chap, x.] MEASUREMENT OF GEOLOGICAL TIME. 
227 
would be increased when the excentricity reached a maximum, 
as already fully explained, and may then have caused glaciers to 
descend into the adjacent sea, carrying those enormous masses 
of rock which are buried in the Upper Miocene of the Superga 
in Northern Italy. An earlier epoch of great altitude in the 
Alps coinciding with the very high excentricity 2,500,000 years 
ago, may have caused the local glaciation of the Middle Eocene 
period when the enormous erratics of the Flysch conglomerate 
were deposited in the inland seas of Northern Switzerland, the 
Carpathians, and the Apennines. This is quite in harmony with 
the indications of an uninterrupted warm climate and rich 
vegetation during the very same period in the adjacent low 
countries, just as we find at the present day in New Zealand a 
delightful climate and a rich vegetation of Metrosideros, fuchsias 
and tree-ferns on the very borders of huge glaciers, descending 
to within 700 feet of the sea-level. It is not pretended that 
these estimates of geological time have any more value than 
probable guesses ; but it is certainly a curious coincidence that 
two remarkable periods of high excentricity should have occurred, 
at such periods and at such intervals apart, as very well accord 
with the comparative remoteness of the two deposits in which 
undoubted signs of ice-action have been found, and that both 
these are localised in the vicinity of mountains which are known 
to have acquired a considerable elevation at about the same 
period of time. 
In the tenth edition of the Principles of Geology , Sir Charles 
Lyell, taking the amount of change in the species of mollusca 
as a guide, estimated the time elapsed since the commencement 
of the Miocene as one-third that of the whole Tertiary epoch, 
and the latter at one-fourth that of geological time since the 
Cambrian period. Professor Dana, on the other hand, estimates 
the Tertiary as only one-fifteenth of the Mesozoic and Palaeozoic 
combined. On the estimate above given, founded on the dates 
of phases of high excentricity, we shall arrive at about four 
million years for the Tertiary epoch, and sixteen million years for 
the time elapsed since the Cambrian, according to Lyell, or sixty 
millions according to Dana. The estimate arrived at from the 
rate of denudation and deposition (twenty-eight million years) 
Q 2 
