish- 

 water invertebrates. The evidence of the fossils and the 

 general stratigraphic and areal relations of the beds together 

 indicate that the coal-bearing formation should be referred 

 to the Mesaverde rather than to the Laramie. The tran- 

 sition from the Maiicos to the Mesaverde is marked litho- 

 logically by the increasing prevalence of sand and paleon- 

 tologically by the change from marine to brackish- and 

 fresh-water conditions. On lithologic grounds all the coal 

 beds would be classed with the sandstone-shale formation, 

 the greater part of which at least is considered to be 

 Mesaverde. 



Two years later (1909) Richardson's fnll 

 report on this field ' was published, and in this 

 report the reasons for referring the coiil-bearing 

 rocks to the Mesaverde v;ere more fully set 

 forth : 



There has been much misapprehension concerning the 

 age of the coal-bearing rocks of the L'inta Basin. In the 

 Book Cliffs field, as already stated, Peale mapped the 

 rocks here referred to the Mesaverde as two formations 

 andcorrelated them respectively with the "Fox Hills" and 

 the "Laramie." Later i^Tifers have considered the 

 entire formation to be Laramie, because it overlies marine 

 ('retaceous beds and in turn is overlain by Wasatch strata, 

 and the fauna and flora were believed to belong to the 

 Laramie. 



The reason for assigning the coal-bearing formation of 

 the Book Cliffs to the Mesaverde is explained in the follow- 

 ing extract from a letter of T. \V. Stanton to the writer: 



■'In northwestern ('olorado. southern Wyoming, and 

 elsewhere many of the coal-bearing rocks previously 

 called Laramie are really older and are overlain by marine 

 Cretaceous formations, thus corresponding with the 

 Mesaverde formation first described in southwestern 

 Colorado. * * * The invertebrate fossils that have 

 been collected from the coal-bearing rocks of the Book 

 Cliffs all occur in the Mesaverde of northwestern Colorado, 

 and Dr. Knowlton finds that this is essentially true of the 

 plants also. It is admitted that most of the fossils in 



» Richardson, O. B., The Book Clids coal Held, between Grand River. 

 Colo., and Sunnyside, Utah: V. S. Geol. Survey Bull. 316, pp. 302-320, 

 1907. 



< Richardson, G. B., Reconnaissance of the Book Clifls coal Held be- 

 tween Grand River, Colo., and Sunnyside, Utah: U. S. Geol. Survey 

 Bull. 371, 1909. 



question from the Book Cliffs would not seem out of place 

 in the Laramie, yet their close agreement with those 

 known to occur in the Mesaverde of a neighboring area 

 and the general stratigraphic and areal relations of the 

 rocks in which they are found make this reference to the 

 Mesaverde most reasonable. The unconformable relations 

 that doubtless exist between these rocks and the o^erhnng 

 Wasatch will explain the absence of the later Cretaceous 

 rocks from the area. " 



GRAND MESA AND WEST ELK MOUNTAINS. COLORADO. 



The Grand Mesa and West Elk Mountains 

 include a ^ery considerable area on the south- 

 ern rim of the Uinta Basin, lying partly in 

 west-central Colorado and partly in eastern 

 Utah. The Grand Mesa field extends west- 

 ward into the Book Cliflis field and eastward 

 into the Anthracite-Crested Butte region and 

 thence westward along the Grand Hogback, 

 which in turn connects in part with the field in 

 Carbon County, Wyo. 



The eastern portion of this field was studied 

 in 1S74 by A. C. Peale,= of the Hayden Survey, 

 before the Laramie had been established. 

 Peale ga\ e many sections of the rocks, espec- 

 ially along Grand and Gunnison rivers, and 

 stated that in some places he had difBculty in 

 distinguishing between Cretaceous Nos. 4 and 

 5, or between Pierre and Fox Hills. He said: 



The upper portion of these beds may possibly have to 

 be referred to the Lignitic group, but for the present I 

 refer them to the Upper Cretaceous. 



On the publication of the Geological Atlas 

 of the Hayden .Survey, in 18S1, the coal-bearing 

 rocks of this region were referred to the Lara- 

 mie, which had then become very generally 

 accepted, and this reference was followed in the 

 main by geologists who had occasion to discuss 

 the area. Thus in 1894 George PL Eltlridge, 

 in the Anthracite-Crested Butte folio, gave a 

 section which, at least from the base of the Fox 

 Hills, corresponds in part with that of the 

 Hayden Survey Atlas. The coal-bearing beds 

 were referred to as the" Jjaramie coal measures" 

 and were said to have a nia.ximuin thickness 

 of 2,000 feet; thej'^ werj described as being 

 overlain unconformably by the Tertiary Ohio 

 formation. 



In 1907 W. T. I.,ee* investigated the Grand 

 Mesa coal field and obtained information which 



' U. S. Oeol. and Oeog. Survey Terr. Ann. Rept. for 1874, pp. 73-179, 

 1876. 



'U.S. Geol. Survey Geol. Atlas. Folio 9, 1894. 



< Lee, W. T., The Grand Mesa coal field, Colo.: U. S. Geol. Survey 

 Bull. 341, p. 303, 1909. 



