CHAP. IX.] 
ANCIENT GLACIAL EPOCHS. 
165 
alternations of cold and warm phases 
every 10,500 years. The diagram 
also shows us another long-continued 
period of high excentricity from 
1,750,000 to 1,950,000 years ago, 
and yet another almost equal to the 
maximum 2,500,000 years back. 
These may perhaps have occurred 
during the Eocene and Cretaceous 
epochs respectively, or all may have 
been included within the limits of 
the Tertiary period. As two of these 
high excentricities greatly exceed 
that which caused our glacial epoch, 
while the third is almost equal to it 
and of longer duration, they seem to 
afford us the means of testing rival 
theories of the causes of glaciation. 
If, as Dr. Croll argues, high excen- 
tricity is the great and dominating 
agency in bringing on glacial epochs, 
geographical changes being subor- 
dinate, then there must have been 
glacial epochs of great severity at all 
these three periods ; while if he is 
also correct in supposing that the 
alternate phases of precession would 
inevitably produce glaciation in one 
hemisphere, and a proportionately 
mild and equable climate in the 
opposite hemisphere, then we should 
have to look for evidence of ex- 
ceptionally warm and exceptionally 
cold periods, occurring alternately 
and with several repetitions, with- 
in a space of time which, geo- 
logically speaking, is very short 
indeed. 
DIAGRAM SHOWING THE CHANGES OB' EXCENTRICITY DURING THE LAST THREE MILLION YEARS. 
