UPPER CRETACEOUS. 



661 



The strata thus described by Cross extend southward in New Mexico around 

 the east side of the San Juan basin, where they have been traced by Gardner : ^^* 



The San Juan coal region is a basin of more or less circular outline, with, an area of about 

 13,500 square miles, about one-seventh of which lies in Colorado and the remainder in New 

 Jilexico. The region receives its name from San Juan River. The two chief coal-producing 

 localities are Durango, in the extreme southwestern part of Colorado, on the Denver & Rio 

 Grande Railroad, and Gallup, in New Mexico, on the Atchison, Topeka & Santa Fe Railway. 



The formations known to occur in the San Juan district are as follows: 

 Generalized section of rocks in coalfield between Gallina and Baton Spring, N. Mex. 



The Dakota or basal formation of the Cretaceous throughout the San Juan region * * * 

 consists chiefly of hard quartzite with intercalated shale toward the top but near the base is 

 made up of reddish sandstone and yellowish shale that grade gradually into the Jurassic-Triassic 

 rocks below. No signs of coal beds were observed in the Dakota, but they are known to occur 

 in other parts of the basin. 



The Mancos shale rests conformably upon the Dakota. In the type-locality at Mancos, 

 Colo., the formation is all shale, but in New Mexico there are transitional sandy beds toward the 

 top. At a point 10 miles north of Gallina a part of the formation becomes arenaceous and 

 forms a hogback in the shale valley. This sandy bed is about 30 feet thick and about 275 

 feet below the top of the formation. It is no doubt the beginning of the sandstone and shale 

 formation that increases in thickness toward the south and is coal bearing on the south side 

 of the basin. In the vicinity of San Miguel, 12 miles down Rio Puerco from Cuba, the writer 

 observed a thickness of 300 feet of argUlaceous sandstone and sandy shale in the Mancos 

 grading upward through a transition zone to the Mesaverde. 



The Mesaverde is the most important coalrbearing formation in the area, as weU as through- 

 out the San Juan region. It forms a prominent hogback across the east side of the GaUina- 

 Raton Spring field. * * * Qoal beds occur at varying intervals between a prominent basal 

 and a capping sandstone. The main coal bed of the area is just below the top sandstone. 



The Lewis is much like the Mancos in appearance. It changes notably in thickness across 

 the field. From about 2,000 feet north of GaUina it thins to 250 feet on the Arroyo Torrejon 

 and in the vicinity of Raton Spring. It may be, however, that the lower part of the shale in 

 that region is replaced by sandstone which has heretofore been considered Mesaverde. 



The Laramie is without workable coal beds at the north limit of the field. In fact, Schrader " 

 maps the entire area northward to the Colorado State line as barren. West of Gallina the Lara- 

 mie and underlying Lewis shale disappear beneath unconformable Eocene beds. The next 



a Schrader, F. C, The Durango-Gallup coal field, Colorado and New Mexico: Bull. U. S. Geol. Survey No. 285 

 1906, p. 243. 



