4. FLORA OF THE FRUITLAND AND KIRTI^VND FORMATIONS. 



By F. H. Knowlton. 



fflSTORICAL SUMMARY. 



San Juan County, in the extreme northwest 

 corner of New Mexico, comprises but a small 

 portion of the hirger more or less clearly defined 

 structural area known as the San Juan Basm. 

 Certain structin-al elements, particularly the 

 upper series of coal-bearing rocks, form a more 

 or less continiious rim aroimd this basin and 

 dip toward the center, where, however, the}' 

 are deeply buried. 



As the object of the present study is to as- 

 certain the bearing of the fossil plants on the 

 age of this series of coal-bearing and related 

 rocks, it is desirable to give first a brief histor- 

 ical setting for the geologic facts here set forth. 

 It is not necessary in this connection, however, 

 to go further back than the beginning of the 

 time embraced within the history of the so- 

 called "Laramie problem" — that is, to about 

 1S75. 



The San Juan district -^ns studied by W. H. 

 Holmes,' of the Hayden Survey, during the 

 field season of 1875. The results of his studies 

 of the rocks in the vaUey of San Juan River are 

 displayed m a generalized section in Plate 

 XXXV of his report. The uppermost member 

 of this section is referred to the Wasatch and is 

 divided into two parts, the lower of which is the 

 Puerco marls of Cope, now included in the 

 Puerco and Torrejon formations. Immediately 

 below the Puerco marls is the so-called " Upper 

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

 stones and marls, which is called Laramie?. 

 This in turn rests on massive sandstone 120 feet 

 thick, caUed the Pictured Cliffs group, and on 

 the evidence of invertebrates was referred to 

 the Fox HlUs unit. 



Except for the preparation of a number of 

 economic reports on the coal of this region, 

 little systematic geologic work was done in the 

 region until 1899, when Ci-oss ^ established the 



' Holmes, W. H., Geological report on the .San Juan district: U. S. 

 Geol, and C.eog. Survey Terr. Ninth Ann. Kept., for 1875, p. 241, 1877. 



.2 Cross, Whitman, U. S. Geol. Survey Geol. Atlas, La Plata folio (No 

 60), 1899. 



units of the Upper Cretaceous section, which 

 subsequently have been so widely identified in 

 Colorado, Wyoming, and Montana. These 

 units, in ascending order, are the Mancos shale, 

 Mesaverde formation, and Lewis shale. Con- 

 cerning the Lewis shale Cross wrote as follows : 



Above the Mesaverde formation occurs another formation 

 of clay shale, reaching an observed thickness of nearly 

 2,000 feet, •wliich 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 that . 

 this division is still apparently below true Fox Hills. This 

 formation is called the Lewis shale. 



Continuing, Cross says : 



Still above the Lewis shale is a second series of sand- 

 stones, shales, and clays, bearing some resemblance to the 

 Mesaverde formation but differing in detail. The lowest 

 member of tliis complex is the "Pictured Cliff sandstone " 

 of Holmes's San Juan section, wliich he placed in the Fox 

 Hills upon the e\ddence of invertebrate remains. The 

 remainder was refeiTed to the Laramie, but without fossil 

 evidence. The present survey has failed to bring t« light 

 valid ground for assigning any of the beds in question to 

 the Laramie, wliile there is some reason to believe that 

 more than the lower sandstone belongs to the Montana 

 group. 



In 1905 Schrader^ made a reconnaissance 

 examination of the Durango-Gallup coal field 

 in which he adopted the stratigraphic classifi- 

 cation established by Cross and described the 

 uppermost coal-bearing sandstone as Laramie ?, 

 though he presented no fossil evidence. 



In 1906 more detailed examinations were 

 made m the region by Taff and Shalcr. Taff * 

 studied the Durango coal district, which lies 

 just off the southern foothills of the San Juan 

 and La Plata mountains, and in his report the 

 upper coal-bearing rocks here under considera- 

 tion were referred without question to the Lara- 

 mie, though he gave no details as to the reason 

 for this reference. Shaler's report •' deals with 

 that part of the Diu-ango-GaUup field lying 



s Schrader, F. C, The Durango-Gallup coal field of Colorado and New 

 Mexico: U. S. Geol. Survey Bull. 2So. pp. 241-258, 1906. 



< Taff, J. A. , The Durango coal district, Colo.: U. S. Geol. Suri^ey Bull 

 316, pp. 321-337, 1907. 



s Shaler, M. K., .\ reconnaissance survey of the western part of the 

 Durango-Gallup coal field of Colorado: Idem, pp. 376-426. 



327 



