CHAPTER X. 
THE EARTH’S AGE, AND THE RATE OF DEVELOPMENT OF 
ANIMALS AND PLANTS. 
Various estimates of Geological Time — Denudation and Deposition of 
Strata as a measure of Time — How to estimate the thickness of the 
Sedimentary Rocks — How to estimate the average rate of deposition of 
the Sedimentary Rocks — The rate of Geological Change probably greater 
in very remote times — Value of the preceding estimate of Geological 
Time — Organic modification dependent on Change of Conditions — 
Geographical mutations as a Motive power in bringing about Organic 
Changes — Climatal revolutions as an agent in producing Organic 
Changes — Present condition of the Earth one of exceptional stability as 
regards Climate — Date of last Glacial Epoch and its bearing on the 
Measurement of Geological Time — Concluding Remarks. 
The subjects discussed in the last three chapters introduce us to 
a difficulty which has hitherto been considered a very formidable 
one — that the maximum age of the habitable earth, as deduced 
from physical considerations, does not afford sufficient time either 
for the geological or the organic changes of which we have evidence. 
Geologists continually dwell on the slowness of the processes of 
upheaval and subsidence, of denudation of the earth’s surface, 
and of the formation of new strata; while on the theory of 
development, as expounded by Mr. Darwin, the variation and 
modification of organic forms is also a very slow process, and 
has usually been considered to require an even longer series of 
ages than might satisfy the requirements of physical geology 
alone. 
As an indication of the periods usually contemplated by geolo- 
gists, we may refer to Sir Charles Ly ell’s calculation in the tenth 
