STRUCTURAL FEATURES IN THE FOOT-HILL REGION. 255 
of the Montana group throughout much of their line of con¬ 
tact; finally , frequent changes in the strike of its beds 
oyer the central portion of the affected area, none of which 
changes are paralleled by corresponding ones in the promi¬ 
nent sandstones at the base of the Laramie, lying but a short 
distance to the east. In the northern half these same pecu¬ 
liarities are again met with, but in some particulars are 
more strongly accented than in the southern : these are the 
more sudden disappearance of the hog-back; the rapidity 
with which the formation thins; the marked crumpling, 
as shown in their strikes, to which its beds have been sub¬ 
jected without the overlying strata being in the least affected. 
In dip the Dakota varies from 45° in its more normal occur¬ 
rence to vertical over the more disturbed, middle portion of 
the field. 
The Fort Benton .—This formation completely disappears 
a short distance north of Coon gulch, and also at a point 
about opposite the middle of the first liog-back north of 
Golden. The southern portion has thinned very gradually 
throughout a distance of three and one-half miles, while for 
the northern member the disappearance has been completed 
in a little less than a mile. In strike and dip the Fort 
Benton conforms to the Dakota, but is overlain, after the 
disappearance of the Niobrara, by successively higher strata 
of the Fort Pierre and Fox Hills formations as the center of 
the disturbed region is approached. 
The Niobrara. —This, like the Fort Benton, disappears 
only from above downward, but its limits are found con¬ 
siderably to the north and south of those of the correspond¬ 
ing members of the older formations. In strike and dip it 
conforms with the Fort Benton and Dakota, and like them, 
in passing from without inward, is overlain by successive 
strata of the Fort Pierre, though nowhere brought in contact 
with the higher member of the Montana group, the Fox 
Hills. The disappearance of this formation is especially 
well shown both in the north and south from the physical 
31—Bull. Phil. Soc., Wash., Vol. 11. 
