112 STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 



The Colorado Group. 



The Niobrara and Benton are sufficiently distinct in the region 

 of the Front Ranges and eastward as far north as Wyoming. The 

 Niobrara consists of black shales and limestones weathering to chalky 

 whiteness ; while Benton is mostly shale, but with bands of sand- 

 stone and more or less persistent limestones. Farther west, how- 

 ever, the deposits answering to the Niobrara-Benton time interval 

 lose the limestones and the mass becomes practically continuous as 

 shale, though varying much at different horizons. The term Colo- 

 rado Shales finds application in those areas, where Niobrara cannot 

 be recognized and where Benton conditions, as shown at some places 

 by the continuing fauna, remained comparatively unchanged. The 

 term Mancos was introduced in southwestern Colorado, to designate 

 the shale mass between the Mesaverde (Middle Pierre) and the 

 Dakota. It is a lithological term for use in the field and includes 

 Lower Pierre as well as Niobrara and Benton. 



The Colorado interval is represented by marine deposits in by 

 far the greater part of the Cretaceous area, but in New Mexico the 

 isolated coal fields give abundant evidence that the mainland was 

 not far distant, as coarse deposits make their appearance, while 

 farther west in the same state as well as in Arizona and Utah one 

 finds conditions such as characterized the Middle Pierre, marking 

 the strife between land and sea, sandstones and coal beds being the 

 especial features. 



The relations of deposits in the southernmost fields of New 

 Mexico are somewhat obscure, the areas being very small and 

 isolated. But there is little room for doubt farther north in the 

 Cerillos and other fields southeast from the San Juan Basin. Lee^'' 

 obtained a detailed section of the Mancos in the Cerillos field. The 

 upper portion is distinctly Pierre and the lower portion, al)out 2.200 

 feet, is certainly Colorado in the lower 1,200 feet. One finds here 

 the several subdivisions of the Benton, as recognized east from the 

 Front Ranges, but the limestones of the Niobrara interval have dis- 

 appeared. A sandstone, Tres Hermanos of Herrick and Johnson,^*"' 



00 W. T. Lee, Bxdl. Gcol. Soc. Amcr., Vol. 23, 1912, pp. 623, 631, 658, 

 651-654. 



1^" C. L. Herrick and D. W. Johnson, Bull. Univ. Nezv Me.v.. Vol. II., p. 13. 



