STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 1L3 



20 feet thick and about 82 feet from the base, is hard, quartzose, 

 cross-bedded and in thin irregular layers, which have rippled surfaces 

 with worm borings and indefinite markings. Of especial interest 

 are impressions very similar to Hah'menitcs major, associated with 

 an ofifshore fauna. At the base of the Benton are conglomerate, 5 

 feet and carbonaceous shale, 5 feet, with a few inches of coal at the 

 top. 



The Tres Hermanos sandstone is 90 feet above the base and only 

 5 feet thick in the Hagan field, west from Cerillos ; though so much 

 thinner, it has the same features. The thin coal bed and its over- 

 lying conglomerate, seen in Cerillos, appear to be wanting. A 

 Benton fauna is present in the lower 670 feet of the section. Con- 

 ditions are practically the same in the Tijeras field. In the Puerco 

 field no coal was seen at base of the Benton, but a conglomerate, 5 

 feet thick, with pebbles of quartz and chert, recalls that overlying 

 the coal in Cerillos. 



In the southwest corner of the San Juan Basin, as Gilbert^"^ has 

 shown, the Colorado is represented by mostly sandstones for 180 

 feet at the base, containing 3 coal seams about midway, while above 

 are 380 feet of carbonaceous and clay shale underlying sandstones 

 and sandy shales. The whole thickness is not far from 850 feet. 

 The coals are not persistent and they were seen in only one section. 

 Elsewhere they are replaced with carbonaceous shale. Winchesters"^ 

 says that in the Zuni Mountain region, a few miles south from the 

 locality of Gilbert's section, the Mancos is 60 per cent, sandstone. 

 This sandstone decreases northwardly as do also the coal seams, 

 which disappear in the northern part of the area examined by him. 



The Mancos shale is thin in the main portion of the San Juan 

 Basin, the whole thickness, according to Gardner,^'*^ being not more 

 than 800 feet. Coal seams occur in the upper 500 feet, where the 

 rocks are sandy ; there are no coals in the lower part, where clayey 

 beds prevail. The coal seams are usually thin, though occasionally 

 reaching 3 feet, are double or triple and often contain much bone. 

 One seam at times becomes workable, with 3 to 5 feet of sub- 



101 G. K. Gilbert, U. S. Geog. Explorations, etc., Vol. III., 1875, pp. 550, 551. 



102 D. E. Winchester, Joiirn. Wash. Acad. Sci., Vol. IV., 1914, p. 300. 

 i'53 J. H. Gardner, Bull. 341, pp. 366, 369, 2>7i, 375; Bull. No. 381, p. 462. 



