Page:Popular Science Monthly Volume 9.djvu/702

674, would accumulate till a cap many thousand feet thick formed at the pole, and would ultimately spread far down into what is now the temperate zone. If such an ice-cap were only equal in density to 1,000 feet of earth, accumulated, say, on the north side of the globe, the centre of gravity would be shifted 500 feet to the north; and as the ocean would accommodate itself to the centre there would be a subsidence at the north pole equal to 500 feet. But this is not all, for at the time the ice-sheet was forming on the northern hemisphere, a sheet of equal size would be melting on the southern. This would double the effect, and produce a total submergence of 1,000 feet at the north pole and a total elevation of 1,000 feet at the south pole.

It is clear that all the upheavals and submergences of land which have so impressed geologists with the immensity of time required for their execution can thus be accounted for within periods, stupendous indeed if compared to historical time, or even to the duration of man on the earth, but still conceivable by human imagination. The nightmare of subsidence and emergence need no longer oppress the geologist. He has only to remark surface-changes and see how far forces now at work are capable of effecting them, and, if so, how long they would take. The discovery of Mr. Croll upsets the whole scale of geological time. Sir Charles Lyell was quite right in saying that the earth could not have subsided and emerged from the sea half a dozen times, in less than a million of years, if it sank or rose in the leisurely manner which has characterized it in recent times: consequently he could not accept as "the glacial epoch" the most recent period of great eccentricity. He was obliged to go back to the next, which happened nearly a million years ago. Sir Charles Lyell's standard of measurement is the date of the age of ice. If, therefore, the age of ice is assigned to a period 200,000 years ago instead of a million years ago, the standard of Sir Charles Lyell is diminished by four-fifths; and, adapting his conclusions to the altered premises, we should have forty-eight millions of years instead of two hundred and forty millions for the age of the fossiliferous rocks.

This change of standard would agree very well with the fact that there are evidences in the Eocene and Miocene periods of ice ages antecedent to the last. These might well be referred to the former periods of high eccentricity.

Enormous as are the periods which have undoubtedly passed since the creation of the world, it need not startle us to be told that every succession of events of which we have any evidence may well have occurred within a manageable number of millions of years. Could we stand, as Mr. Croll says, upon the edge of a gorge a mile and a half in depth, that had been cut out of the solid rock by a tiny stream scarcely visible at the bottom of this fearful abyss, and were we informed that the little streamlet was able in one year to wear off only one-tenth of an inch of its rocky bed, what would be our