FORMATIONS OF THE NACIMIENTO GROUP 725 



under discussion except those resulting from secondary deposition 

 or weathering. There are occasional concretions of barite observed 

 by the writer in the Torrejon shale ten miles northeast of Encina 

 Spring, which may be nearly contemporaneous with the rocks which 

 inclose them. These are rough, irregular-shaped, often discoidal 

 aggregations of impure barium sulphate; they lie imbedded in clay 

 shale with which they harmonize closely in color, being usually 

 dark gray with a slightly bluish tinge. It is quite possible that they 

 represent the segregation of this slightly soluble salt at the time of 

 deposition or while the shale was in a semiplastic state. Neither 

 the Puerco nor Torrejon, so far as known, contains any primary 

 deposits of salt, gypsum, or Hmestone; rarely there are present thin 

 lenses of dark-colored limonite. 



The beds of the Nacimiento group show evidences of the existence 

 of currents from time to time during their deposition. Thin layers 

 of small quartz pebbles and cross-bedding are not uncommon. The 

 sudden termination along the bedding of massive sandstones is quite 

 common and in the absence of faults can scarcely be accounted for 

 except by the effects of stream-channels. The presence of smooth, 

 globular forms of siliceous sandstone within sandstone of a similar 

 matrix may be accounted for by either concretionary forces or by 

 the action of currents. The variegated shale layers common to the 

 upper beds and the overlying Wasatch formation probably represent 

 varying degrees of oxidation at or near the surface during deposition. 



It is the writer's opinion that the materials of the Nacimiento 

 group in this region were transported by broad, shallow streams and 

 laid down in deltas, lagoons, and shallow fresh-water lakes over a 

 broad peneplaned surface. At the close of the Cretaceous, this por- 

 tion of the continent was possibly slightly above sea-level, the eleva- 

 tion having been gradual from the time of marine deposition of the 

 Lewis shale on through a time of brackish and fresh-water accumula- 

 tion to land conditions at the close of the "Laramie." There was 

 then a period of widespread orogenic movement, mountain growth, 

 and attendant structure, accompanied and followed by erosion and 

 accumulation of local, fresh- water sediments; at this time the forma- 

 tions of the Shoshone group, including the Arapahoe arid Denver 

 formations, of the Denver Basin, and the Animas formation of the 



