44 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
probably of fresh-water origin. In the interval between the deposition of 
the red sandstones and the Gunnison sandstones and shales there was 
probably some folding and perhaps a great amount of erosion. The red 
sandstones appear to be missing in part or wholly in certain areas which 
closely adjoin the Aspen district. 
This uplift was followed by a depression, so that the Cretaceous beds 
above the Dakota are of marine formation, through the Fort Benton, Nio- 
brara, Montana, and a large part of the Laramie. The greater part of these 
marine sediments, as illustrated by the thick Montana shales, were depos- 
ited in comparatively quiet waters, and were derived from a land surface 
which was being actively, but not enormously eroded. At the top of the 
Cretaceous section there recur beds of sandstone, at first intercalated at 
wide intervals in the shales, but finally forming beds of greater purity and 
thickness. These indicate an elevation corresponding to the preceding 
depression. This elevation, further carried on, is shown by the fresh-water 
deposits of the late Cretaceous, and culminated in the violent uplift and 
voleanic disturbance toward the close of the Cretaceous and the beginning 
of the Tertiary. 
CRETACEOUS-TERTIARY UNCONFORMITY. 
Near the close of the Cretaceous, in the Laramie, there began a series 
of disturbances which has probably lasted up to the present day, although 
the amount of disturbance has varied considerably at different times. This 
disturbance toward the close of the Cretaceous was manifested by the lifting 
above the sea of the whole mass of the Rocky Mountains im Colorado; and 
as if this uplifting were accompanied by the accumulation of molten rock 
beneath the earth’s crust, at intervals great masses of lava were thrust upward 
into the sedimentary rocks, or were poured out on the surface. The dynamic 
strains which arose in this disturbance were relieved partly by folding of the 
rocks and partly by faulting. The main uplift of the Rocky Mountains, 
producing the lofty structures which excite our admiration at the present 
day, began at this time. The greatest disturbance seems to have been about 
at the end of the Cretaceous and the beginning of the Tertiary; the existing 
Tertiary beds were deposited after this maximum disturbance, and therefore 
lie unconformably upon the folded Cretaceous strata. Such Tertiary beds 
are not found in the Aspen district; but to understand the history of the 
