TJie Evolution of Climates. — Manson. 167 
energy. When by the chilHng of the oceans to about 31' 
Fahr., and by the glaciation of continental areas, the air was 
cleared of obscuring clouds and fogs, the uniform series of 
climates was at an end. 
The transition epoch — the date of passage from the control 
of earth heat to that of solar heat is the Ice age. The transi- 
tion is still in progress, for solar energy has not yet fullv estab- 
lished its control, nor has the Ice age fully departed from the 
earth. 
As in both plant and animal life certain types foreshadowed 
through long ages the forms which were to follow, so doubt- 
less, too, in earlier ages glaciers formed upon the mountains, 
and foreshadowed the Ice age which was to follow when earth 
heat failed. 
When the ruptured and denuded crust ceased to yield heat, 
to be held and conserved by the high specific heat of water, 
then was the Ice age inaugurated in earnest, to culminate when 
the last effective remnant of earth heat was wrung from that 
agent most capable of holding it. 
As the oceans approached their point of maximum density 
then did earth heat cease to dominate surface temperatures, 
and thenceforth climatic development has been dominated bv 
solar energy. It is reasonably certain that under the vertical 
rays of equatorial latitudes, solar control was first established 
in the torrid zone and is still being extended as marked bv 
the vertical retreat of dwarfed glaciers left at great altitudes: — 
that over temperate and polar regions this power is yet being 
extended as similarly marked by progressive glacial retreat. 
With the dominion of solar heat there dawned upon our 
planet an era of climatic zones whose lines sensibly follow 
parallels of latitude; then also began seasons of spring, sum- 
mer, autumn and winter, with the varying changes due to the 
earth's annual round. 
The climatic changes during the control of earth heat, and 
within the range of geological research, extended over eras: 
1. An era of torrid heat. 
2. An era of tropical heat. 
3. An era of temperate heat. 
4. An era of glacial cold. 
Each merged gradually into the others, but each recorded 
