much discussed and controverted topic. Seven 
theories have been presented from time to time, 
accounting for these important epochs, chief of 
which is the very interesting— 
ASTRONOMIC AI, THEORY. 
This theory, first presented by Mr. Croll, and en¬ 
dorsed by Professor Geikie and many other English 
geologists, “attributes the Glacial Age to the com¬ 
bined influence of precession of equinoxes and 
secular changes in the eccentricity of the earth's 
orbit,” whereby the seasons, summer and winter, 
would have a disparity of nearly five weeks instead 
of one week, as at present: this disparity, it is 
claimed, would produce Glacial and Thermal 
Ages alternately, every 21,000 years. 
However, Professor Le Conte, America’s most 
renowned geologist, controverts this theory, in 
part, quoting from many authorities, showing that 
but one Glacial Age can be proven. Referring to 
the researches of Professor Wallace and others he 
asserts that the phenomenon is the result of several 
agencies—astronomical, geological, and geograph¬ 
ical—producing a severe Glacial Age of great 
length, with two cumulative periods of greatest 
severity and a Sub-Thermal period between; the 
Age commencing about 240,000 years ago, continu¬ 
ing 160,000 years, and ending 80,000 years ago. 
GREAT DESTRUCTION BY COED. 
During this Glacial Age the plants were driven 
slowly, generation after generation, a few feet at a 
time, down across the North Temperate Zone, by 
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