124 THE JOURNAL OF GEOLOGY. 
somewhat regular period of about 91,000 years, but the successive 
maxima are of so unequal values that they cannot well be corre- 
lated with the relatively uniform cycles of deposition, 
The precession of the equinoxes seems better qualified to 
explain the Colorado phenomena. As the earth’s axis slowly 
describes its circle on the celestial sphere the relation of the 
seasons to perihelion is steadily shifted, so that the winter of the 
northern hemisphere, for example, occurs during one epoch 
when the earth is nearest the sun, and during another when it is 
farthest away. The terrestrial consequences of this cycle of 
change have been discussed by Adhémar, Herschel, Croll, 
Murphy, Pilar Hill, McGee, Penck, Ramsay, Wallace, Woeikof, 
Blytt, Ball, Becker and others, and, though there is wide differ- 
ence of opinion as to the character and amount of the climatic 
variations which may thus be brought about, these writers are 
in substantial agreement that the distribution of climates may 
be materially affected. The precessional period is about 26,000 
years, but the position of perihelion also moves—for the most 
part in a direction opposite to that of the equinoxes—and the 
resultant of the two motions has an average period of about 
21,000 years. It is not absolutely regular, but ranges ordinarily 
within 10 per cent. of its mean value, and exceptionally to 50 
per cent. above and below. 
I shall make no attempt to determine what were the climatic 
oscillations affecting Cretaceous sedimentation in Colorado nor 
how their influence was exerted. For the purposes of the present 
discussion it seems sufficient to point out that the local character 
of sedimentation might be influenced by changes in the local 
distribution of terrestrial climates: 
1. A periodic change in the circulation of the winds might 
modify the currents of the Cretaceous sea in such way as to bring 
to this district at one time argillaceous material and at another 
time calcareous material. 
2. A general change of climate producing glaciation about 
the two poles in alternation, as inferred by Croll and others, 
might shift the center of gravity of the earth in such way as to 
