228 
ISLAND LIFE. 
[part I. 
is nearly midway between these, and it is, at all events, 
satisfactory that the various measures result in figures of the 
same order of magnitude, which is all one can expect on so 
difficult and exceedingly speculative a subject. 
The only value of such estimates is to define our notions of 
geological time, and to show that the enormous periods, of 
hundreds of millions of years, which have sometimes been 
indicated by geologists, are neither necessary nor warranted 
by the facts at our command ; while the present result places us 
more in harmony with the calculations of physicists, by leaving a 
very wide margin between geological time as defined by the 
fossiliferous rocks, and that far more extensive period which 
includes all possibility of life upon the earth. 
Concluding Remarks . — In the present chapter I have endea- 
voured to show that, combining the measured rate of denudation 
with the estimated thickness and probable extent of the known 
series of sedimentary rocks, we may arrive at a rude estimate of 
the time occupied in the formation of those rocks. From 
another point of departure — that of the probable date of the 
Miocene period, as determined by the epoch of high excentricity 
supposed to have aided in the production of the Alpine glaciation 
during that period, and taking the estimate of geologists as to 
the proportionate amount of change in the animal world since 
that epoch — we obtain another estimate of the duration of geolo- 
gical time, which, though founded on far less secure data, agrees 
pretty nearly with the former estimate. The time thus arrived 
at is immensely less than the usual estimates of geologists, and 
is so far within the limits of the duration of the earth as cal- 
culated by Sir William Thomson, as to allow for the develop- 
ment of the lower organisms an amount of time anterior to the 
Cambrian period several times greater than has elapsed between 
that period and the present day. I have further shown that, in 
the continued mutations of climate produced by high excentri- 
city and opposite phases of precession, even though these did 
not lead to glacial epochs, we have a motive power well calcu- 
lated to produce far more rapid organic changes than have 
hitherto been thought possible ; while in the enormous amount 
of specific variation (as demonstrated in an earlier chapter), we 
