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104 The American Geologist. February, 100 
out with some degree of certainty. 1. The Carboniferous 
strata of Perry park together with the mottled beds of doubt- 
ful age, abut against the crystallines which form the southern 
base of the arch. 2. The Red Beds also thin from bottom up- 
ward as they approach the crest. The gypsum at the top of 
the Red Beds approaches the crystallines and is found close to 
them near the crest of the arch. 3. The Dakota sandstone is 
absent for a distance of about six miles across the crest—un- 
less a limited exposure of vertical sandstone just north of the 
crest is in part Dakota. It was not determined whether the 
absence of the Dakota is due to non-deposition or to erosion. 
4. On the north side, the basal stratum of the Niobrara lime- 
stone extends beyond the older formations and comes nearly 
if not quite in contact with the crystallines. 5. There is a 
notable development of Ft. Pierre shales both north and south 
of the crest. The maximum thickness for the Denver basin 
is estimated at 7,700 feet. It is probably thicker in Perry park 
than at any point within the Denver basin. This shale thins 
toward the crest of the arch from either direction. 6. The 
strike of the Laramie south of the arch renders it doubtful 
whether that formation ever extended over the crest. 7. The 
Arapahoe lies across the truncated edges of the older forma- 
tions from the Laramie to the Red Beds. Beyond this, in the 
region of the crest, it could not be definitely followed ow. ng 
to the surface debris. 
If the physical conditions be reconstructed by which these 
relations were brought about, we should probably have some- 
thing like the following,—An east-west elevation of the crys- 
talline rocks existed as early as the Carboniferous period, 
against which the sediments of that period were deposited. 
This elevation remained at least during the early part of the 
Red Beds period. There was an arching either near the be- 
ginning of the Dakota period resulting in the non-deposition 
of the Dakota, or after its close, resu'ting in the removal of the 
L.Dakota from the crest. It is probable that the first alternative 
is the correct one and that the reg’on was affected by the “early 
Cretaceous movement” of Mr, Emmons.* At the close of the 
(Niobrara epoch, a notable re-elevation occurred. This move- 
ment has been called by Mr. Emmons, the ‘“mid-Cretaceous 
*S. F. Emmons, U. S. Geol. Surv., Mon, 27, p. 23. 
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