--During
the year he had much correspondence on the subject of the subsidences
on Blackheath.
The following letter was written in reply to a gentleman who had asked
whether it could be ascertained by calculation how long it is since
the Glacial Period existed:
_1882, July 4_.
SIR,
I should have much pleasure in fully answering your questions of July
3 if I were able to do so: but the subject really is very obscure.
(1) Though it is recognized that the glacial period (or periods) is
late, I do not think that any one has ventured to fix upon a rude
number of years since elapsed.
(2) We have no reason to think that the mean distance of the earth
from the sun has sensibly altered. There have been changes in the
eccentricity of the orbit (making the earth's distance from the sun
less in one month and greater in the opposite month), but I do not
perceive that this would explain glaciers.
(3) I consider it to be certain that the whole surface of the earth,
at a very distant period, was very hot, that it has cooled gradually,
and (theoretically and imperceptibly) is cooling still. The glaciers
must be later than these hot times, and later than our last
consolidated strata: but this is nearly all that I can say.
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