FORMATIONS OF THE NACIMIENTO GROUP 719 



carbonaceous shale very similar to the dark layers in the underlying 

 Puerco; but on the Puerco River the carbonaceous layers are con- 

 fined to the Puerco formation almost entirely. 



Both the formations of the Nacimiento group consist essentially 

 of variegated clay shale, arenaceous shale, and soft, coarse-grained 

 sandstone of white, gray, and tan colors. Such beds are of that 

 consistency necessary to the development of badland topography, 

 with the exception of local, siliceous sandstones hard enough to form 

 mesas or dip slopes through which intermittent streams cut canyons 

 into the underlying shale. The latter is more particularly the type of 

 topography in the vicinity of Nacimiento. The softer sandstones 

 which commonly alternate with the arenaceous and clay shale usually 

 weather to soft incoherent sand at the outcrop. These beds have 

 been said to consist in part of unconsolidated sand but the writer 

 has found the term applicable only to weathered exposures; for in 

 every case the sand is solid at a short distance from the surface. 



There are local conglomerate layers in both the Puerco and 

 Torrejon but they are of minor importance and exceptional. No 

 one member of either of the formations is a true conglomerate such 

 as the basal conglomerate of the Wasatch formation. However, 

 occasional lenses of small quartz and chert pebbles are to be seen 'in 

 the more massive, coarse-grained, siliceous sandstones. 



The basal sandstone of the Wasatch is strongly conglomeratic, 

 consisting of pebbles averaging the size of an egg and of varied compo- 

 sition; the pebbles consist of quartz and chert, of red, black, brown, and 

 white colors, and various crystalline rocks. The matrix is composed 

 chiefly of coarse, brown quartz grains. This sandstone is a promi- 

 nent horizon-marker over a wide area on the south and east sides of 

 the San Juan Basin. On the north side of the basin, in southwest 

 Colorado, where the Wasatch rests on the Animas formation, this 

 conglomerate member is absent. 



With the exception of the conglomerate above mentioned the forma- 

 tions of the Nacimiento group are lithologically distinguished only in 

 a slight degree from the overlying Wasatch. The Wasatch contains 

 a larger percentage of highly colored shales and softer sandstones 

 than the Nacimiento, but otherwise the composition is not materially 

 different. The contrast of the Nacimiento group with the "Laramie" 



