66 SECTIONAL ADDRESSES. 
variations in the rate of decay under different conditions, variation in the 
rate of deposition and the occurrence of unrecorded intervals either evident 
or obscure. 
The rate of erosion has received much attention, but as this factor is 
obviously dependent on the shape and condition of the land surface, its 
average for all time is difficult to estimate. Barrell considers that denudation 
by solution lowers the land surface one foot in 30,000 years, and that 
mechanical degradation accomplishes this result in 13,800 years. The two 
forces acting together require 9,000 years to effect one foot of erosion. 
Barrell’s estimate of 250,000,000 years since the beginning of the Pale- 
ozoic has been mentioned already ; this estimate has been arrived at by 
a study of details of deposition under the hypothesis of rhythms in 
geological time. According to this author, time is to be measured by 
rhythms or pulsations, the greater rhythms having shorter rhythms im- 
posed upon them. The longer are to be measured in terms of the smaller, 
and the smaller in terms of years. A single rhythm is an erosion cycle and 
small partial rhythms are superimposed on it. 
Present erosion and sedimentation owing to the Plocene-Pleistocene 
uplift is unduly high, with the result that estimates of time based on the 
present rate of erosion are much too short. Barrell would further increase 
the time by restricting the area of deposition to the zone immediately 
below the local base level, and making the accumulation dependent on 
upward oscillations of the base level or downward oscillations of the 
bottom. The control of sedimentation by base level is summed up in 
three principles as follows : 
1. The rate of sedimentation is determined by the rate of the 
discontinuous depression of the surface of deposition. 
2. Subsidence of the sedimentary floor is not initiated by the load 
of sediment, but its further sinking may be facilitated thereby. 
3. The deposition of beds is not always a continuous process, but 
is often broken. The process of sedimentation is scour and fill with a 
balance in favour of the fill; in consequence, apparently continuous beds 
are broken by minor gaps to which the name ‘diastem’ is given. These 
gaps tend to increase the estimate of time, and they probably represent 
unrecorded intervals as long as the total indicated by the apparent 
unconformities. In view of all the factors, Barrell concludes that the 
geological ages may be ten to fifteen times as long as methods based 
on uniformitarian principles indicate. 
In connexion with the rate of sedimentation and its bearing on the age 
of the earth, it is apparent that the intimate structure of the stratified 
rocks must be looked to for data bearing on the problem. To this end the 
character and mode of formation of these rocks are now receiving an in- 
creasing degree of attention. A better understanding of sedimentation is 
being obtained by direct observation on the formation of modern sedi- 
ments, determination of the precipitating value of alge and bacteria, 
studies on coral reefs, deep-sea investigations, studies on colloidal solutions, 
investigations of chemical deposits, and a better appreciation of the value 
of facies and vegetal terrestrial deposits. Direct investigation of the rocks 
themselves is leading to an increased use of the petrographic microscope 
and of analytical methods. Secondary features of stratified rocks are 
receiving greater attention, horizontal transitional stages are better under- 
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