STRUCTURAL FEATURES IN THE FOOT-HILL REGION. 263 
the other formations, but, on the contrary, disappears by 
the sudden truncation of its strata in almost their full nor¬ 
mal thickness, clearly the effect of subsequent erosion. 
The movement which brought about the elevation of the 
Triassic strata must be regarded as synchronous with at least 
a portion of that more prolonged or extensive movement by 
which the sea was sooner or later shut out from certain areas 
in the Rocky Mountain region of Colorado, causing either a 
partial or an entire absence of marine beds, according to cir¬ 
cumstances, with a succeeding deposition of fresh-water strata 
in which a lacustrine life appeared. In the area under dis¬ 
cussion the fresh-water Jurassic alone was laid down. 
General subsidence of the entire region continued during 
the deposition of the Jura upon and against the sides of the 
Triassic eminence, and at its close the second period in the 
geological development of the area was completed. 
Third period .—This opened with still another uplift of all 
the pre-existing sediments into the fold traced in profile III, 
the rise of the arch in this case being approximately 1,000 
feet. The figure shows the character of the fold on the line 
of profile given to have been that of a long gentle slope from 
the confines well towards the center, where, on further yield¬ 
ing to the compressive forces, a clearly denned median ridge 
was produced. Erosion naturally went on in a more or less 
irregular manner, but the general position of the hill and 
its component strata relative to erosive forces was apparently 
such as to cause the disappearance from the top of the Jura 
over those parts of the slopes of gentle inclination of only 
the most insignificant amounts of material, while over the 
central or sharper portion of the fold the probable effect 
was the complete removal of the beds of the Jura and Upper 
Trias together with a partial removal of those of the Lower 
Trias from the crown of the arch, and from the adjoining 
flanks the material to the gently sloping line of union shown 
between these formations and the Dakota lying across their 
edges. Whether erosion reached an extent sufficient to per¬ 
mit the deposition of the Dakota and the lower part of the 
32—Bull. Phil. Soe., Wash., Vol. 11. 
