STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 87 



embraces practically the whole of the Pierre, while in others it but 

 the middle portion. Mancos is another lithological term, designat- 

 ing the mass of shale underlying the Mesaverde, so that in many 

 districts it includes the Lower Pierre as well as the Niobrara and 

 Benton. The significance of the several terms wnll appear in de- 

 scription of the districts. 



The Pierre in the Parks of Colorado and east from the meridian 

 of the Front Ranges of Colorado consists mostly of shales, becom- 

 ing sandy toward the top, with irregular lenses of limestone and, in 

 the upper portion, huge calcareous and ferruginous concretions. 

 Sandstone is wholly unimportant except in the Boulder district of 

 the Denver Basin, where Fenneman saw^,^" at one third way from 

 the base, the Hygiene sandstone, which is several hundred feet thick 

 west from Berthoud, but only 250 feet at the north end of the dis- 

 trict. The thickness of Pierre in this region is not fully determined; 

 Eldridge gives 7,700 to 7,900 feet in the Denver Basin, but Fenne- 

 man gives only 5,000 in the Boulder district of that basin. Near 

 Canyon City on the Arkansas River, oil-borings found 4,500 feet, 

 while farther south on the eastern border of the Raton-Trinidad 

 coal field, the thickness appears to be considerably less. 



But the change is startling between the southern termination of 

 the Raton field and Cerillos, a distance of about 100 miles in west 

 of south direction. At Cerillos, one is on the same meridian with 

 the Park area of Colorado, where the Pierre is almost wholly shale, 

 whereas here it is largely sandstone. Some small isolated coal 

 fields remain farther south. The Engle, unimportant from the 

 economic standpoint, has coal-bearing rocks, which as Lee^' has 

 shown, rest on deposits of Benton age. Wegemann found similar 

 conditions in the Sierra Blanca field about 80 miles west-northwest 

 from the last. Both authors are inclined to refer the coals and asso- 

 ciated rocks to ithe Mesaverde, because the general conditions re- 

 semble those observed farther north in the Cerillos field. In the 

 absence of conclusive information, the writer is inclined to sviggest 

 that the coals may be of Benton age. The Sierra Blanca area is 

 not far from 120 miles south from the Cerillos field and by so much 



56 N. M. Fenneman, Bull. 265, 1905, pp. 31, 2^. 



5" W. T. Lee, Bull. 285, 1905, p. 240; C. H. Wegemann, Bull. 541-/, p. 10. 



