June, 1892 . 
THE ICE AGES, PAST AND FUTURE. 
123 
from north to south, and 800 miles in width in the northern 
portion, covered entirely with a continuous ice sheet, excepting 
only a narrow inhabited fringe on the south-western coast. 
The thickness of the ice in the ancient glaciers of the Ice 
Age is indicated by the height above the floor of the valleys 
to which the glacier groovings and striations are seen ; in 
Llanberis Pass, for instance, on the flank of Snowdon, these 
show a thickness of several hundred feet in the glacier that 
flowed down the pass, and in the Norway Fiords a thickness 
is shown of more than 1,000 feet. 
The fact of the previous existence of an Ice Age has long 
been established ; but all the suggestions made as to the 
producing cause of the Ice Age were found insufficient to 
account for so gigantic a phenomenon, until the remarkable 
investigation of Croll, showing that the cause was not in anv 
changes in the earth itself, such as changes in the ocean 
currents, by volcanic agency altering the levels of the earth’s 
surface and disturbing the currents; but that the cause was 
entirely external to the earth, and due to disturbances in the 
motion of the earth, that were produced by the action of 
other members of the solar system, and had the effect of 
altering the position of the earth in relation to the sun, which 
is the source of all the heat received by the earth. These 
changes in the motion of the earth are exceedingly slow in 
their progress, and extend over enormous periods of time. 
There are two entirely distinct sets of perturbations in 
the earth’s motion that are combined in bringing about an 
Ice Age, and their combined effect is exceedingly irregular in 
amount at different periods ; but the total possible disturbance 
is limited to an amount which, though very large in its effects 
upon the climate of the earth at the different periods, is in 
itself only a small fractional disturbance of the earth’s 
standard or normal motion. 
One set of the perturbations is caused by the attraction of 
the sun and moon conjointly ; this change is periodic and 
regular, all the change being passed through in a definite time, 
and thus the whole change is exactly repeated. The period 
of this change is as great as 21,000 years ; but this period, 
though so long a time in relation to the inhabitants of the 
earth, sinks into insignificance when compared with the other 
set of perturbations, which extend over the enormous periods 
of a quarter of a million years to more than a million years. 
The second set of the perturbations is caused by the 
attraction of the two planets Jupiter and Venus, and to a 
slight extent also by the other planets of the solar system. 
The discovery by Croll of this cause of the Ice Age has been 
further worked out by Geikie and others, and recently by 
