264 
ELDRIDGE. 
Fort Benton clear across this rise is doubtful, but from the 
rate of disappearance of the Dakota from below, it is prob¬ 
able that neither this formation nor the lower half of the 
Fort Benton was here laid down. 
The evidence for the conclusions given in the preceding 
statements is clearly brought out in the strikes (ancient dips) 
and surface relations of the formations to each other: notably, 
in the divergence in strike and the truncation of the edges 
of the Jura by the Dakota on the southern side of the gap 
(profile III); in the thinning of the Dakota in such a man¬ 
ner as to eventually leave the fire-clay in its upper half in 
contact with the older sediments at the two points where the 
formation appears to end, in the south bank of Clear creek 
and the north one of Gold run; and in the ready reproduc¬ 
tion by graphic methods of the structural conditions observed 
in the field and the natural sequence of events based thereon. 
Sedimentation of the Dakota, Fort Benton, and Niobrara 
continued uninterruptedly to the close of the latter time, 
subsidence probably keeping pace. With this the third 
period of development ended. 
Fourth period .—The fourth period embraces the time dur¬ 
ing which the great elevation shown in profile IV was cre¬ 
ated, and in which the sediments of the Montana and over- 
lying formations were laid down. The uplift of this time 
was of much greater vertical and areal extent than any of 
those which preceded it, the rise of the arch on the line of 
section given reaching at least 9,500 feet, while its lateral 
extent was not far from 21 miles. It is broadly symmetrical, 
though there are several sub-flexures of a more or less pro¬ 
nounced curvature. The two of greatest prominence occur 
midway either flank. The others, of minor development, 
are confined chiefly to the higher part of the arch, and rep¬ 
resent a crumpling of a secondary nature along this portion 
of the fold. This crumpling is well shown upon the present 
surface of the region in the changes in strike of the affected 
beds, which are in strong contrast with the unbroken direc¬ 
tion to which the strata of younger age hold. The possi- 
