+50 Climate of the Glacial Period. (October, 
earth having once been ina state of fusion, I can find nothing 
to warrant the conclusion that for long geological ages it 
has cooled in any appreciable degree. Laplace, reasoning 
from astronomical observations made in the time of Hip- 
parchus, calculated that during the last 2000 years there has 
been no appreciable contraction of the earth by cooling, for 
the length of the day has not been sensibly shortened, not 
even to the amount of 1-300th of a second, so that the con- 
traction of the globe must have been inappreciably small or 
none at all, as it could not take place without affecting the 
length of the day. We may therefore ask how an amount 
of contraction inappreciable in 2000 years can have resulted 
in the great amount of movement of the earth’s crust and 
the vast volcanic energy now apparent, or why should its 
tendency be to lengthen the polar and shorten the equatorial 
diameters ? and are not such movements more in accordance 
with the cause I have suggested ? 
It is true that the earth must radiate heat into space; but 
it is not evident that it radiates more annually than it re- 
ceives from the sun, and if it does not it is not a cooling 
globe. If earthquakes and volcanoes are the result of move- 
ments of the earth’s crust—produced, not by contraction, 
but by the strains set in action by the melting of the ice 
caps of the Glacial period—so probably is what we call the 
internal heat of the earth increasing in depth in our mines. 
The usually accepted theory that the increased temperature 
in depth is due to a greatly heated or even fluid fused nucleus, 
slowly giving off its heat towards the surface, does not explain 
the irregular distribution of the heat. To my mind it is 
much more conceivable and more probable that the centre of 
the earth is as cold as space, and that the movements of its 
upper strata and the heat they give rise to are confined to 
a comparatively shallow envelope, say not more than 500 
miles thick. 
The insufficiency of the theory of central heat was strongly 
impressed upon me when I was studying the facts connected 
with the frozen soil of Northern Siberia. At Yakutsk the 
soil—excepting a few feet at the surface which is thawed 
every summer—is permanently frozen to a depth of about 
400 feet. This frozen soil extends to the shores of the Arctic 
Sea, and in many places the cliffs bordering the rivers are 
composed of alternate layers of soil and ice. It is in these 
cliffs that the bodies of the Arétic rhinoceros and mammoth 
have been found with their flesh still preserved. As Lyell 
as remarked, since they were entombed, the soil cannot have 
thawed for a single season or their flesh would have 
