114 THE GLACIER EPOCH OF AUSTRALASIA. 
Australia and Tasmania, and the more intense glacier period 
of New Zealand, indicating a still more marked refrigeration 
of climate, occurred anterior to this, namely, in the plio- 
cene period, it would prove that geographical and physical 
eauses must be added to the astronomical before we can 
adequately account for the special refrigeration of Northern 
and Central Europe at a later epoch. 
From independent reasoning, based mainly on the absence 
of evidence of glaciation in the earlier tertiary rocks o 
Europe, corresponding to former cycles of even greater in- 
tensity of the eccentricity of the earth’s orbit than occurred 
during the pleistocene period, Dr. Wallace and Mr. Searles 
V. Wood, jun., arrive at an exactly similar conclusion, viz., 
the necessity of the coneurrence of geographical and astro- 
nomical causes. 
There is one remarkable point connected with the supposed 
earlier occurrence of the glacier period of Australasia, which, 
if confirmed, will be a strong argument in favour of the 
potency of the astronomical theory, considered at any rate as 
a constant and necessary major element or great co-efficient 
in the causation of great glacial or glacier epochs. In the 
extended calculation of the cycles of eccentricity of the 
earth’s orbit, Dr. Croll places the highest limit reached im 
three million years at a point of time nearly 850,000 years 
ago, or about 550,000 years anterior to the last great eycle, 
extending from 80,000 to 250,000 years ago, which is sup- 
posed to correspond with the glacial and inter-glacial epochs 
of Europe and North America. Now, as already stated by 
me :—-* ‘“‘ Although it be admitted that the primary cause of 
the glacial epoch in the Northern Hemisphere in the pleisto- 
cene period may be due to the high phase of eccentricity of 
the earth’s orbit in combination with winter in aphelion—the 
effect of precession—it does not necessarily follow 
that the extreme effects of glaciation have been 
produced in both hemispheres, or in different. epochs 
by the recurrence of such astronomical causes alone. 
It is admitted that warm ocean currents have such an 
important bearing upon the question that, if they were not 
debarred to a great extent from the hemisphere specially 
affected by the astronomical causes referred to, glaciation of 
an extraordinary character would not be appreciable. Now 
the preponderance and the nature of the distribution of the 
land in the Northern Hemisphere render the latter more 
liable to the obstruction or diversion of the warm equatorialk 
ocean currents produced by geographical changes, while, with 
the smaller extent of elevated land.and its insular position, 
* Geol. of Tasmania, p. 255. 
