ISLAND LIFE. 
[t-ART I. 
226 
Date of last Glacial Epoch and its bearing on the Measurement of 
Geological Time. — Directly we go back from this stable period 
we come upon changes both in the forms and in the distribution 
of species ; and when we pass beyond the last glacial epoch into 
the Pliocene period we find ourselves in a comparatively new 
world, surrounded by a considerable number of species altogether 
different from any which now exist, together with many others 
which, though still living, now inhabit distant regions. It seems 
not improbable that what is termed the Pliocene period, was really 
the coming on of the glacial epoch, and this is the opinion of 
Professor Jules Marcou. 1 According to our views, a considerable 
amount of geographical change must have occurred at the change 
from the Miocene to the Pliocene, favouring the refrigeration of 
the northern hemisphere, and leading, in the way already pointed 
out, to the glacial epoch whenever a high degree of excentricity 
prevailed. As many reasons combine to make us fix the height 
of the glacial epoch at the period of high excentricity which 
occurred 200,000 years back, and as the Pliocene period was 
probably not of long duration, we must suppose the next great 
phase of very high excentricity (850,000 years ago) to fall within 
the Miocene epoch. Dr. Croll believes that this must have 
produced a glacial period, but we have shown strong reasons 
for believing that, in concurrence with favourable geographical 
conditions, it led to uninterrupted warm climates in the tem- 
perate and northern zones. This, however, did not prevent the 
occurrence of local glaciation wherever other conditions led to 
its initiation, and the most powerful of such conditions is a great 
extent of high land. Now we know that the Alps acquired 
a considerable part of their elevation during the latter part of 
the Miocene period, since Miocene rocks occur at an elevation of 
over 6,000 feet, while Eocene beds occur at nearly 10,000 feet. 
But since that time there has been a vast amount of denudation, 
so that these rocks may have been first raised much higher 
than we now find them, and thus a considerable portion of the 
Alps may have been once more elevated than now. This would 
certainly lead to an enormous accumulation of snow, which 
1 Explication (Tune seconde Edition de la Carte Geologique de la Terre 
(1875), p. 64. 
