But in
the matter of the Glacial Epoch, a comparatively late and almost
historical event, geologists have broken through their usual reserve on
this chronological question and condescended to give us a numerical
determination. And here is how Dr. Croll gets at it.
Every now and again, geological evidence goes to show us, a long cold
spell occurs in the northern or southern hemisphere. During these long
cold spells the ice cap at the poles increases largely, till it spreads
over a great part of what are now the temperate regions of the globe,
and makes ice a mere drug in the market as far south as Covent Garden or
the Halles at Paris. During the greatest extension of this ice sheet in
the last glacial epoch, in fact, all England except a small
south-western corner (about Torquay and Bournemouth) was completely
covered by one enormous mass of glaciers, as is still the case with
almost the whole of Greenland. The ice sheet, grinding slowly over the
hills and rocks, smoothed and polished and striated their surfaces in
many places till they resembled the _roches moutonnees_ similarly ground
down in our own day by the moving ice rivers of Chamouni and
Grindelwald. Now, since these great glaciations have occurred at various
intervals in the world's past history, they must depend upon some
frequently recurring cause. Such a cause, therefore, Dr. Croll began
ingeniously to hunt about for.
He found it at last in the eccentricity of the earth's orbit.
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