Art. IX.] Herrick-Johnson, Geology of the Alhnquevqiie Sheet 187 
the south or is rendered less apparent by the disappearance of 
the Cretaceous strata under the Tertiary on the east side of the 
stream. Above San Francisco the stream is flanked by Creta- 
ceous hills leaving a rather narrow valley. The lowest horizon 
of the Cretaceous appears above the river level near the point 
where the valley enters the portion mapped. 
The Cretaceous Area. 
The entire portion of the sheet west of the Rio Puerco is 
composed of the fringe of the great Cretaceous and Jura-triassic 
series of the Mt. Taylor region. In the immediate vicinity of 
the Rio Puerco these shales and limestones dip gently away 
from the river to the south-west while in many places the ex- 
posures east of the river dip to the south-east and farther north 
the dip is to the north-east. The bottom of the Cretaceous is • 
nowhere well exposed upon our sheet but the white and yellow 
sandstone lying above the vermilion division of the red beds 
appears to the east of the river near where it leaves the sheet 
to the north-west. No single exposure affords an opportunity 
to measure the thickness of the Cretaceous in our region but 
the sequence has been pretty well made out. 
At the bottom is a well-defined bed of granular sandstone 
which is often quite pulverent though upon occasion it may 
become well indurated. The lower portion is white and attains 
a thickness of from 25 to 50 feet while the upper portion is 
more indurated and of a yellowish color. These two we shall 
include under the term “basal sandstone beds ” and find them 
a useful bench-mark for the base of the Cretaceous.^ The 
basal sandstone is followed by a series of dark, sometimes lig- 
nitic shales with a thickness of over forty feet which near the 
top contain bands of flags or sandstone impregnated with iron. 
Usually one of these layers at least is fossiliferous and has been 
termed the Gasteropod zone. Among the fossils so far identi- 
fied in this bed are the following : Ostrea translucida, Exogyra 
^ Farther to the westward this band reposes on the Dakota Sandstone or 
may perhaps be said to form a part of it. 
