MEASUREMENT OF CRETACEOUS TIME. 128 
apparently irregular recurrence of physical conditions leading to 
the disposition of calcareous matter in this district, there was a 
relatively rapid and remarkably regular alternation of conditions 
determining the deposition of alternately more and less calcare- 
ous matter. The regularity of this minor alternation suggested 
the possibility that its cause might be discovered, for of the 
various causes known or supposed to modify sedimentation those 
which recur with uniform rhythm are comparatively rare. So 
far as we have definite evidence, the purely terrestrial causes, 
such, for example, as upheaval and subsidence, the shifting of 
waterways or divides, and the removal of oceanic barriers, are of 
irregular sequence; but certain astronomic causes are compara- 
tively regular. 
There are many astronomic cycles, and their periods vary 
widely in extent, but there are only a few to which it is reason- 
able to appeal for explanation of a rhythm in sedimentation. 
There are, in fact, but three to which geologists have made such 
appeal, and my own inquiry has discovered no others. I refer 
to the period of the earth’s revolution about the sun, the preces-. 
sional period, and the variation of the eccentricity of the earth’s 
orbit. Each of these is known or supposed to have an influence 
on climates, and the nature of sedimentation may in various. 
ways be influenced by climate. 
The period of the earth’s revolution does not seem applicable 
to the sedimentary rhythm under consideration, because a year 
is too short a time for the accumulation of the sediment. Doubt- 
less eighteen inches of sediment are often added in a year to the 
sea bottom near the mouths of rivers; but when we consider that 
many centuries are required to degrade the land to an average 
depth of eighteen inches, that areas of marine sedimentation are 
in a broad way commensurate with those of terrestrial degrada- 
tion, and that the Cretaceous sediments under consideration 
were accumulated scores and perhaps hundreds of miles from the 
land, we cannot for a moment imagine that they were deposited 
at so rapid a rate. 
The variation of the eccentricity of the earth’s orbit has a 
