2 
W. J. Solla-s — On Evolution in Geology. 
temperature of its own, or what temperature it has is generally 
taken as practically constant. 
Tlius by raisiug tlie temperature of our luminary we increase the 
difference between its temperature and that of surrounding space. 
It will consequently cool quicker tlian wben its temperature is lower 
and the difference in temperature correspondingly less. 
In radiating its beat away more rapidly, it will supply our earth 
witb a greater quantity of energy, wbicb only comes to saying what 
is obvious enough, that when the sun was hotter, it shone with 
increased intensity on the earth. 
The Earth . — The same conclusion may be held to be true as 
regards the earth, likewise considered as a body cooling in space. 
It is now very hot inside, but at one time it must have beeu much 
hotter ; every Century that has passed over it, has left it more and 
more impoverished in heat. It becoines hotter not only as we 
descend its crust, but also as we pass backwards in time. 
The rate at which it cools can only be determined by making use 
of data confessedly imperfect, and from these Sir Wm. Thomson 
finds that 100 or 200 millions of years ago it first began to be 
crusted over by a solid film of rocks ; that 10,000 years after its 
first crusting over the temperature of the crust increased 2° F. for 
every lft. vertically desceuded below the zone of constant tempera- 
ture. The rate of increase for descent with successive periods is 
shown in the following table : 
Period after crusting over, 
Years. 
10,000 
40,000 
160,000 
4,000,000 
100,000,000 
Rate of increase in temperature 
for every lft. descended. 
2° F. 
T 
x° 
2 
1 a 
TU 
vr° 
yj of a degree being the present rate of increase. 
Various estimates have been made by geologists of the time which 
has elapsed since the deposition of the earliest recorded strata, from 
the time the Laurentian were first laid down to the present day. 
From 500 to 50 millions of years would, I think, include the least 
extravagant of them, a period which brings the period of formation 
of the earliest known strata into very close proximity with that 
when the sun first began bis existence as our luminary, and when 
our earth was first permanently crusted over. 
Prof. Tliomson’s estimates may be wrong, and the estimates of 
the geologists may be wrong, either or both may be erroneous to a 
considerable degree ; but even if we put Sir Wm. Thomson’s 
periods farther back, twice as far back, or bring the Laurentian 
period nearer to us, there will yet remain a very considerable excess 
of energy, both in the sun and in our planet, beyond what they 
at present possess, and the influence of which must have been 
distinctly sensible on the rate of early geologic change. It is there- 
fore not only legitimate, but it even becomes necessary, to consider 
what must have been the precise effects of this excess of energy on 
the earlier physical geograpliy of our globe. 
