674 THE POPULAR SCIENCE MONTHLY. 



mer, 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 cen- 

 tre 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 night- 

 mare of subsidence and emergence need no longer oppress the geolo- 

 gist. 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 geo- 

 logical 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 an- 

 tecedent 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 in- 

 formed 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 concep- 



