256 
ELDRIDGE. 
character of its sediments, the upper, bright yellow or buff, 
sand}^ measures, which often form a well marked outcrop, 
first being lost, followed by the destruction of the argilla¬ 
ceous middle part of the series, and finally by the cutting 
out of the prominent basal limestones themselves. The dis¬ 
appearance occurs in the south only a short distance north 
of Bear creek, and in the north a quarter mile north of Van 
Bibber creek. 
The Montana .—This group in strike conforms strictly with 
the overlying Laramie, the basal sandstones of which afford 
a prominent and reliable key to the relations of these upper 
formations with the ones already considered. The dip of the 
component beds of the group, where all are present and in 
normal occurrence, shows a gradual increase from 45° to 90° 
as the distance increases from the base towards the summit. 
Over the middle of the anomalous tract, however, the vertical 
dip prevails, as in the case of nearly all the formations in 
this portion of the area. 
The fact of chief interest regarding the Montana group is 
its remarkable and rapid disappearance between Bear and 
Coal creeks as distance is gained from either of these streams 
towards the center of the region at Golden. Immediately 
north of Bear creek its strike relations with the underlying 
formations are rather more exaggerated than at most other 
points, and consequently moref clearly brought out in the 
surface exposures there occurring. In this vicinity successive 
beds of the Fort Pierre may be traced over their general line 
of strike by means of their lithological characteristics and 
the general prevalence of certain fossils at particular hori¬ 
zons, from points a thousand feet or more to the east of the 
Niobrara at the bluffs of Bear creek to others within only 
two or three hundred feet of the older formation, one or two 
miles to the north. The angle thus made by the difference 
in strike between the Niobrara and Fort Pierre is on an 
average about 15°, but decreases to the north, opposite the 
middle of the Dakota hog-back, beyond which the strikes are 
for a considerable distance more nearly parallel. 
