CHAPTEB 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 Lyell's calculation in the tenth 



