126 THE JOURNAL OF GEOLOGY. 
a relatively rapid or a relatively slow deposition to the dark 
shales, but the fact that the shale body is not entirely uniform 
in character tends to increase the probable error of an esti- 
mate of its rate of deposition. It appears to me that an allow- 
ance of four feet of local sedimentation for each astronomic 
cycle should afford a somewhat conservative estimate for the 
corresponding portion of geologic time. Upon this basis the 
3900 feet of sedimentation required about twenty million years, 
and this estimate covers the Benton, Niobrara and Pierre epochs. 
These epochs constitute a part of the Cretaceous period, being 
preceded in the chronology of the Great Plains province by the — 
Dakota and Comanche epochs and followed by the Fox Hills and 
Laramie. As the sediments representing those epochs are of 
different character from the shale to which computation is here 
applied, the estimate cannot be extended to cover the entire 
Cretaceous period without materially increasing its probable 
error. 
The reasoning here employed is strictly parallel and partly 
identical with that of Blytt in his discussion of ‘‘The Probable 
Cause of the Displacement of Beach Lines” (Christiania, 1889). 
It differs most conspicuously in the interpretation of the influences 
of dry and moist climates. He correlates fragmental sediments 
with warmth and moisture, and chemical with coolness and dry- 
ness. In discussing the Cenozoic sedimentation of various 
European countries he finds the alternation of clay and lime 
carbonate to have an average thickness of 51 inches, nearly three 
times that observed in the Cretaceous of Colorado. 
On the authority of Geelmuyden, Blytt states that the pre- 
cession period should theoretically have been relatively short in 
earlier geologic eras because then the axial rotation was more 
rapid and the oblateness of the spheroid greater; and to what- 
ever extent this was true in Cretaceous time the preceding esti- 
mate of twenty million years should be diminished. 
That the logic of this discussion may be quite clear, some of 
its leading points are briefly restated. Certain parts of a shale 
body are found to exhibit a rhythm of sedimentation, the cycles of 
