466 RECENT ADVANCES IN GEOLOGY. 
attaining an extreme height of between four hundred and 
five hundred feet, contain marine remains; but when we 
pass over into the Great Lake-basin, these remains disap- 
pear. Hence it has been inferred that, at that time, as now, 
the Great Lakes were filled with fresh water; but the dis- 
coveries of Dr. Stimpson, I think, disprove the correctness 
of this inference; and further discoveries may show that 
these lakes formerly had communication, not only with the 
Atlantie through the St. Lawrence, but with the Arctic 
Ocean through Hudson Bay. 
We are now led to the inquiry: What has caused these 
great changes of temperature, affecting the whole economy 
of terrestial life? Between the Arctie an Antarctic regions, 
there are great diversities of climate and physical conditions. 
The one is characterized by a vast expanse of land, and the 
other by a vast expanse of ocean. The one enjoys a short- 
lived summer in which the flowers blossom and fructify ; in 
the other reigns unmitigated winter, and. even mosses and 
lichens are absent. In the one the reindeer and musk-ox 
are hunted to the verge of the sea; in the other, animal life 
disappears below latitude 56 deg. Man has been able to 
penetrate North to 82 deg., 40 min., 30 sec., or within 
nearly five hundred miles of the pole; but to the south he 
has only reached 78 deg., 10 min., or about eight hundred 
and fifty miles. 
There are several causes which combine to produce this 
result. The great continental masses which characterize the 
northern hemisphere, warmed by the summer sun, radiate 
heat into surrounding space, while the narrow expanse of 
land in the Antaretie circle, bathed by chilled waters, and 
encased in ice, acts as a refrigerator of the atmosphere. Be- 
sides, as we shall hereafter show, owing to the earth’s move- 
ment, the southern summer is shorter by at least eight days, 
and the amount of heat received during that period by the 
northern hemisphere cannot but exert an appreciable influ- 
ence. The Arctic region, then, enjoys a milder climate than 
