HISTORICAL REVIEW OF THE LARAMIE PROBLEM. 



69 



f erred to the Montana in my " Flora of the 

 Montana formation." " 



The economic resources of this region were 

 briefly examined by J. A. Taff " in 1906, but he 

 made only incidental mention of the geologic 

 relations, taken mainly from the results of the 

 earlier students. 



The Coalville section was also briefly sum- 

 marized by A. C. Veatch" in his paper on the 

 "Geography and geology of a portion of south- 

 wester Wyoming," published in 1908, for the 

 purpose of comparing it with the Wyoming 

 section, of which it doubtless represents a 

 part. He suggested that the Colorado group 

 may extend higher in the section so as to in- 

 clude the plant-bearing beds. Veatch also 

 gave a very complete bibliography relating to 

 the exploration and geology of this general 

 region. The latest report on this field is that 

 by Carroll H. Wegemann,*'' published in 1915. 

 In this report the Coalville section, with a 

 thickness of about 9,000 feet, was described as 

 a slightly overturned anticline. Folding and 

 faulting here further complicated matters until 

 in the absence of adequate paleontologic data, 

 it is difficult to draw satisfactory formational 

 boundaries; in fact, Wegemann stated that 

 " although the area has been studied by several 

 geologists, the formation boundaries are by no 

 means definitely determined." 



According to Wegemann, the workable coal 

 of the region is of Colorado age, but the thick- 

 ness that is to be assigned to the Colorado part 

 of the section is indefinite. Its thickness is at 

 least 1,000 feet, and there is a tendency to 

 extend the Colorado upward to include an addi- 

 tional 1,000 feet, but the evidence for this was 

 not regarded as conclusive. It would then 

 include the plant horizon already mentioned, 

 which contains a flora correlated with the 

 Montana (Mesaverde) at Point of Rocks, Wyo. 



The Laramie is not known to be present in 

 this section, but in this connection Wegemann 

 pointed out that there are over 4,000 feet of 

 beds between those last mentioned and the 

 unconformity at the base of the Tertiary 

 (Wasatch formation) that are still unplaced. 

 This great thickness of beds was divided into 



« Knowlton, F. H., U. S. Geol. Survey Bull. 163, pp. 8-9, 1900. 



« Notes on the Weber River coal field, Utah: U. S. Geol. Survey Bull. 

 2S5, pp. 285-288, 1906. 



« U. S. Geol. Survey Prof. Paper 56, p. 103, 1907 [1908). 



•1 The Coalville coal field, Utah: U. S. Geol. Survey Bull. 581, pp. 161- 

 18i, 1915. 



two parts, the lower of which (No. 3 of Wege- 

 mann's section on p. 163), 1,650 feet thick, is 

 marine, while the upper (No. 2), 2,500 feet 

 thick, contains leaves and fresh-water shells, 

 though they are not sufficiently abundant or 

 sufliciently well preserved to admit of definite 

 stratigraphic determination. 



SAN JUAN BASIN AND ADJACENT AREAS IN 

 COLORADO AND NEW MEXICO. 



The area included in the San Juan Basin and 

 vicinity extends from the vicinity of Durango, 

 Colo., on the north, beyond Gallup and Mount 

 Taylor, N. Mex., on the south, and from a point 

 near the Arizona-New Mexico line on the west 

 to Chama, Elvado, and the Sierra Nacimiento, 

 near the longitude of Albuquerque on the east, 

 or nearly 150 miles from north to south and 

 100 miles from east to west. The coal-bearing 

 rocks form a more or less continuous rim 

 around this basin and dip toward the center, 

 where, however, they are deeply buried. 



The San Juan district was studied by W. H. 

 Holmes," of the Hayden Survey, in 1875. In 

 Plate XXXV of his report he gave a general 

 section of the rocks in the valley of San Juan 

 River. The uppermost member was referred 

 to the Wasatcli, which he divi(U>d into two 

 parts, the lower being the Pucrco marl of Cope. 

 Immediately below this is the so-called " upper 

 coal group," made up of 800 feet of soft sand- 

 stones and marls, wliich was referred to as 

 Laramie?. This in turn rests on the Pictured 

 Clifl[ sandstone, which with the underlying 

 1,500 to 2,000 feet of rocks Holmes referred to 

 the Fox Hills. 



E.xcept for a number of economic reports on 

 the coal of the region, little systematic work 

 was done in this area until 1899, when Whitman 

 Cross," in the La Plata folio, established the 

 units of the Upper Cretaceous section of the 

 region, which have subsequently been so widely 

 identified in Colorado, Wyoming, and Montana. 

 These units in ascending order are Mancos shale, 

 Mesaverde formation, and Lewis shale. Con- 

 cerning the Lewis Cross wrote as follows : 



Above the Mesaverde formation occurs another formation 

 of clay shale, reaching an observed thickness of nearly 

 2,000 feet, which is very much like the Mancos shale but 

 contains fewer fossils. The only identifiable form thus 

 far found in this shale occurs also in the Mancos shale, so 



" Geological report on the San Juan district: U. S. Geol. and Geog. 

 Survey Terr. Ninth Ann. Rept., p. 241, 1877. 

 « U. S. Geol. Survey Geol. Atlas, La Plata foUo (No. 60), p. 4, 1899. 



