i62 TJic American Geologist. September. i899 
from the outer surface of this cloud sphere, witli no partial 
return of heat from exterior sources. 
It therefore follows that the isotherm equally heated by 
both exterior and interior sources was colder than 32° Fahr., 
or below that temperature at which snow and ice form. [It 
is well known that solar energy cannot maintain a temperature 
as high as 32° Fahr., except in the lower and denser regions 
of the atmosphere.] 
The isothermal surfaces nearest the earth were spheroidal 
in shape, and by reason of the conditions their surfaces were 
practically parallel with that of the earth ; those most remote 
from the earth, by reason of solar influences, flattened at the 
equator and protruded at the poles, so as to be slightly less 
oblate than the earth. Hence at the equator the direct action 
of the sun was first felt and established. 
As the effective and controlling earth heat was a finite 
quantity exposed to loss, it was in time exhausted. As this 
loss proceeded, the spheroidal isothermal shells of mean tem- 
perature shrunk in upon the earth, and their contact with its 
surface marked the lines of corresponding climates prevailing 
during the dual source of heat. Since the inner isotherms were 
largely independent of equatorial or polar exposure to solar 
energy their contacts with the planetary surface established 
climates practically independent of equatorial or polar position, 
or in other words of latitude; and not until those depending 
upon solar energy shrunk to the surface could climates ranged 
in latitudinal zones be established. As the climates estab- 
lished by the contact of the isotherms inside of the one equally 
heated by earth and sun were independent of direct solar heat, 
they varied from the climates established later by solar heat 
alone; hence the marked difference between climates ante- 
dating and succeeding the Ice age. The isotherms preceding 
this age were dependent almost entirely upon elevation above 
sea level, fractures, and conductivity of the earth's crust; those 
succeeding it are dependent upon proximity to the equator, 
elevation above sea level, and the distribution of heat by ocean 
currents. 
At the expiration of a succeeding period of time, the 
earth lost sufficient heat to cause the isothermal surface of 90° 
Fahr., to shrink to the surface except at fractures, and a uni- 
