MEASUREMENT OF CRETACEOUS TIME. 125 
make the sea alternately advance against and recede from a coast. 
Even a small oscillation of this sort might render the principal 
load transported by streams from a coastal plain alternately 
chemical and fragmental; and a great oscillation, by causing the 
coast line to migrate, might periodically revolutionize the distri- 
bution of sediments in the sea. 
3. Ifthe climate of a broad peneplain were by precession made 
alternately moist and dry, then during moist epochs it would be 
densely clothed with vegetation, subterranean waters would be 
highly charged with organic acids so as to dissolve much lime 
carbonate, and mechanical degradation would be impeded by 
the vegetal mat. During dry epochs vegetation would be sparse, 
water would have little power of solution, and relatively rapid 
mechanical degradation would cause the residual clays to be 
transported to the ocean. 
Adopting 21,000 years as the time unit corresponding to each 
sedimentary alternation in the calciferous portions of the great 
shale bed, it remains to estimate the rate of deposition of the 
more argillaceous portions. As already stated, the sedimentary 
cycle repeats itself every eighteen inches where the principal 
deposit is limestone; it also repeats itself every eighteen inches 
where the limestone makes but one-fourth of the total deposit ; and 
it repeats itself in about 2.7 feet where the calcareous material 
suffices only to modify an otherwise argillaceous shale. It would 
appear, then, that the shale was on the whole deposited more 
rapidly than the limestone, so that in the great bodies of shale 
something more than 2.7 feet of sedimentation should be cor- 
related with a unit of the time scale. It is moreover true that 
certain portions of the shale are of different type from those 
associated with the limestone. This difference does not find 
definite expression in the chemical composition but appeals to 
the eye. All shales near the calcareous passages are pale gray 
in color, while there are important beds in the upper and lower 
portions of the Benton series and in the upper part of the Pierre 
series which are dark gray. These constitute about one-tenth 
of the entire series. It is not clear whether we should ascribe 
