SOME GENERAL OBSERVATIONS ON THE WHOLE QUATERNARY. 593 



2. Time involved in the Quaternary Period. — If we accept CrolPs 

 and Wallace's view, then it is possible to estimate with accuracy the 

 length of the Glacial epoch and the time elapsed since its close, for it 

 is needless to say that astronomical cycles are calculable with great 

 certainty. The following diagram, taken from Mr. Wallace, shows the 



200 150 100 * 50 



THOUSAND YEARS AGO FROM 



Fig. 964.— Diagram of Eccentricity and Precession: Absciss represents time, and ordinates, degrees 

 of eccentricity and also of cold. The dark and light shades show the warmer and colder win- 

 ters, and therefore indicate each 10,500 years, the whole representing a period of 300,000 years. 



degrees of eccentricity during the last 300,000 years, and the recurring 

 cycles of precession during that period. If, as he thinks, the cold was 

 mainly due to eccentricity and geographical changes, the precessional 

 changes having little effect, then this figure will also represent the 

 degrees of cold. It is seen that, according to Croll and Wallace, the 

 Glacial period commenced 240,000 years ago, lasted 160,000 years, and 

 80,000 years have elapsed since its decline. It is seen also that Mr. 

 Wallace makes but one interglacial period instead of eight, the effect 

 of the shorter precessional cycles being tided over by the effect of the 

 accumulated ice. 



On any view as to the cause of the glacial climate, there can be no 

 doubt that the changes which produced it were effected very slowly, 

 and therefore involved long periods of time, so slowly that they would 

 probably be unobserved by contemporaneous man, if such existed. 

 There are changes by elevation and depression now going on in various 

 parts of the earth which are probably as rapid as those of the Glacial 

 and Champlain epochs. The shores of the Baltic and of Norway are 

 now rising at an average rate of two and a half feet per century. Con- 

 tinue this rate for 800 centuries, and Norway would attain an elevation 

 as great as that of the Glacial epoch, and, if such elevation produces 

 cold, would be again ice- sheeted. Depression at a similar rate for the 

 same time would bring about a condition similar to that of the Champ- 

 lain epoch. Yet these changes are unremarked, except by the eye of 

 Science. The only difference, if any, between what is in progress now 



38 



