Page:Popular Science Monthly Volume 27.djvu/118

106 giving you a distinct answer. He knows that the chalk is older than the London clay, and the oölite than the chalk, and the red marl than the oölite; and he knows also that each of them took a very long time indeed to lay down, but exactly how long he has no notion. If you say to him, "Is it a million years since the chalk was deposited?" he will answer, like the old lady of Prague, whose ideas were excessively vague, "Perhaps." If you suggest five millions, he will answer oracularly once more, "Perhaps"; and if, you go on to twenty millions, "Perhaps," with a broad smile, is still the only confession of faith that torture will wring out of him. 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 southwestern 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 moutonnées 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. This world of ours, though usually steady enough in its movements, is at times decidedly eccentric. Not that I mean to impute to our old and exceedingly respectable planet any occasional aberrations of intellect, or still less of morals (such as might be expected from Mars and Venus); the word is here to be accepted strictly in its scientific or Pickwickian sense as implying merely an irregularity of movement, a slight wobbling out of the established path, a deviation from exact circularity. Owing to a combination of astronomical revolutions, the precession of the equinoxes and the motion of the aphelion (I am not going to explain them here; the names alone will be quite sufficient for most people; they will take the rest on trust)—owing to the combination of these profoundly interesting causes, I say, there occur certain periods in the world's life when for a very long time together (10,500 years, to be quite precise) the northern hemisphere is warmer than the