FORMATIONS OF THE NACIMIENTO GROUP 725 



known at certain points and the inconstant thickness of the interval 

 between this horizon and the base of the Puerco indicates the uncon- 

 formable relationships between the two formations. For instance, 

 on the Rio Puerco and at other points where the full section of the 

 Nacimiento group is represented, the Torrejon formation is more 

 than 500 feet stratigraphically above the base of the Puerco. From 

 the Rio Puerco, where the full section of the group is present, the 

 base of the Puerco formation was traced westward very definitely 

 to the Arroyo Torrejon. In the latter district some of the upper 

 members of the Puerco are notably absent and the Torrejon fossil 

 mammals were secured at a little over 200 feet above the base of the 

 Puerco, thus indicating an unconformity at that point unless a very 

 notable thinning takes place in only the upper member of the Puerco- 

 Since the members in the lower portion of the Puerco are persistent 

 across the area and since the vertical distance between the base of 

 the Puerco and the base of the Torrejon is scarcely sufficient to repre- 

 sent the great time-interval indicated by the evolution of the Torrejon 

 fauna, it seems highly probable that the Torrejon is unconformable 

 here. The stratigraphic section as prepared on the east fork of the 

 Arroyo Torrejon is given below: 



SECTION OF THE NACIMIENTO GROUP ON THE EAST FORK OF 



ARROYO TORREJON, FOUR MILES EAST OF THE 



FARMINGTON ROAD 



(Wasatch, massive, conglomeratic sandstone at base) in Feet 



(Unconformity) 



Shale, drab and gray 14 



Shale, dark, carbonaceous and thin sandstone 20 



rr^ ■ I Sandstone, soft, gray and reddish shale 16 



Torrejon ( ) > & .7 



Shale, black, yellow, drab, and gray 35 



Sandstone, soft, tan-colored 20 



Shale, gray, strongly arenaceous (fossil mammals) 35 



(Unconformity ?) Thickness of formation 140 



/ Shale and soft sandstone 100 



) Sandstone, massive, coarse-grained, tan-colored 30 



Puerco \ 



) Shale, red, purple, drab, and white 30 



\ Shale, sandy, gray, dark, and yellowish 50 



(Unconformity) Thickness of formation 210 



"Laramie," sandstone, massive, brown Thickness of group 350 



