92 STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 



It would appear from the observations by Lee and Gardner that, 

 in this portion of the basin, the Mesaverde is again Middle Pierre. 

 The sea area extended as a gulf southward as far as Cabezon's 

 latitude and the sandy member of the Pierre must have disappeared 

 at only a little way east from Gallina. 



Shaler"" examined the western outcrop in the San Juan Basin. 

 He reports that Lewis shale, only 250 feet thick where first recog- 

 nized at the south, becomes 2,000 feet farther north but diminishes 

 to 1,600 feet at the northern outcrop. The Mesaverde, massive 

 sandstone and thin interbedded shales and sandstones with coal 

 seams at the south, shows the triple succession at the north, where 

 the thickness is from 750 to 1,450 feet. He observed " horsebacks " 

 and " rolls " in a Mesaverde seam near Gallup. Along the northern 

 outcrop in Colorado, Cross and Spencer*'^ found the highest member 

 of the Pierre, named by them the Lewis shale, well defined. The 

 IVIesaverde, named by W. H. Holmes, is triple, the two great escarp- 

 ment sandstones with between them a coal group of sandstones, 

 marls and coal seams. The whole thickness in the La Plata quad- 

 rangle is barely 1,000 feet, that of the coal group being 600. The 

 coal seams are variable and the authors look irpon them as a series 

 of lenses. The Mancos shale named by Cross, has Pierre fossils 

 in the upper " several hundred feet," so that here also, one has the 

 condition observed on the opposite side of the area, at Cerillos, where 

 Mesaverde is the Middle Pierre. In the southern part of the San 

 Juan Basin, it would appear that Mesaverde and Pierre are prac- 

 tically synonymous terms. Gardner's*''* observations are of interest 

 in this connection. He traced the Mesaverde around the northern 

 border from Durango, Colorado, to Monero, New Mexico. It is 

 about 1,000 feet thick near Durango but decreases eastwardly, so 

 that it is only 400 feet at the Piedra River, 60 miles from Durango. 

 This is in accord with Schrader's observations and with those of 

 Gardner in the Gallina area. One seems to be justified in suggest- 

 ing that the Mesaverde disappears at a short distance east from the 

 San Juan basin, giving place to the shales, which are present in 



^^ M. K. Shaler, Bull. 316, Part 2, 1907, pp. 378, 414. 



^'' W. Cross, " Telluride Folio, No. 57," 1899; W. Cross and A. C. Spencer, 

 " La Plata Folio, No. 60," 1899. 



"^ J. H. Gardner, Bull. 341, p. 353. 



