dation and deposition
(twenty-eight million years) 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 when discussing 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 {125} far more extensive period which
includes all possibility of life upon the earth.

_Concluding Remarks._--In the present chapter I have endeavoured 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 geological 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 calculated
by Sir William Thomson, as to allow for the development 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
excentricity and opposite phases of precession, even though these did not
lead to glacial epochs, we have a motive power well calculated 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 have ample material for that power to act upon, so as
to keep the org