110 THE GLACIER EPOCH OF AUSTRALASIA. 
strated by the late Dr. Croll, and as lately confirmed and 
amplified in some particulars by Sir. Robert Ball and 
others, would no doubt cause the limits of the polar ice cap 
to extend considerably towards the lower latitudes. But this 
result would not of itself involve the dynamic effects of glacia- 
tion without (1) the agency of highlands, and (2) without 
sufficient precipitation of moisture to turn the balance in each 
separate locality against the loss by melting during the short 
but excessive heat of summer. Inasmuch also as the now 
depressed isochional of the névé or snow line, the result 
of the astronomical cause, would still continue to rise 
in height towards the KHquator, there would (8) still 
e€ a point or isothermal line towards the Equator, beyond 
which the lowered temperature, from astronomical causes, 
would be inadequate to produce the freezing of water vapour 
near to the earth’s surface, even in cases where the latter 
attained to a considerable height, yet falling short of its 
still more elevated snow line isochional. 
On the other hand, the evidences of former glaciation in 
any one place are only direct proof that the particular area 
affected was formerly subjected to conditions involving a 
temperature at or below 32° F., but they do not directly deter- 
mine whether the area so affected was mainly influenced by 
(1) a general change in the regional temperature of that place, 
due to astronomical causes; or (2) by geographical and 
physical causes which, subsequent to the glacial action, of 
Which evidence remains, may have operated in lowering the 
mountain cap below the influence of normal temperature of 
the existing snow line isochional, above which it may have 
formerly reared; or (8) by alteration in the distribution of 
land and water in the same region, and thereby altering the 
thermal currents of air and ocean, whereby the isotherm of a 
former climate, as in Labrador, may have been changed— 
with or without the conjunction of astronomical causes—to an 
isotherm like that of Ireland. Notwithstanding all such 
qualifications which arise in the mind when dealing with the 
milder and detached examples of former glaciations which 
occur on the border line of latitudes lying almost beyond the 
scope of the astronomical cause to produce intense 
meteorological changes—as on lowlands in lower lati- 
tudes—it is almost conclusive that it would require 
a general astronomical cause, in combination with 
the geographical, to account for that intense form of 
glacial action which repeated itself after a small geologicak 
interval of time in Northern Europe, and in North America, 
during the pleistocene period. How far any one of these 
causes would fail to account for all the known evidence. of 
glacial phenomena in the younger rocks of both hemispheres 
is best realised when we consider the matter more closely. 
