EVIDENCES OF THE AGE OF ICE. 
115 
our islands by the more temperate sub-arctic and arctic charac- 
ter of their faunas, and what traces remain* of their floras also. 
Then came the Glacial period, first initiated perhaps by the 
diversion of the Gulf-Stream, caused by the subsidence of the 
Isthmus of Panama, or more probably by the opening up of a 
course for its waters up the great central valley of North 
America, down which the Mississippi river now flows, and which, 
save a narrow strip between Lake Superior and Hudson’s 
Bay, is nowhere more than 800 feet above the level of the sea ; 
this, if lowered, would give a direct course for the Gulf-Stream 
up to the north-west coast of Greenland and to Smith’s Sound. 
Afterwards, by the elevation of the land only 600 feet, this 
island would be united to the continent on the one hand, and 
to Ireland on the other ; whilst its shores would extend out- 
wards to the margin of the plateau of Ireland, seventy miles to 
the west, and from beyond the Shetlands in the north to near 
the north-west of Spain in the south. 
Probably the elevation was far greater, for the British Isles 
have a powerful line of volcanic disturbance running down about 
the meridian of 6° west longitude, which in the western High- 
lands and the north of Ireland was active down to an exceed- 
ingly late geological period (Miocene). 
If it be necessary to call in extra-mundane causes to explain 
the great increase of ice at this glacial period, I would prefer 
the theory propounded by Dr. Robert Hooke, in 1688 ; since by 
Sir Richard Phillips and others ; and lastly by Mr. Thomas Belt, 
C.E., F.G-.S. ; namely, a slight increase in the present obliquity 
of the ecliptic — a proposal in perfect accord with other known 
astronomical facts, and the introduction of which involves no 
disturbance of that harmony which is essential to our cosmical 
condition as a unit in the great solar system.* 
Such an increase in the obliquity of our earth’s axis would 
result in an increase of ice, not at one pole at a time, as pro- 
posed by Mr Croll, but at both poles simultaneously ; a condition 
which accords with the fact that with our present obliquity we 
have ice at both poles now ; the larger supply at the antarctic 
being purely caused by the fact that in the southern hemisphere 
we have a polar continent surrounded by a circumpolar ocean 
(fig. 4), whereas in the arctic we have a polar sea surrounded 
by circumpolar land (fig. 3). 
* In Jupiter the axis is nearly perpendicular to the plane of its orbit. In 
Saturn the obliquity is 29°. In Mars it is 30f° ; in Venus it reaches the 
extreme of 75°, so that its tropics actually overlap its arctic circle, and there 
are no temperate zones. The earth has an inclination of 23^°. It is esti- 
mated that its axis may have been inclined as much as 35^° during the 
glacial period. 
