STRATIGRAPHY. 
15 
occur at several horizons, but chiefly in the upper few hundred feet of 
the formation. This part of the formation is also characterized by 
beds of buff sandstone, usually thin bedded, by which the Mancos 
shale grades into the overlying Mesaverde formation. 
From the nature of the exposures the thickness of the Mancos shale 
is difficult to determine. The best measurement was obtained from 
a drill hole near the upper terminus of the Book Cliffs Railroad, about 
10 miles northeast of Grand Junction. This began about 200 feet 
below the top of the formation, and was still in shale when drilling 
was stopped at a depth of 2,600 feet, showing a minimum thickness of 
2,800 feet. The dip of the shale in the valley can be only roughly 
estimated; but assuming an average of 2° and allowing 1,200 feet 
for the difference in elevation between the outcrops of base and top 
of the shale, and a width of outcrop of 1 1 miles, gives a thickness of 
about 3,200 feet. 
Marine shells have been found in the shale at two general horizons, 
one near the base and the other near the top. The lower collections 
were obtained at several localities within 200 feet of the base of the 
formation, and among these T. W. Stanton has identified the follow- 
ing forms: 
Fossils from lower part of Mancos shale. 
Anomia sp. 
Modiola sp. 
Ostrea lugubris Conrad. 
Inoceramus dimidius White. 
Inoceramus fragilis H. and M. 
Scaphites warreni M. and II. 
Prionocyclus macombi Meek. 
Callista sp. 
Pyropsis? sp. 
Baculites gracilis Shumard? 
Gryphsea newberryi Stanton. 
Most of the fossils from the upper part of the shale were found in 
limestone lenses about 250 feet below the lowest coal bed, but a few 
specimens were obtained in sandstone only 50 feet below the coal. 
They include the following forms, identified by T. W. Stanton: 
Fossils from upper part of Maricos shale. 
Lucina sp. 
Baculites compressus Say. 
Baculites ovatus Say. 
Inoceramus cripsi var. barabini Morton. 
Concerning those fossils Stanton reports as follows: 
The fossils of the shale between the Dakota and the coal-bearing rocks indicate thai 
two distinct faunas are represented, one, in the lower part, being characteristic of the 
Benton shale of the Colorado group, and the other, near the top, equally characteristic 
of the Montana group. As this agrees perfectly with the fauna of the Mancos shale in 
the type locality, and as there is also essential agreement in other respects, such ;is 
stratigraphic position, lithologic character, and thickness, it seems justifiable to apply 
the name Mancos to this shale. 
