158 The Americaii Geologist. September, ]89'.> 
tures, we will therefore accept as a premise that at the dawn of 
geological time two sources of heat influenced the climatic 
conditions of the earth. 
It is necessary to note the peculiar function of each and to 
mark the date of the sensible extinction of one and the es- 
tablishment in control of the other. 
The mode of heat distribution during the existence of earth 
heat must have been so different from the distribution after it 
ceased to be a controlling cause, that nature may be depended 
upon to have definitely recorded within reasonable and posi- 
tive limits the epoch when it ceased to act as the dominant 
agent, and although there may have been"fluctuations in this 
dying energy," it may be given us to plainly fix the epoch of 
the transition. We must note also that there were two dis- 
tinct cooling surfaces, one land and the other water; the latter 
having a very high specific heat, the former a comparatively 
low specific heat. A marked difference in the rate and time 
of cooling, must therefore, be looked for, water cooling much 
more slowly than earth and rocks, and it was also a prime 
agent through which the land parted with its heat. In addi- 
tion to this water appears in three distinct forms each of which 
possesses remarkable properties in its relation to heat and cold. 
It is universally admitted that this original heat has been 
lost to such an extent that it is no longer the controlling factor 
in the surface temperature of the earth, and that solar energy 
is now the controlling source of heat.* 
There can be then, no mistaking the first nor the present 
condition of the earth as regards its exposure to the two 
known sources of heat — (i) solar and stellar heat,f and (2) 
resident internal or earth heat. There must have been two 
marked eras of climatic control — (i) a past era, during which 
both sources were active; (2) the present era, in which the 
greater exterior source only remains, the local and lesser 
source having been largely exhausted. 
Or in other words, we have, first, a heated globe having 
resident in its mass a finite quantity of heat undergoing loss, 
*See also Manual of Geology, Dana, 4th Edition, p. 258. 
fStellar heat having the same function as solar heat, and being 
sensibly a constant of unknown amount but much less than solar heat, 
need not be separately considered in the further discussion of the 
question. 
