254 
ELDRIDGE. 
or over-lying one is apparent until upon near approach to 
the confines of the region presenting the anomalies just 
described for the Trias. Any decrease in the thickness of 
the Jura beyond is little more than is usually met with from 
point to point along the range. From about a mile south 
and a mile and one-half north of Clear creek, however, the 
beds of the formation disappear in rapid succession as the 
center of the region is gained. Their strike is, moreover, at 
variance with the formations both above and below: in the 
southern part it is in noticeable contrast with that of the 
Dakota, being some 10° or 15° to the east of the latter; in 
the northern portion not only is the same discrepancy prob¬ 
able between these two formations, but an equal one also 
appears between the beds of the Jura and those of the Trias 
below. The thinning of the Jura is in part probably due to 
the absence of some of its lower beds, while the cause of its 
sudden and final thinning is found in the rapid and succes¬ 
sive disappearance of, first, its upper beds, followed in turn 
by those lying beneath. 
The Dakota .—As ascent is gained in the series of forma¬ 
tions, the region of anomalies becomes more and more ex¬ 
tended in north and south directions. The Dakota begins 
to display irregularities as far south as the northern end of 
the high hog-back just south of Coon gulch, and in the 
north at the southern end of the chain of hog-backs north 
of Golden. The noticeable points in the behavior of the 
southern half of the formation are: first , the disappearance 
of the characteristic hog-back; second, the gradual decrease 
in thickness, which the outcrops of the remaining portions 
show to be both from above and from below, the fireclays in 
the middle of the formation being the last to disappear, as 
evidenced at the bluffs of both Clear creek and Gold run • 
third, the discrepancy in strike between this formation and 
those below and above, its beds in the region of more pro¬ 
nounced irregularity lying across the edges of the former, 
and abutted from above by the ends of the successive strata 
