JOURNAL OF GEOLOGY 
FEBRUARY-MARCH, 1895. 
SEDIMENTARY MEASUREMENT OF CRETACEOUS 
TIME. 
Ir is the purpose of this paper to describe certain regular 
alternations of strata observed in Colorado, to correlate these 
with an astronomic cycle of known period, and to deduce from 
this correlation an estimate in years of a portion of Cretaceous 
time. 
Along the base of the Rocky Mountains, and eastward for 
many miles, the basin of the Arkansas river is occupied by Cre- 
taceous rocks. At bottom are the Dakota sandstones, several 
hundred feet in thickness; and above these a great body of 
shales, constituting the Benton, Niobrara and Pierre groups and 
having a total thickness of 3900 feet. In the main these shales 
are argillaceous; but at a few horizons they are calcareous, and 
at one level a sandstone appears, accompanied by a few feet of 
arenaceous shale. The sandy passage is best developed near the 
mountains, and disappears altogether toward the east. The cal- 
careous passages are more persistent and have been recognized 
throughout the district. At least two of them occur many miles 
farther to the north. As the shales and the associated lime- 
stones approach the mountains they do not assume the character 
of littoral deposits, but remain practically unchanged; and it is 
thence inferred that the sea in which they were deposited 
extended to a remote western shore. 
tRead before the Geological Society of America, December 28, 1894. 
Vou. IIIL., No. 2. 121 
