BY R. M. JOHNSTON, F.L.S. 109 
THE Causes oF GruactaAL Epocus. 
Mr. Scarles V. Wood, jun., enumerates no less than seven 
different causes which have been advanced, more or less 
strenuously at various times by different persons, to account 
for the marked changes in climate of which the record of the 
rocks bear unmistakable evidence. These are— 
. A decrease in the original heat of the planet. 
. Changes in the obliquity of the ecliptic. 
Changes in the position of the earth’s axis of 
rotation. 
. A variation in the amount of heat radiated from 
the Sun. 
A variation in the temperature of space. 
. The combined effect of the precession of the equinoxes 
and of the eccentricity of the earth’s orbit. 
. Changes in the distribution of land and water. To 
these we must add another, or rather the combina- 
tion of the last two, as ably put forward by 
Dr. Wallace in his “ Island Life,” viz. : 
8. A particular distribution of geographical and 
physical conditions operating concurrently with 
high eccentricity of the earth’s orbit with winter 
in aphelion. 
As regards the first five supposed causes, I follow Dr. 
Wallace in rejecting them on the grounds that they are 
either inadequate, taken singly or in combination, to explain 
the whole of the known phenomena associated with glacial 
epochs, or there is no geological evidence which reveal their 
occurrence. 
There remain, therefore, only two, taken singly with the 
third, which demand their causal concurrence. 
Before entering upon a discussion as to the efficiency or 
otherwise of the principal hypotheses which would at least 
explain the observed dynamic effects of former glaciation, and 
which, after all, are the only phenomena that require special 
explanation from a geologist’s point of view, it is necessary 
to mark strongly the distinction between (1) a period of 
abnormally Jow temperature in a given region, and (2) glacial 
action, fer se, in the same region. In ordinary discussions on 
probable causes of glaciation the two ideas are mixed up, or 
appear to signify the same thing, and consequently not a 
little of the antagonism between some of the advocates of the 
geographical theory and the astronomical theory respectively is 
due to the lack of precision with which they grasp the 
essential differences which exist between them. 
The mere lowering of temperature throughout the various 
zones of isotherms of any one hemisphere during a period of 
extreme eccentricity with winter in aphelion, as first demon- 
3° Dore & Op 
