172 The American Geologist. September, isgy 
cold depths of the oceans, the conservative influence of which 
cannot be estimated; for besides the difficuhies of heating 
water from the surface downwards, there yet remains the cool- 
ing effect of surface evaporation. There is thus presented the 
extreme slowness of the methods by which vast changes are 
wrought. Here are agencies whose results are so slight as not 
to be detected by thermometric methods — yet recording their 
effects in the progressive removal of glacial conditions, and in 
the establishment and maintenance in their stead of zones of 
tropical and temperate climates. 
In this view it becomes possible to trace the steps by which 
glacial conditions were removed and zones of climate estab- 
lished. 
Solar energy first established its control in that zone most 
exposed to its power — namely, the torrid zone. From this 
zone, glacial conditions were first removed, and this removal 
continued north and south upon lines sensibly parallel with 
present isotherms but modified by precipitation and glacial 
flow. 
In considering the astronomical causes, and the physical 
results thereby brought about, it has been argued that these 
causes tended to heat the northern hemisphere more rapidly 
than the southern. Dr. Croll and other physicists have so fully 
discussed this question that there remains but little to be 
added. 
The prime reason, however, seems to have been omitted, 
which is simply this : the northern hemisphere, containing so 
large a predominance of land area, is more easily warmed than 
the southern. This unequal heating once inaugurated would 
establish currents both of air and water tending to perpetuate 
this action, reinforced as it is by geographical and cosmical 
agencies. 
By the action of the same causes and upon the same prin- 
ciples, the Atlantic ocean has warmed more rapidly than the 
great Pacific ocean, as shown by the higher temperatures on 
the easterly coasts of the Atlantic compared with those of the 
Pacific. 
When, by this gradual accession of heat, conditions and 
temperatures resembling those existing prior to the Ice age, 
were established, we find these new conditions restricted to 
