MEASUREMENT OF CRETACEOUS TIME. W2y7, 
deposition being repeated in from eighteen to thirty-three inches. 
After making certain allowances, the average unit of deposition for 
the whole body of shale is assumed to be four feet. From the reg- 
ularity of the sedimentary rhythm and the large “number of its 
cycles, it is assumed to have been occasioned by a regular rhythm 
of conditions. The cycle of deposition is correlated with the 
precession-perihelion cycle—because this alone, of the various 
cycles known to the writer, appears competent to explain the 
phenomena. In discussing its competence, the ability of the pre- 
cessional cycle to produce climatic oscillations is postulated with- 
out argument (because it has already been treated at great length 
by others), and ways are suggested in which climatic oscilla- 
tions might result in the observed cycle of sedimentation. 
Assuming that the general inference is valid, the specific esti- 
mate is qualified chiefly by the uncertainty in passing from those 
portions of the sedimentary column where rhythm finds expres- 
sion in the alternate abundance and scarcity of lime carbonate to 
the other and greater portions of the column from which lime 
carbonate is nearly absent. This uncertainty is believed to be 
represented by the number 2 as a factor of safety; that is, the 
true period may be either twice or only one-half the estimated 
period of twenty million years. 
G. K. GILBERT. 
